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Title: The Chemistry of Plant Life
Author: Thatcher, Roscoe Wilfred, 1872-1933
Language: English
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THE CHEMISTRY OF PLANT LIFE
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THE CHEMISTRY OF PLANT LIFE
BY
ROSCOE W. THATCHER, M.A., D.AGR.
DEAN OF THE DEPARTMENT OF AGRICULTURE
AND DIRECTOR OF THE AGRICULTURAL EXPERIMENT STATIONS.
UNIVERSITY OF MINNESOTA
(FORMERLY PROFESSOR OF PLANT CHEMISTRY. UNIVERSITY OF MINNESOTA)
FIRST EDITION
SECOND IMPRESSION
MCGRAW-HILL BOOK COMPANY, INC.
NEW YORK: 370 SEVENTH AVENUE
LONDON: 6 & 8 BOUVERIE ST., E. C. 4
1921
COPYRIGHT 1921; BY THE McGRAW-HILL BOOK COMPANY, INC.
PREFACE
The author has had in mind a two-fold purpose in the preparation of this
book. First, it is hoped that it may serve as a text or reference book for
collegiate students of plant science who are seeking a proper foundation
upon which to build a scientific knowledge of how plants grow. The late
Dr. Charles E. Bessey, to whom I owe the beginning of my interest in plant
life, once said to me: "The trouble with our present knowledge of plant
science is that we have had very few chemists who knew any botany, and no
botanists who knew any chemistry." This may have been a slightly
exaggerated statement, even when it was made, several years ago. But it
indicated a very clear recognition by this eminent student of plants of the
need for a better knowledge of the chemistry of plant cell activities as a
proper foundation for a satisfactory knowledge of the course and results of
plant protoplasmic activities. It is hoped that the present work may
contribute something toward this desired end.
Second, the purpose of the writer will not have been fully accomplished
unless the book shall serve also as a stimulus to further study in a
fascinating field. Even the most casual perusal of many of its chapters
cannot fail to make clear how incomplete is our present knowledge of the
chemical changes by which the plant cell performs many of the processes
which result in the production of so many substances which are vital to the
comfort and pleasure of human life. Studies of the chemistry of animal life
have resulted in many discoveries of utmost importance to human life and
health. It requires no great stretch of the imagination to conceive that
similar studies of plant life might result in similar or even greater
benefit to human life, or society, since it is upon the results of plant
growth that we are dependent for most of our food, clothing, and fuel, as
well as for many of the luxuries of life.
The material presented in the book has been developed from a series of
lecture-notes which was used in connection with a course in
"Phyto-chemistry" which was offered for several years to the students of
the Plant Science Group of the University of Minnesota. In the preparation
of these notes, extensive use was made of the material presented in such
general reference works as Abderhalden's "Biochemische Handlexicon" and
"Handbuch der Biochemischen Arbeitsmethoden," Oppenheimer's "Handbuch der
Biochemie des Menschen und der Tiere," Czapek's "Biochemie der Pflanzen,"
Rohmann's "Biochemie," Frankel's "Descriptive Biochemie," and "Dynamische
Biochemie," Euler's "Pflanzenchemie," and Haas and Hill's "Chemistry of
Plant Products"; as well as of the most excellent series of "Monographs on
Biochemistry," edited by Plimmer, several numbers of which appeared in
print prior to and during the period covered by the preparation of these
lectures. Frequent use was made also of the many special treatises on
individual groups of compounds which are mentioned in the lists of
references appended to each chapter, as well as of articles which appeared
from time to time in various scientific journals.
Hence, no claim is made of originality for the statements presented herein,
except in an insignificant number of studies of enzyme action, and of the
possible physiological functions of certain specific compounds. The only
contributions which the writer has felt qualified to make to this general
subject are those of an intense personal interest in the chemistry of plant
processes and a viewpoint with reference to the relation of chemical
processes to vital phenomena which will be apparent as the various subjects
are presented.
The text has been prepared upon the assumption that the students who will
use it will have had some previous training in elementary inorganic and
organic chemistry. A systematic laboratory course in organic preparations,
such as is required of students who are preparing to become professional
chemists, is not at all a necessary requisite to the understanding of the
chemistry of the different groups of plant compounds as here presented; but
it is assumed that the student will have had such previous training as is
now commonly given in a one-year collegiate course in "General Chemistry,"
or a year's work in general inorganic chemistry followed by a brief course
in "Types of Carbon Compounds" or "Elements of Organic Chemistry," such as
is usually required of students who are preparing for advanced work in
agricultural science, in animal or human nutrition, etc.
An attempt has been made to arrange the material in such a way as to
proceed from simpler chemical principles and substances to those of more
complex structures. This results in an arrangement of the groups to be
studied in an order which is quite different than their biological
significance might suggest. It is believed, however, that in the end a more
systematic understanding and a more orderly procedure is obtained in this
way than would result from the treatment of the groups in the order of
their relative biological importance.
CONTENTS
INTRODUCTION PAGE
Development of biological science; characteristics of protoplasm;
plant and animal life, similarities and differences; protoplasmic
activity essentially chemical changes; objects of study of the
chemistry of plant life xiii-xvi
CHAPTER I--PLANT NUTRIENTS
Definitions; the plant food elements; available and unavailable
forms; the value of the different soil elements as plant foods;
functions of the different plant food elements in plant growth;
inorganic plant toxins and stimulants; references 1-15
CHAPTER II--ORGANIC COMPONENTS OF PLANTS
Plants as synthetic agents; types of changes involved in plant
growth; groups of organic compounds found in plants; physiological
use and biological significance defined; physiological uses of
organic groups 16-20
CHAPTER III--PHOTOSYNTHESIS
Definitions; physiological steps in photosynthesis; formaldehyde,
the simplest carbohydrate structure; the condensation of
formaldehyde into sugars; theories concerning photosynthesis;
the production of starches and sugars; references 21-29
CHAPTER IV--CARBOHYDRATES
Importance, nomenclature, and classification; groups of
carbohydrates; isomeric forms of monosaccharides; chemical
constitution of monosaccharides; characteristic reactions of
hexoses; the occurrence and properties of monosaccharides;
disaccharides; trisaccharides; tetrasaccharides; the relation of
molecular configuration to biochemical properties; polysaccharides,
dextrosans, levulosans, mannosans, and galactosans; physiological
uses and biological significance of carbohydrates; references 30-66
CHAPTER V--GUMS, PECTINS, AND CELLULOSES
Relation to carbohydrates; groups; the natural gums and pentosans;
mucilages; pectins; celluloses; physiological uses of celluloses;
references 67-75
CHAPTER VI--GLUCOSIDES
Definition; general structure; hydrolysis of the natural
glucosides; general properties; the phenol glucosides; the alcohol
glucosides; the aldehyde glucosides; the oxycumarin glucosides;
the cyanophoric glucosides; the mustard-oil glucosides; the pigment
glucosides; the digitalis glucosides; the saponins; physiological
uses; biological significance; references 76-93
CHAPTER VII--TANNINS
General properties; occurrence; chemical constitution; classes;
some common tannins; physiological uses; biological significance
of tannins in fruits; references 94-101
CHAPTER VIII--PIGMENTS
Types and classes; the chlorophylls, chemical constitution,
similarity of chlorophyll and hæmoglobin, properties of the
chlorophylls; the carotinoids, carotin, xanthophyll, lycopersicin,
and fucoxanthin; phycoerythrin and phycophæin; the anthocyans;
the anthoxanthins; the production of ornamental pigments in
flowers, etc.; the functions of pigments; references 102-123
CHAPTER IX--ORGANIC ACIDS, ACID SALTS, AND ESTERS
Chemical constitution; some common organic acids; physiological
uses of organic acids; biological significance of fruit acids and
esters 124-128
CHAPTER X--FATS AND OILS, WAXES, AND LIPOIDS
General composition; fats and oils, occurrence, chemical
constitution, acids which occur in natural fats, alcohols which
occur in natural fats, hydrolysis and synthesis of fats,
extraction of oils from plant tissues, identification of fats and
oils, physiological use; the waxes; the lipoids, lecithin, other
plant phosphatides, plant cerebrosides, physiological uses of
lipoids; references 129-145
CHAPTER XI--ESSENTIAL OILS AND RESINS
Definitions, classes, occurrence; the essential oils; the
resins; physiological uses and biological significance of
essential oils; references 146-150
CHAPTER XII--THE VEGETABLE BASES
Composition and groups; the plant amines; alkaloids; the purine
bases; the pyrimidines; the nucleic acids, composition and uses;
references 151-163
CHAPTER XIII--PROTEINS
Importance; general composition; amino-acids and peptid units;
individual amino-acids; composition of the plant proteins; general
properties of proteins; classification; differences between plant
and animal proteins; extraction of proteins from plant tissues;
synthesis in plants; physiological uses; references 164-180
CHAPTER XIV--ENZYMES
Reaction velocities; enzymes as catalysts; general properties;
extracellular and intracellular enzymes; chemical nature;
nomenclature and classification; occurrence and preparation;
general and individual enzymes; nature of enzyme action;
accelerators and inhibitors; coenzymes and antienzymes; zymogens;
physiological uses; further studies needed; references 181-201
CHAPTER XV--THE COLLOIDAL CONDITION
"Colloids" and "crystalloids"; the colloidal condition a
dispersion phenomenon; nomenclature and classification;
conditions necessary to the formation of sols; gel-formation;
general properties of colloidal solutions; suspensoids and
emulsoids; adsorption; catalysis affected by the colloidal
condition; industrial applications of colloidal phenomena;
natural colloidal phenomena; references 202-220
CHAPTER XVI--THE PHYSICAL CHEMISTRY OF PROTOPLASM
Heterogeneous structure of protoplasm; protoplasm a colloidal
gel; water; salts; osmotic pressure; surface boundary phenomena;
electrical phenomena; acidity and alkalinity; summary; vital
phenomena as chemical and physical changes; references 221-238
CHAPTER XVII--HORMONES, AUXIMONES, VITAMINES, AND TOXINS
External and internal stimulants; hormones; vitamines; auximones,
toxins 239-248
CHAPTER XVIII--ADAPTATIONS
General discussion; adaptations, accommodations, and adjustments;
chromatic adaptations; morphological adaptations; accommodations;
concluding statements 249-258
INDEX 259-268
INTRODUCTION
The history of biological science shows that the conceptions which men have
held concerning the nature of plant and animal growth have undergone a
series of revolutionary changes as the technique of, and facilities for,
scientific study have developed and improved. For a long time, it was
thought that life processes were essentially different in character than
those which take place in inanimate matter, and that the physical sciences
had nothing to do with living changes. Then, too, earlier students had only
vague notions of the actual structure of a living organism. Beginning with
the earliest idea that a plant or an animal exists as a unit organism, to
be studied as such, biological science progressed, first to the recognition
and study of the individual organs which are contained within the organism;
then to the tissues which make up these organs; then (with the coming into
use of the microscope as an aid to these investigations) to the cells of
which the tissues are composed; then to the protoplasm which constitutes
the cell contents; and finally to the doctrine of organic evolution as the
explanation of the genealogy of plants and animals, and the study of the
relation of the principles of the physical sciences to the evolutionary
process. The ultimate material into which organisms are resolved by this
process of biological analysis is the cell protoplasm. But protoplasm is
itself made up of a complex system of definite chemical compounds, which
react and interact according to the laws of physical science. Hence, any
study of the chemistry of plant growth is essentially a study of the
chemical and physical changes which take place in the cell protoplasm.
Protoplasm differs from non-living matter in three respects. These are (1)
its chemical composition; (2) its power of waste and repair and of growth;
and (3) its reproductive power. From the standpoint of chemical
composition, protoplasm is the most complex material in the universe. It
not only contains a greater variety of chemical elements, united into
molecules of enormous size and complexity, but also a greater variety of
definite chemical compounds than exist in any other known mixture, either
mineral or organic in type. One of the first problems in the study of
protoplasm is, therefore, to bring this great variety of complex compounds
into some orderly classification and to become familiar with their
compositions and properties. Again, living matter is continually undergoing
a process of breaking down as a result of its energetic activities and of
simultaneously making good this loss by the manufacture of new protoplasm
out of simple food materials. It also has the power of growth by the
production of surplus protoplasm which fills new cells, which in turn
produce new tissues and so increase the size and weight of individual
organs and of the organism as a whole. Hence, a second field of study
includes the chemical changes whereby new protoplasm and new
tissue-building material are elaborated. Finally, living material not only
repairs its own waste and produces new material of like character to it,
but it also produces new masses of living matter, which when detached from
the parent mass, eventually begin a separate existence and growth.
Furthermore, the plant organism has acquired, by the process of evolution,
the ability not only to produce an embryo for a successive generation but
also to store up, in the tissues adjacent to it, reserve food material for
the use of the young seedling until it shall have developed the ability to
absorb and make use of its own external sources of food material. So that,
finally, every study of plant chemistry must take into consideration the
stored food material and the germinative process whereby this becomes
available to the new organism of the next generation. Also, the chemistry
of fertilization of the ovum, so that a new embryo will be produced, and
the other stimuli which serve to induce the growth phenomena, must be
brought under observation and study.
A further step in the development of biological science has been to
separate the study of living things into the two sciences of botany and
zoology. From the standpoint of the chemistry of the processes involved
this segregation is unfortunate. It has resulted in the devotion of most of
the study which has been given to life processes and living things to
animal chemistry, or "physiological chemistry." As a consequence,
biochemistry, which deals with the living processes of both plants and
animals, is yet in its infancy; while phytochemistry is almost a new
science, yet its relation to the study of plants can scarcely be less
vital than is that of physiological chemistry to studies of animal life.
The common conception that plant life and animal life are antithetical or
complementary to each other has much to justify it. Animals breathe in
oxygen and exhale carbon dioxide; while plants use the carbon dioxide of
the air as a part of the raw material for photosynthesis and exhale oxygen.
Plants absorb simple gases and mineral compounds as raw food materials and
build these up into complex carbohydrates, proteins, fats, etc.; while
animals use these complex compounds of plant origin as food, transforming
parts of them into various other forms of structural material, but in the
end breaking them down again into the simple gases and mineral compounds,
which are expelled from the body through the excretory organs. Thus it
would seem that the study of the chemistry of plant life and of animal life
must necessarily deal with opposite types of phenomena.
But one cannot advance far into the study of the biochemistry of plants and
animals before he discovers marked similarities in the chemical principles
involved. Many of the compounds are identical in structure, undergo similar
changes, and are acted upon by similar catalysts. Plant cells exhibit
respiratory activities, using oxygen and giving off carbon dioxide, in
exactly the same way that animal organisms do. The constructive
photosynthetic processes of green plants are regulated and controlled by a
pigment, chlorophyll, which is almost identical with the blood pigment,
hæmatin, which regulates the vital activities in the animal organism,
differing from the latter only in the mineral element which links the
characteristic structural units together in the molecule. Many other points
of similarity in the chemistry of the life processes of plants and animals
will become apparent as the study progresses. It is sufficient now to call
attention to the fact that these vital processes, in either plants or
animals, are essentially chemical in character, and subject to study by the
usual methods of biochemical investigations.
The protoplasm of the cell is the laboratory in which all the changes which
constitute the vital activities of the plant take place. All of the
processes which constitute these activities--assimilation, translocation,
metabolism, and respiration--involve definite chemical changes. In so far
as it is possible to study each of these activities independently of the
others, they have been found to obey the ordinary laws of chemical
reactions. Thus, the effect of the variations in intensity of light upon
photosynthesis causes increase in the rate of this activity which may be
represented by the ordinary responses of reaction velocities to external
stimuli. Similarly, the effect of rises in temperature upon the rate of
assimilation and upon respiration are precisely the same as their effect
upon the velocity of any ordinary chemical reaction. Within certain
definite ranges of temperature, the same statement holds true with
reference to the rate of growth of the plant, although the range of
temperature within which protoplasm lives and maintains its delicate
adjustment to the four vital processes of life is limited; beyond a certain
point, further rise in temperature does not produce more growth but rather
throws the protoplasmic adjustment out of balance and growth either slows
up markedly or stops altogether.
Hence, we may say that the methods by which the plant machine (protoplasm)
accomplishes its results are essentially and definitely chemical in
character and may be studied purely from the standpoint of chemical
reactions, but the maintenance of the machine itself in proper working
order is a vital phenomenon which is largely dependent upon the external
environmental conditions under which the plant exists. A study of the
phenomena resulting from the colloidal condition of matter is throwing a
flood of light upon the mechanism by which protoplasm accomplishes its
control of vital activities. But we are, as yet, a long way from a complete
understanding of how colloidal protoplasm acquires and maintains its unique
ability of self-regulation of the conditions necessary to preserve its
colloidal properties and of how it elaborates the enzymes which control the
velocity of the chemical reactions which take place within the protoplasm
itself and which constitute the various processes of vital activity.
The object of this study of the chemistry of plant growth is to acquire a
knowledge of the constitution of the compounds involved and of the
conditions under which they will undergo the chemical changes which, taken
all together, constitute the vital processes of cell protoplasm.
CHEMISTRY OF PLANT LIFE
CHAPTER I
PLANT NUTRIENTS
There is some confusion in the use of the terms "nutrient," "plant food,"
etc., as applied to the nutrition and growth of plants. Strictly speaking,
these terms ought probably to be limited in their application to the
organized compounds within the plant which it uses as sources of energy and
of metabolizable material for the development of new cells and organs
during its growth. Botanists quite commonly use the terms in this way. But
students of the problems involved in the relation of soil elements to the
growth of plants, including such practical questions as are involved in the
maintenance of soil productivity and the use of commercial fertilizers for
the growing of economic plants, or crops, are accustomed to use the terms
"plant foods," or "mineral nutrients," to designate the chemical elements
and simple gaseous compounds which are supplied to the plant as the raw
material from which its food and tissue-building materials are synthetized.
Common usage limits these terms to the soil elements; but there is no
logical reason for segregating the raw materials derived from the soil from
those derived from the atmosphere.
The essential difference between these raw materials for plant syntheses
and the organic compounds which are produced within the plants and used by
them, and by animals, as food, is that the former are inorganic and can
furnish only materials but no energy to the organism; while the latter are
organic and supply both materials and potential energy. It would probably
be the best practice to confine the use of the word "food" to materials of
the latter type, and several attempts have been made to limit its use in
this way and to apply some such term as "intake" to the simple raw
materials which are taken into the organism and utilized by it in its
synthetic processes. But the custom of using the words "food," or
"nutrient," to represent anything that is taken into the organism and in
any way utilized by it for its nourishment has been followed so long and
the newer terms are themselves so subject to criticism that they have not
yet generally supplanted the loosely used word "food."
If such use is permitted, however, it is necessary to recognize that only
the green parts of green plants can use this inorganic "food," and that the
colorless plants must have organic food.
To avoid this confusion, the suggestion has recently been made that all of
the intake of plants and animals shall be considered as food, but that
those forms which supply both materials and potential energy to the
organism shall be designated as _synergic foods_, while those which contain
no potential energy shall be known as _anergic foods_. On this basis,
practically all of the food of animals, excepting the mineral salts and
water, and all of the organic compounds which are synthetized by plants and
later used by them for further metabolic changes, are synergic foods; while
practically all of the intake of green plants is anergic food.
It is with the latter type of food materials that this chapter is to deal;
while the following and all subsequent chapters deal with the organic
compounds which are synthetized by plants and contain potential energy and
are, therefore, capable of use as synergic food by either the plants
themselves or by animals. It will be understood, therefore, that in this
chapter the word "food" is used to mean the anergic food materials which
are taken into and used by green plants as the raw materials for the
synthesis of organic compounds, with the aid of solar energy, or that of
previously produced synergic foods. In all later chapters, the term "food"
will be used to mean the organic compounds which serve as the synergic food
for the green parts of green plants and as the sole supply of nutrient
material for the colorless parts of green plants and for parasitic or
saprophytic forms (see page 16).
PLANT FOOD ELEMENTS
The raw materials from which the food and tissue-building compounds of
plants are synthetized include carbon dioxide, oxygen, water, nitrogen,
phosphorus, sulfur, potassium, calcium, magnesium, and iron. The two gases
first mentioned are derived directly from the air, through the respiratory
organs of the plant. Water is taken into the plant chiefly from the soil,
through its fibrous roots. All the other elements in the list are taken
from the soil, nitrogen being derived from decaying organic matter (the
original source of the nitrogen is, however, the atmosphere, from which the
initial supply of nitrogen is obtained by direct assimilation by certain
bacteria and perhaps other low forms of plant life), and the remaining ones
from the mineral compounds of the soil.
Carbon dioxide and oxygen, being derived from the air, are always available
to the leaves and stems of growing plants in unlimited supply; but the
supply available to a seed when germinating in the soil, or to the roots of
a growing farm crop, may sometimes become inadequate, especially in soils
of a very compact texture, or "water-logged" soils. In such cases, the
deficiency of these gaseous food elements may become a limiting factor in
plant growth.
Water is often a limiting factor in plant growth. Experiments which have
been repeated many times and under widely varying conditions show that when
water is supplied to a plant in varying amounts, by increasing the
percentage of water in the soil in which the plant is growing by regular
increments up to the saturation point, the growth of the plant, or yield of
the crop, increases up to a certain point and then falls off because the
excess of water reduces the supply of air which is available to the plant
roots. Hence, abundance of water is, in general, a most essential factor in
plant growth.
Under normal conditions of air and moisture supply, however, the plant food
elements which may be considered to be the limiting factors in the
nutrition and growth of plants are the chemical elements mentioned in the
list above.
AVAILABLE AND UNAVAILABLE FORMS
The plant food materials which are taken from the soil by a growing plant
must enter it by osmosis through the semi-permeable membranes which
constitute the epidermis of the root-hairs, and circulate through the plant
either carried in solution in the sap or by osmosis from cell to cell.
Hence, they must be in water-soluble form before they can be utilized by
plants. Obviously, therefore, only those compounds of these elements in the
soil which are soluble in the soil water are _available_ as plant food. The
greater proportion of the soil elements are present there in the form of
compounds which are so slightly soluble in water as to be _unavailable_ to
plants. The processes by which these practically insoluble compounds become
gradually changed into soluble forms are chiefly the "weathering" action of
air and water (particularly if the latter contains carbonic acid) and the
action of the organic acids resulting from decaying animal or vegetable
matter or secreted by living plants.
THE VALUE OF THE SOIL ELEMENTS AS PLANT FOOD
Analyses of the tissues of plants show that they contain all of the
elements that are to be found in the soil on which they grew. Any of these
elements which are present in the soil in soluble form are carried into the
plants with the soil water in which they are dissolved, whether they are
needed by the plant for its nutrition or not. But in the case of those
elements which are not taken out of the sap to be used by the plant cells
in their activities, the total amount taken from the soil is much less than
is that of the elements which are used in the synthetic processes of the
plant. Hence, much larger proportions of some elements than of others are
taken from the soil by plants. The proportions of the different elements
which are used by plants as raw materials for the manufacture of the
products needed for their growth varies with the different species; but a
certain amount of each of the so-called "essential elements" (see below) is
necessary to every plant, because each such element has a definite rôle
which it performs in the plant's growth. A plant cannot grow to maturity
unless a sufficient supply of each essential element comes to it from the
soil.
From the standpoint of their relative value as raw materials for plant
food, the elements which are present in the soil may be divided into three
classes; namely, the _non-essential_, the _essential and abundant_, and the
_critical_ elements.
The first class includes silicon, aluminium, sodium, manganese, and certain
other rarer elements which sometimes are found in soils of some special
type, or unusual origin. These elements seem to have no rôle to play in
the nutrition of plants; although silicon is always present in plant ash
and sodium salts are found in small quantities in all parts of practically
all plants. Nearly all species of plants can be grown to full maturity in
the entire absence of these elements from their culture medium. Occasional
exceptions to this statement in the case of special types of plants are
known, and are of interest in special studies of plant adaptations, but
need not be considered here.
The second group includes iron, calcium, magnesium, and, generally, sulfur.
All of these elements are essential for plant growth, but are usually
present in the soil in ample quantities to insure a sufficient supply in
available form for all plant needs. Recent investigations have shown,
however, that there are many soils in which sulfur is present in such
limited quantities that many agricultural crops, when grown on these soils,
respond favorably to the application of sulfur-containing fertilizers. In
such cases, sulfur is a "critical" element.
The "critical" elements are those which are essential to the growth of all
plants and which are present in most soils in relatively small proportions
and any one may, therefore, be the limiting factor in plant growth so far
as plant food is concerned. These are nitrogen, phosphorus, potassium, and
(possibly) sulfur.
RÔLE OF PLANT FOOD ELEMENTS IN PLANT GROWTH
The use which a plant makes of the elements which come to it from the soil
has been studied with great persistency and care by many plant
physiologists and chemists. Many of the reactions which take place in a
plant cell are extremely complicated, and the relation of the different
chemical elements to these is not easily ascertained. It is probable that
the same element may play a somewhat different rôle in different species of
plants, in different organs of the same plant, or at different stages of
the plant's development. But the usual and most important offices of each
element are now fairly well understood, and are briefly summarized in the
following paragraphs. It should be understood that a thorough and detailed
discussion of these matters, such as would be included in an advanced study
of plant nutrition, would reveal other functions than those which are
presented here and would require a more careful and more exact method of
statement than is suitable here. However, the general principles of the
utilization of soil elements by plants for their nutrition and growth may
be fairly well understood from the following statements.
=Nitrogen= is a constituent of all proteins (see Chapter XIII). Proteins
are apparently the active chemical components of protoplasm. Since it is in
the protoplasm of the green portions, usually foliage, of plants that the
photosynthesis of carbohydrates and the synthesis of most, or all, of the
other tissue-building materials and reserve food substances of the plant
takes place, the importance of nitrogen as a plant food can hardly be
over-emphasized. Nitrogen starvation produces marked changes in the growth
of a plant. Leaves are stunted in growth and a marked yellowing of the
entire foliage takes place; in fact, the whole plant takes on a stunted or
starved appearance. Abundance of nitrogen, on the other hand, produces a
rank growth of foliage of a deep rich color and a luxuriant development of
tissue, and retards the ripening process. In the early stages of growth,
the nitrogen is present most largely in the leaves; but when the seeds
develop, rapid translocation of protein material into the seeds takes
place, until finally a large proportion of the total supply is deposited in
them.
Nitrates are the normal form of nitrogen in the soil which is available to
plants. During germination and early growth, the young seedling uses
amino-acids, etc., derived from the proteins stored in the seed, as its
source of nitrogen; and experiments have shown that similar forms of
soluble organic nitrogen compounds can be successfully fed to the seedling
as an external food supply. Soluble ammonium salts can be utilized as
sources of nitrogen by most plants during later periods of growth,
particularly by the legumes. But for most, if not all, of the common farm
crops whose possibilities in these respects have been studied, it has been
found that a unit of nitrogen taken up as a nitrate is very much more
effective in promoting growth, etc., than is the same unit of nitrogen in
the form of ammonium salts.
While the proteins are finally stored up largely in the seeds, or other
storage organs, they are actively at work during the growing period in the
cells of the foliage parts of the plant. Hence, the popular statement that
"nitrogen makes foliage" is a fairly accurate expression of its rôle.
Inordinate production of straw in cereal crops and of leaves in root crops
often results from liberal supplies of available nitrogen in the soil early
in the growing season. If the crops develop to normal maturity, this
excessive foliage growth has no harmful results, as the surplus material
which has been elaborated is properly translocated into the desired storage
organs; but, unfortunately, the retarding effect of the surplus nitrogen
supply upon the date of maturing of the crop is often associated with
premature ripening of the plants from other causes, with the consequence
that too large a proportion of the valuable food material is left in the
refuse foliage material of the crop. Crops which are grown solely for their
leaves, such as hay crops, lettuce, cabbage, etc., profit greatly by
abundant supplies of available nitrogen; although when foliage growth is
stimulated in this way the tissue is likely to be thin-walled and soft
rather than firm and solid.
=Phosphorus= is likewise an extremely important element in plant nutrition.
But phosphorus starvation produces no such striking visible effects upon
the growth of the plant as does lack of nitrogen. Abundance of available
phosphorus early in the plant's life greatly stimulates root growth, and
later on it undoubtedly hastens the ripening process; hence, this element
seems to act as the exact antithesis of nitrogen.
The rôle of phosphorus, or of phosphates, in the physiological processes of
the cell seems to be difficult to discover. The element itself is a
constituent of some protein complexes and of the lecithin-like bodies (see
page 141) which are supposed by some investigators to play an important
part in determining the rate of chemical changes which take place in the
cell and the movement of materials into and out of it. It is an essential
constituent of the nucleus, and a meager supply of phosphorus retards, or
inhibits, mitotic cell-division. Photosynthesis of sugars and the
condensing of these into starch or cellulose takes place in plants in the
absence of available phosphorus; but the change of these insoluble
carbohydrates back again into soluble and available sugar foods does not.
Phosphorus is taken from the soil by plants in the form of phosphates. Much
study has been given to the problem of the proper supply of available soil
phosphates for economic crop production. Any discussion of soil fertility
and fertilization which did not devote large attention to the conditions
under which phosphates become available as plant food would be wholly
inadequate; but such a discussion would be out of place here.
The final result of an ample supply of phosphates in hastening the ripening
process and stimulating seed production, as contrasted with that of an
over-supply of nitrogen, has led to the popular statement that "phosphates
make seeds." This statement, while not strictly accurate, is a fairly good
summary of the combined results of the rôle of phosphorus in the plant
economy. Large amounts of phosphorus are stored in the seeds. The two facts
that large amounts of these compounds are thus available to the young
seedling and that relatively large proportions of phosphates are taken from
the soil by the plant during its early stages of growth are undoubtedly
connected with the need for rapid cell-division at these periods in the
plant's life.
=Potassium.=--The popular expression that "potash makes sugars and starch"
is a surprisingly accurate description of the rôle of this element in plant
metabolism. Either the photosynthesis of starch, or the changes necessary
to its translocation (it is not yet certain which) is so dependent upon the
presence of potassium in the cell sap that the whole process stops at once
if an insufficient supply is present. The production and storage of sugar,
or starch, in such root crops as beets, potatoes, etc., diminishes in
direct proportion with a decreasing supply of potassium as plant food. The
grains of the cereal crops become shrunken as a result of potassium
starvation; and are plump and well filled with starch in the endosperm when
sufficient potassium is available for the crop's needs.
The general tone and vigor of growth of the plant is largely dependent upon
an ample potassium supply; potash-hungry plants, like those which have been
weakened by any other unfavorable conditions, have been found to be more
susceptible to injury by disease, than those which are well nourished with
this food element. But potassium-starvation does not produce any
pathological condition of the cell contents; its absence simply prevents
the possibility of the development of the necessary carbohydrates for
vigorous growth.
There is no known difference in the availability, or effectiveness, of
potassium from the different forms of compounds containing it which may be
present in the soil. Apparently, the only essential is that the compound
shall be soluble so that it can be absorbed into the plant through the
root-hairs. Of course, the acid radical to which the basic potassium ion is
attached may, in itself, have some beneficial or deleterious influence
which gives to the compound as a whole some important effect in one case,
which might not follow in the case of another type of compound; but the
relative efficiency as plant food of a given unit of potassium seems to be
the same regardless of the nature of the compound in which it is present.
=Calcium= is an essential plant food element but its physiological use has
not yet been definitely established. It seems to stimulate
root-development, and certainly gives vigor and tone to the whole plant. It
is commonly believed that calcium is in some way connected with the
development of cell-wall material. It has been reported that the stems of
grasses and cereal plants become stiffer in the presence of ample calcium,
but this may be due to greater turgidity rather than to strengthened
cell-walls. Calcium remains in the leaves or stem as the plant ripens, but
it is not clear that this has anything to do with the stiffness or weakness
of the stem, or straw, of the plant. Experiments with algæ have shown that
in the absence of calcium salts mitotic cell division takes place, showing
that the nucleus functions properly, but the formation of the new
transverse cell-wall is retarded. This is the only direct evidence that has
been reported that calcium has any connection with cell-wall formation.
Certain species of plants, notably many legumes, require such large amounts
of calcium salts for their growth as to give to them the popular
appellation of "lime-loving plants." Other plants, known as "calciphiles,"
while not actually showing abnormally large percentages of calcium in their
ash, flourish best on soils rich in lime. On the other hand, certain other
species, known as "calcifuges," will not grow on soils which are even
moderately rich in lime; in what respect these differ in their vital
processes from others which demand large amounts of calcium, or those which
flourish on soils rich in lime, has not been determined, however.
The beneficial effect of alkaline calcium compounds in the soil, in
correcting injurious acidity, in improving the texture of clay soils, and
in promoting the proper conditions for bacterial growth, is well known; but
this has no direct connection with the rôle of calcium as plant food.
Furthermore, calcium salts in the soil have a powerful influence in
overcoming the harmful, or toxic, effects of excessive amounts of soluble
salts of magnesium, sodium, or potassium, in the so-called "alkali soils"
(i.e., those which contain excessive amounts of water-soluble salts). The
probable explanation for this fact is pointed out in a later paragraph of
this chapter (see page 14); but this property of calcium probably has no
connection with its physiological uses as plant food.
=Magnesium=, like phosphorus, is finally stored up mostly in the seeds, not
remaining in the leaves and stems, as do calcium and potassium. This fact,
together with other evidence obtained from experiments in growing plants in
culture solutions containing varying amounts of this element, has led
certain investigators to the conclusion that the rôle of magnesium is to
aid in the transport of phosphorus, particularly from older to more rapidly
growing parts of the plant. More recent investigations have shown, however,
that magnesium has other rôles which are probably more specific and more
important than this one. It is now known that magnesium is a definite
constituent of the chlorophyll molecule serving, as will be shown (see
Chapter VIII), as the means of linkage between its essential component
organic groups. Because of this fact, magnesium-starvation produces
etiolated plants, which cannot function normally. Further, magnesium seems
to be necessary for the formation of fats, apparently standing in a similar
relation to fat-formation to that of potassium to carbohydrate-formation.
This view is supported by the observations that when algæ are grown in
magnesium-free solutions they contain no fat globules and that oily seeds
are richer in magnesium than are those which store up starch as their
reserve food material. Observers of the second of these phenomena
have failed to note, however, that oily seeds are likewise richer in
phosphorus than are starchy ones, and that the presence of larger
proportions of magnesium in such seeds may, perhaps, be related to
phosphorus-translocation rather than to fat-formation.
Whatever relation magnesium may have to fat-formation, or to the
translocation of phosphorus, it is evident that these are rôles quite apart
from its use as a constituent element in chlorophyll. As yet, no
explanation of how it aids in these other synthetic processes has been
advanced.
On the other hand, an excess of soluble magnesium salts in the soil
produces definite toxic effects upon plants, magnesium compounds being
known to be among the most destructive of the "alkali soil" salts. Calcium
salts are remarkably efficient in overcoming these harmful effects of
magnesium salts. On this account, a large amount of experimental study has
been given to the question of the calcium-magnesium ratio in plants.
Numerous analyses of plant ashes have established the fact that there is a
fairly definite ratio of this kind, which ratio, however, varies with the
species of plant and is not correlated with the ratio of these elements
present in the soil on which the plant grows, as was formerly believed.
Cereal plants, as a rule, contain approximately twice as much lime as
magnesia; while leafy plants (tobacco, cabbage, etc.) usually contain about
four times as much calcium oxide as magnesium oxide.
Iron is essential to chlorophyll-formation. It is not a constituent of the
chlorophyll molecule, as is magnesium; but in the absence of iron from the
culture solution, a plant fails to produce chlorophyll and a green plant
which is deprived of a supply of iron rapidly becomes etiolated. The way in
which iron is related to chlorophyll-formation is not known.
Iron is taken from the soil by plants in the smallest proportions of any of
the essential elements. Only soluble _ferric_ compounds seem to serve as a
suitable source of supply of the element; _ferrous_ compounds being usually
highly toxic to plants.
=Sulfur= is an essential element of plant food. The amounts required by
plants were supposed, until recently, to be relatively small. This was due
to the fact that earlier studies took account only of the sulfur which, on
analysis, appeared as sulfates in the ash. Improved methods of analysis,
which insure that the sulfur which is present in the plant tissue in
organic combinations is oxidized under such conditions that it is not lost
by volatilization during the combustion of the material, have shown that
the total sulfur which is present in many plants approaches the quantity of
phosphorus which is present in the same tissue. Furthermore, recent field
and pot experiments have shown that at least a considerable part of the
beneficial effects of many fertilizers, which has previously been
attributed to the calcium, potassium, or phosphorus which they contain, is
actually due to the sulfur present as sulfates in the fertilizers used.
Sulfur occurs in the organic compounds of plants, associated with
phosphorus. It seems probable that its physiological uses are similar to
those of the latter element; but there is as yet no experimental evidence
to establish its exact rôle in the economy of plant growth. It appears to
be needed in largest proportion by plants which contain high percentages of
nitrogen in their foliage, such as the legumes. There is some evidence that
sulfur has a particular rôle in promoting the growth of bacteria, and it
may be that the percentages of total sulfur which are found in the tissues
of legumes are due to the presence of the symbiotic nitrogen-gathering
bacteria in the nodules on the roots of these plants. This point has not
yet been investigated, however.
=Sodium= is probably not essential to plant growth, although it is present
in small proportions in the ash from practically all plants. In cases of
insufficient supply of potassium, sodium can apparently perform at least a
part of the rôle of the former element; but this seems not to be a normal
relationship or use.
=Chlorine= is found in small amounts in the sap and in the ash of nearly
all plants. However, it does not appear to be essential to the growth of a
plant, except possibly in the case of certain species, such as asparagus,
buckwheat, and, perhaps, turnips and some other root crops. Whether the
benefit which these crops derive from the application of common salt to the
soil in which they are growing is due to the direct food value of either
the chlorine, or the sodium, or to some indirect effect, is not yet known.
The presence of chlorine in the sap of plants is undoubtedly due to the
inevitable absorption of soluble chlorides from the soil and apparently has
no connection with the nutritional needs of the plant.
=Silicon= is always considered as a non-essential element, although it
occurs in such large proportions in some plants as to indicate that it
cannot be wholly useless. It accumulates in the stems of plants, chiefly in
the cell-wall, and has sometimes been supposed to aid in giving stiffness
to the stems. But large numbers of analyses have failed to show any direct
correlation between the stiffness of straw of cereal plants and the
percentage of silicon which they contain. Further, plants will grow to full
maturity and with erect stems when no silicon is present in the mineral
nutrients which are furnished to them. On the other hand, certain
experiments appear to indicate that silicon can perform some of the
functions of phosphorus, if soluble silicates are supplied to
phosphorus-starved plants. But under normal conditions of plant nutrition,
it seems to have no such function.
INORGANIC PLANT TOXINS AND STIMULANTS
Much study has been given during recent years to the question of the
supposed poisonous, or toxic, effects upon plants of various soil
constituents. There seems to be no doubt that certain _organic_ compounds
which are injurious to plant life are often present in the soil, either as
the normal excretions of plant roots or as products of the decomposition of
preceding plant growths. A consideration of these supposedly toxic organic
substances would be out of place in this discussion of mineral soil
nutrients. But there seems to be no doubt that there may also be mineral
substances in the soil which may sometimes exert deleterious influences
upon plant growth. In fact, most metallic salts, except those of the few
metals which are required for plant nutrition, appear to be toxic to
plants. The exact nature of the physiological effects which are produced by
these mineral toxins is not clearly understood; indeed, it is probably
different in the case of different metals. Further, it is certain that both
the stimulating and the toxic effect of metallic compounds upon low forms
of plants is quite different from the effects of the same substances upon
the more complex tissues of higher plants, a fact which is utilized to
advantage in the application of fungicides for the control of parasitic
growths on common farm crops.
Among the elements whose physiological effects upon higher plants, such as
the cereal crops, etc., when their soluble compounds are present in the
soil, have been carefully studied, there are three fairly distinct types of
injurious mineral elements. The first of these, represented by copper,
zinc, and arsenic, apparently exert their toxic effect regardless of the
proportion in which they are present in the nutrient solution which is
presented to the plant; although the degree of injury varies with the
amount of injurious substance present, of course. The second type, of which
boron and manganese are representatives, apparently exerts a definite
stimulating effect upon plants when supplied to them in concentrations
below certain clearly defined limits; but are toxic in concentrations above
these. The third includes many soluble salts of magnesium, sodium,
potassium, etc., which while either innocuous or else definite sources of
essential plant foods when in lower concentrations, become highly toxic, or
corrosive, when present in the soil solution in concentrations above the
limits of "toleration" of individual plants for these soluble salts. The
tolerance shown by the different species of plants toward these soluble
salts (the so-called "alkali" in soils) varies widely; indeed, there seems
to be considerable variation in the resistance of different individual
plants of the same species to injury from this cause.
With reference to the toxic effect of the third type of substances, i.e.,
the common soluble salts, it is known that single salts of potassium,
magnesium, sodium, or calcium, in certain concentrations, are toxic to
plants, while mixtures of the same salts in the same concentrations are
not. Thus, solutions of sodium chloride, magnesium sulfate, potassium
chloride, and calcium chloride which, when used singly, killed plants whose
roots were immersed in them for only a few minutes, formed when mixed
together a nutrient solution in which the same plants grew normally. The
remarkable remedial effect of calcium salts in overcoming the injurious
effects of other soluble salts has already been mentioned. One explanation
of these relationships between mineral soil constituents and the living
plant is that the life phenomena depend upon a balanced adjustment between
the compounds of these different mineral elements with the proteins
(producing the so-called "metal proteids") which constitute the active
material of the cell protoplasm. According to this theory, any excess or
deficiency of any one or more of these elements in the plant juices which
surround a given cell will, of course, cause an interchange with the
mineral components of the supposed "metal proteids" which upsets the
assumed essential balance between them, with disastrous results. A more
recent, and much more satisfactory, explanation of the "antagonism" between
mineral elements in their toxic effects upon plants, which has both
theoretical and experimental confirmation, is that single salts disturb the
colloidal condition (see Chapter XV) of the protoplasm of the plant cells
in such a way as to destroy its permeability to nutrient substances, while
mixtures of salts restore the proper state of colloidal dispersion and
permit the normal functioning of the protoplasm.
It is apparent from the above brief discussions that the rôle of the
different soil elements as plant food, and their relations to the complex
processes which constitute plant growth, afford an interesting and
promising field for further study.
References
BRENCHLEY, WINIFRED E.--"Inorganic Plant Poisons and Stimulants," 106
pages, 18 figs., Cambridge, 1914.
HALL, A. D.--"Fertilizers and Manures," 384 pages, 7 plates, London, 1909.
HALL, A. D.--"The Book of the Rothamsted Experiments," 294 pages, 49 figs.,
8 plates, London, 1905.
HOPKINS, C. G.--"Soil Fertility and Permanent Agriculture," 653 pages,
Chicago, 1910.
HILGARD, E. W.--"Soils," 593 pages, 89 figs., New York, 1906.
LOEW, O.--"The Physiological Rôle of Mineral Nutrients," U. S. Department
of Agriculture, Bureau of Plant Industry, _Bulletin_ No. 45, 70 pages,
Washington, D. C., 1903.
RUSSELL, E. J.--"Soil Conditions and Plant Growth," 243 pages, 13 figs.,
_Monographs_ on Biochemistry, London, 1917. (3d ed.)
WHITNEY, M.--"A Study of Crop Yields and Soil Composition in Relation to
Soil Productivity," U. S. Department of Agriculture, Bureau of Soils,
_Bulletin_ No. 57, 127 pages, 24 figs., Washington, D. C., 1909.
CHAPTER II
THE ORGANIC COMPONENTS OF PLANTS
From the standpoint of their ability to synthetize synergic foods (see page
2) from inorganic raw materials, plants may be divided into two types;
namely, the _autotrophic_, or self-nourishing, plants, and the
_heterotrophic_ plants.
Strictly speaking, only those plants whose every cell contains chlorophyll
are entirely self-nourishing; and some parts, or organs, of almost any
autotrophic plant are dependent upon the active green cells of other parts
of the plant for their synergic food. Furthermore, if the term is used in a
very wide sense, green plants are more than self-nourishing, they really
nourish all living things. But the general significance of the term
"autotrophic plants" is apparent.
"Heterotrophic plants" must, of necessity, get food, either directly or
indirectly, from some other plant which can synthetize synergic foods or,
in a few cases, from animal organic matter. If they do this by feeding upon
the organic compounds of other living organisms, they are known as
"parasites"; while if they secure their organic food from the tissues or
debris of dead organisms, they are called "saprophytes." The heterotrophic
plants are chiefly the bacteria and fungi; although a few seed-plants are
devoid of chlorophyll or have nutritive habits similar to those of the
non-green plants, and a few species are semi-parasitic or semi-saprophytic.
It is obvious that the metabolic processes of the autotrophic plants are
very different from those of the heterotrophic type of plants. These
differences constitute a most interesting field of study for plant
physiologists. But the nature of the chemical compounds themselves and of
the chemical changes involved in their transformations is not radically
different in the two types of plants, the essential difference being in the
preponderance of one kind of activities, or chemical reactions, over
another in bringing about the metabolic processes which are characteristic
of each particular species. Hence, it does not seem necessary, or
desirable, in this study of the chemistry of plant growth, to present as
detailed a consideration of the differences in metabolic activity of the
different types of plants as complete accuracy of statement in all cases
might demand. We will, instead, discuss the organic chemical components of
plant tissues and the reactions which they undergo, using the more common
type of autotrophic plants as the illustrative material in most cases.
Hence, it will be understood that in all the following discussions of plant
activities, except where specific exceptions are definitely mentioned, it
is the green, or autotrophic, plants to which reference is made in each
case.
From the standpoint of the sum total of its activities, a green plant is
essentially an absorber of solar energy and a synthetizer of organic
substances. Each individual autotrophic plant takes up certain amounts of
the anergic foods which are discussed in the preceding chapter and
manufactures from them a great variety of complex organic compounds, using
the energy of the sun's rays, absorbed by chlorophyll, as the source for
the energy necessary to accomplish these synthetic reactions. The ultimate
object of these processes is to produce seeds, each containing an embryo
and a sufficient supply of food for the young plant of the next generation
to use until it has developed its own synthetic organs; or (in the case of
perennials) to store up reserve food materials with which to start off new
growth after a period of rest and often of defoliation. To be sure, animals
and men often interfere with the completion of the life cycle of the plant,
and utilize the seeds or stored food material for their own nutrition, but
this is a biological relation which has no influence upon the nature of the
plant's own activities.
Since all of these synthetic reactions must go on at ordinary temperatures,
active catalyzers are necessary. These the plant provides in the form of
enzymes (see Chapter XIV) which are always present in active plant
protoplasm. Proper conditions for rapid chemical action are further assured
by the colloidal nature (see Chapter XV) of the protoplasm itself.
TYPES OF CHEMICAL CHANGES INVOLVED IN PLANT GROWTH
The whole cycle of chemical changes which is involved in plant growth
represents the net result of two opposite processes; the first of these is
a constructive one which has at least three different phases: namely, a
synthesis of complex organic compounds, the translocation of this
synthetized material to the centers of growth, and the building up of this
food material into tissues or reserve supplies; and the second is a
destructive process of respiration whereby carbohydrate material is broken
down, potential energy is released, and carbon dioxide is excreted.
The synthetic processes which take place in plants are of two types;
namely, photosynthesis, in which sugars are produced, and another, which
has no specific name, whereby proteins are elaborated. The translocation of
the synthetized material involves the change of insoluble compounds into
soluble ones, effected by the aid of enzymes. For storage purposes, the
soluble forms are usually, though not always, condensed again into more
complex forms, these latter changes requiring much less energy than do the
original syntheses from raw materials.
The destructive process, respiration, is characteristic of all living
matter, either plant or animal organisms. It takes place continuously
throughout the whole life of a plant. During rapid growth it is
overshadowed by the results of the synthetic process, but during the
ripening period in which the seed is matured, and during the germination of
the seed itself, growth is practically at a standstill and the respiratory,
destructive action predominates, so that the plant actually loses weight.
GROUPS OF ORGANIC COMPOUNDS FOUND IN PLANTS
As a result of their various synthetic and metabolic activities, a great
variety of organic compounds is produced by plants. Certain types of these
compounds, such as the carbohydrates and proteins, are necessary to all
plants and are elaborated by all species of autotrophic plants. Other types
of compounds are produced by many, but not all, species of plants; while
still others are found in only a few species. It is fairly easy to classify
all of these compounds into a few, well-defined groups, based upon
similarity of chemical composition. These groups are known, respectively,
as the carbohydrates and their derivatives, the glucosides and tannins; the
fats and waxes; the essential oils and resins; organic acids and their
salts; the proteins; the vegetable bases and alkaloids; and the pigments. A
consideration of these groups of compounds, as they are synthetized by
plants, constitutes the major portion of the study of the chemistry of
plant life as presented in this book. Following the discussion of the
compounds themselves, the chapters dealing with enzymes, with the colloidal
nature of protoplasm, and with the supposed accessory stimulating agencies,
aim to show how the manufacturing machine known as the plant cell
accomplishes its remarkable results, so far as the process is now
understood.
PHYSIOLOGICAL USES AND BIOLOGICAL SIGNIFICANCE
In connection with the discussion of each of the above-mentioned groups of
organic components of plants, an attempt will be made to point out what
significance these particular compounds have in the plant's life and
growth. Certain terms will be used to designate different rôles, which it
is probably necessary to define.
There may be two possible explanations of, or reasons for, the presence of
any given type of compound in the tissues of any particular species of
plant. First, it may be supposed that this particular type of compounds is
elaborated by the plant to satisfy its own physiological needs, or for the
purpose of storing it up in the seeds as synergic food for the growth of
the embryo, in order to reproduce the species. For this rôle of the various
organic food materials, etc., we will employ the term "physiological use."
On the other hand, it is often conceivable that certain types of compounds,
which have properties that make them markedly attractive (or repellent) as
a food for animals and men, or which are strongly antiseptic in character,
or which have some other definite relationship to other living organisms,
have had much to do with the survival of the particular species which
elaborates them, in the competitive struggle for existence; or have been
developed in the plant by the evolutionary process of "natural selection."
For this relation of the compound to the plant's vital needs, we will use
the term "biological significance." Such a segregation of the rôles which
the different compounds play in the plant's economy may be more or less
arbitrary in many cases; but it will be clear that when _physiological
uses_ are discussed, reference is being made to the plant's own internal
needs; while the phrase _biological significance_ will be understood to
refer to the relation of the plant to other living organisms.
PHYSIOLOGICAL USES OF THE ORGANIC COMPONENT GROUPS
From the standpoint of the rôle which each plays in the plant economy, the
several groups of organic compounds may be roughly divided into three
classes. These are: (_a_) the framework materials, including gums, pectins,
and celluloses; (_b_) synergic foods, including carbohydrates, fats, and
proteins; and (_c_) the secretions, including the glucosides, volatile
oils, alkaloids, pigments, and enzymes.
The _framework material_, as the name indicates, constitutes the cell-wall
and other skeleton substances of the plant. It is made up of carbohydrate
complexes, produced by the cell protoplasm from the simpler carbohydrates.
The _synergic foods_, or "reserve foods" as they are sometimes called,
produced by the excess of synthetized material over that needed for the
immediate use of the plant, are accumulated either in the various storage
organs, to be available for future use by the plant itself or by its
vegetative offspring, or in the seed, to be available to the young seedling
of the next generation. Proteins not only serve as reserve food materials
but also make up the body of the living organism itself. Carbohydrates and
fats serve as synergic and reserve foods.
The _secretions_ may be produced either in ordinary cells and found in
their vacuoles, or in special secretory cells and stored in cavities in the
secreting glands (as in the leaves of mints, skin of oranges, etc.), or in
special ducts (as in pines, milkweeds, etc.) or on the epidermis (as the
"bloom" of plums, cabbages, etc., the resinous coating of many leaves,
etc.). As a general rule, the glucosides, pigments, and enzymes are the
products of unspecialized cells and have some definite connection with the
metabolic processes of the plant; while the volatile oils and the alkaloids
are usually secreted by special cells and have no known rôle in
metabolism.
CHAPTER III
PHOTOSYNTHESIS
Photosynthesis is the process whereby chlorophyll-containing plants, in the
presence of sunlight, synthetize organic compounds from water and carbon
dioxide. The end-product of photosynthesis is always a carbohydrate.
Chemical compounds belonging to other groups, mentioned in the preceding
chapter, are synthetized by plants from the carbohydrates and simple raw
materials; but in such cases the energy used is not solar energy and the
process is not photosynthesis.
Under the ordinary conditions of temperature, moisture supply, etc.,
necessary to plant growth, photosynthesis will take place if the three
essential factors, chlorophyll, light, and carbon dioxide are available.
PHYSIOLOGICAL STEPS IN PHOTOSYNTHESIS
There are five successive and mutually dependent steps in the process of
photosynthesis, as follows:
(1) There must be a gas exchange between the plant tissue and the
surrounding air, by means of which the carbon dioxide of the air may reach
the protoplasm of the chlorophyll-containing cells.
(2) Radiant energy must be absorbed, normally that of sunlight, although
photosynthesis can be brought about by the energy from certain forms of
artificial light.
(3) Carbon dioxide and water must be decomposed by the energy thus
absorbed, and the nascent gases thus produced combined into some synthetic
organic compound, with a resultant storage of potential energy.
(4) This first organic synthate must be condensed into some carbohydrate
suitable for translocation and storage as reserve food.
(5) The oxygen, which is a by-product from the decomposition of the water
and carbon dioxide and the resultant synthetic process, must be returned to
the air by a gas exchange.
Of the five steps in this process, the first two and the last are
essentially purely physical phenomena, the chemical changes involved being
those of the third and fourth steps. Hence, it is only these two parts of
the process which need be taken into account in a consideration of the
chemistry of photosynthesis.
FORMALDEHYDE, THE SIMPLEST CARBOHYDRATE STRUCTURE
The simplest carbohydrates known to occur commonly in plant tissues are the
hexoses (see Chapter IV) having the formula C_{6}H_{12}O_{6}, which is just
six times that of formaldehyde, CH_{2}O. Also, it is known that
formaldehyde easily, and even spontaneously, polymerizes into more complex
forms having the general formula (CH_{2}O)_n_; trioxymethylene,
C_{3}H_{6}O_{3}, being a well-known example. Further, both trioxymethylene
and formaldehyde itself can easily be condensed into hexoses, by simple
treatment with lime water as a catalytic agent. Hence, it is commonly
believed that formaldehyde is the first synthetic product resulting from
photosynthesis, that this is immediately condensed into hexose sugars, and
that these in turn are united into the more complex carbohydrate groups
which are commonly found in plants (see Chapter IV).
There is considerable experimental confirmation of the soundness
of this view. The whole photosynthetic process takes place in
chlorophyll-containing plant tissues with astonishing rapidity, sugars,
and even starch, appearing in the tissues almost immediately after their
exposure to light in the presence of carbon dioxide. Hence, any
intermediate product, such as formaldehyde, is present in the cell for
only very brief periods and in very small amounts. But small amounts of
formaldehyde can often be detected in fresh green plant tissues and, as
will be pointed out below, the whole process of photosynthesis,
proceeding through formaldehyde as an intermediate product, can be
successfully duplicated _in vitro_ in the laboratory.
Assuming, then, that formaldehyde is the first photosynthetic product in
the process of the production of carbohydrates from water and carbon
dioxide, the simple empirical equation for this transformation would be
H_{2}O + CO_{2} = CH_{2}O + O_{2}.
It is apparent, however, that the process is not so simple as this
hypothetical reaction would indicate, as water and carbon dioxide can
hardly be conceived to react together in any such simple way as this.
Various theories as to the exact nature of the steps through which the
chemical combinations proceed have been advanced. A discussion of the
experimental evidence upon which these are based and of the conclusions
which seem to be justified from these experimental studies is presented
below. The only value which may be attached to the empirical equation just
presented is that it does accurately represent the facts that a volume of
oxygen, equal to that of the carbon dioxide consumed in the process, is
liberated and that formaldehyde is the synthetical product of the reactions
involved.
It should be noted, in this connection, that formaldehyde is a powerful
plant poison and that few, if any, plant tissues can withstand the toxic
effect of this substance when it is present in any considerable
concentration. Hence, it is necessary to this whole conception of the
relation of formaldehyde to the photosynthetic process, to assume that,
however rapidly the formaldehyde may be produced in the cell, it is
immediately converted into harmless carbohydrate forms.
THE CONDENSATION OF FORMALDEHYDE INTO SUGARS
As has been mentioned, it is easily possible to cause either formaldehyde,
or trioxymethylene, to condense into C_{6}H_{12}O_{6}, using milk of lime
as a catalyst. Of course, no such condition as this prevails in the plant
cell, and the mechanics of the protoplasmic process may be altogether
different from those of the artificial syntheses. Furthermore, the hexose
produced by the artificial condensation of these simpler compounds is, in
every case, a non-optically active compound, while all natural sugars are
optically active (see Chapter IV). Emil Fischer has succeeded, however, by
a long and round-about process which need not be discussed in detail here,
in converting the artificial hexose into glucose and fructose, the
optically-active sugars which occur naturally in plant tissues. The
condensation of formaldehyde directly into glucose and fructose in the
plant cell is brought about by some process the nature of which is not yet
understood. Probably synthetic enzymes (see Chapter XIV), whose nature and
action have not yet been discovered, come into play. It is a noteworthy
fact, however, that the mechanics of this apparently simple chemical
change, upon which the whole nutrition of the plant depends, and which
furnishes the whole animal kingdom, including the human race, with so large
a proportion of its food supplies, is as yet wholly unknown.
It is the common practice to represent the whole results of the
photosynthetic action by the empirical equation
6H_{2}O + 6CO_{2} = C_{6}H_{12}O_{6} + 6O_{2};
but here again the only value to be attached to such an algebraic
expression is that it accurately represents the gaseous exchange of carbon
dioxide and oxygen involved in the process. Certainly, it throws no light
upon the nature of the process itself.
THEORIES CONCERNING PHOTOSYNTHESIS
The many theories which have been advanced concerning the nature of the
chemical changes which are involved in photosynthesis have served as the
basis for much experimental study of the problem. The following brief
summary will serve to point out the general trend of these investigations
and the present state of knowledge concerning the chemistry of
photosynthesis.
Von Baeyer, in 1870, advanced the hypothesis that the first step in the
process is the breaking down of carbon dioxide into carbon monoxide and
oxygen and of water into hydrogen and oxygen; that the carbon monoxide and
hydrogen then unite to produce formaldehyde, which is immediately
polymerized to form a hexose. These theoretical changes may be represented
by the following equations:
{ CO_{2} = CO + O
1. {
{ H_{2}O = H_{2} + O
2. H_{2} + CO = CH_{2}O
3. 6(CH_{2}O) = C_{6}H_{12}O_{6}
In the investigations and discussions of this hypothesis, it has been
ascertained: first, that carbon monoxide has never been found in the free
form in plant tissues; second, that when _Tropaeolum_ plants were
surrounded with an atmosphere in which there was no carbon dioxide, but
which contained sufficient carbon monoxide to give a concentration of this
gas in the cell-sap equivalent to that in which CO_{2} is normally present,
the plants grew normally and apparently elaborated starch; third, other and
more extensive experiments indicated, however, that green plants in general
cannot make use of carbon monoxide gas for photosynthesis, although this
does not prove that von Baeyer's idea that CO is a step in the process is
necessarily erroneous; and finally it was shown that carbon monoxide, in
sufficient concentration to produce the results with _Tropaeolum_ mentioned
above, usually acts as a powerful anæsthetic towards most other plants.
While these considerations do not positively prove that von Baeyer's
hypothesis is incorrect, they render it so improbable that it has generally
been abandoned in favor of others which are described below.
Erlenmeyer, even before the experimental work mentioned in the preceding
paragraph had been reported, suggested that instead of assuming a separate
breaking down of the carbon dioxide and water, it is easier to conceive
that they are united in the cell-sap into carbonic acid and that this is
reduced by the chlorophyll-containing protoplasm into formic acid and then
to formaldehyde, as indicated by the following equations:
1. H_{2}CO_{3} = H_{2}CO_{2} + O
2. H_{2}CO_{2} = CH_{2}O + O
Like von Baeyer's hypothesis, this assumes that formaldehyde and oxygen are
the first products of photosynthesis.
Proceeding upon this assumption, many investigators have studied the
question as to whether formaldehyde actually is present in green leaves.
Several workers have reported successful identification of formaldehyde in
the distillate from green leaves; while others have criticized these
results and have maintained that formaldehyde can likewise be obtained by
distilling decoctions of dry hay, etc., in which the photosynthetic process
could not possibly be conceived to be at work. Other investigators, notably
Bach and Palacci, reported that they had succeeded in artificially
producing formaldehyde from water and carbon dioxide, in the presence of a
suitable catalyzer or sensitizer. Euler, however, later showed
conclusively that under the conditions described by these investigators,
formaldehyde can be obtained even if no carbon dioxide is present, being
apparently produced by the action of water upon the organic sensitizer
which was used.
These conflicting reports led Usher and Priestley, in a series of studies
reported between 1906 and 1911, to submit the whole matter to a critical
review. Briefly, these investigators showed that the photolysis of carbon
dioxide and water results in the formation of formaldehyde and hydrogen
peroxide, as represented by the equation
CO_{2} + 3H_{2}O = CH_{2}O + 2H_{2}O_{2}.
The formaldehyde is then condensed by the protoplasm into sugars, while the
hydrogen peroxide is decomposed, by an enzyme in the plant cell, into water
and oxygen. If the formaldehyde is not used up rapidly enough by the
protoplasm, it kills the enzyme and the undecomposed hydrogen peroxide
destroys the chlorophyll, which stops the whole photosynthetic process.
Usher and Priestley were able to cause the photolysis of carbon dioxide and
water into formaldehyde outside of a green plant, in the presence of a
suitable catalyzing agent which continually destroys the hydrogen peroxide
as fast as it is formed; to show the actual bleaching effect of an excess
of hydrogen peroxide in plant tissues which had been treated in such a way
as to prevent the enzyme from decomposing it; and, finally, to demonstrate
the condensation of formaldehyde into starch by the action of protoplasm
which contained no chlorophyll.
In the meantime, Fenton, in 1907, found that in the presence of magnesium
as a catalyst (it will be shown in Chapter VIII that magnesium is a
constituent of the chlorophyll molecule) formaldehyde may be obtained from
a solution of carbon dioxide in water, especially if weak bases are
present.
Further, Usher and Priestley's later results showed that radium emanations,
acting upon a solution of carbon dioxide in water, produce hydrogen
peroxide and formaldehyde, and the latter polymerizes but not up to the
point represented by the hexose sugars; also, that the ultra-violet rays
from a mercury vapor lamp are very effective in bringing about the
production of hydrogen peroxide and formaldehyde from a saturated aqueous
solution of carbon dioxide, the reaction taking place even in the absence
of any "sensitizer," but much more readily if some "optical" or "chemical"
sensitizer is present. Finally, these investigators were able to duplicate
all their results, using green plant tissues, and to show that the
temperature changes which take place in a film of chlorophyll when it is
exposed to an atmosphere of moist carbon dioxide in the sunlight are such
as would be required by the formation of formaldehyde and hydrogen peroxide
from carbonic acid.
More recently, Ewart has showed that formaldehyde can combine chemically
with chlorophyll; from which fact, Schryver deduces the theory that if for
any reason the condensation of formaldehyde into carbohydrates by the cell
protoplasm does not proceed as rapidly as the formaldehyde is produced by
photosynthesis, the excess of the latter enters into combination with the
chlorophyll, and that if condensation into sugar uses up all the free
formaldehyde which is present in the active protoplasm, the compound of
formaldehyde with chlorophyll is broken down setting free an additional
supply for further sugar manufacture. According to this conception there
are, in the chlorophyll-bearing protoplasm, not only the agencies for the
production of formaldehyde from carbon dioxide and water and for the
condensation of this into carbohydrates, but also a chemical mechanism by
means of which the amount of free formaldehyde in the reacting mass may be
regulated so that at no time will it reach the concentration which would be
injurious to the cell protoplasm or fall below the proper proportions for
sugar-formation. This explanation affords a satisfactory solution of the
difficulty which formerly confronted the students of photosynthesis,
namely, the fact that free formaldehyde is powerfully toxic to cell
protoplasm. Without some such conception, it was difficult to imagine how
the presence of formaldehyde in the cell contents, even as a transitory
intermediate product, could be otherwise than injurious.
As a result of these studies, the nature of the chemical changes which
result in the production of formaldehyde as the first product of
photosynthesis, with the liberation of a volume of oxygen equal to that of
the carbon dioxide consumed, seems to be fairly well established.
THE PRODUCTION OF SUGARS AND STARCHES
The next step in the process, the conversion of formaldehyde into sugars
and starches, is not necessarily a _photo_synthetic one, as it can be
brought about by protoplasm which contains no chlorophyll or other
energy-absorbing pigment. It is, however, a characteristic synthetic
activity of living protoplasm. There is little definite knowledge as to how
the cell protoplasm accomplishes this important task. As has been pointed
out, the polymerization of formaldehyde into a sugar-like hexose, known as
"acrose," can be easily accomplished by ordinary laboratory reactions, and
acrose can be converted into glucose or fructose by a long and difficult
series of transformations. But such processes as are employed in the
laboratory to accomplish these artificial synthesis of optically-active
sugars from formaldehyde can have no relation whatever to the methods of
condensation which are used by cell protoplasm in its easy, almost
instantaneous, and nearly continuous accomplishment of this transformation.
Furthermore, these simple hexoses are by no means the final products of
cell synthesis, even of carbohydrates alone. In many plants, starch appears
as the final, if not the first, product of formaldehyde condensation. At
least, the transformation of the simple sugars, which may be supposed to be
the first products, into starch is effected so nearly instantaneously that
it is impossible to detect measurable quantities of these sugars in the
photosynthetically active cells of such plants. Other species of plants
always show considerable quantities of simple sugars in the vegetative
tissues, and some even store up their reserve carbohydrate food material in
the form of glucose or sucrose. Attempts have been made to associate the
type of carbohydrate formed in cell synthesis with the botanical families
to which the plants belong, but with no very great success. For each
individual species, however, the form of carbohydrate produced is always
the same, at least under normal conditions of growth. For example, the
sugar beet always stores up sucrose in its roots, although under abnormal
conditions considerable quantities of raffinose are developed. Similarly,
potatoes always store up starch, but with abnormally low temperatures
considerable quantities of this may be converted into sugar, which becomes
starch again with the return to normal conditions.
While it is impossible, with our present knowledge, to even guess at the
mechanism by which protoplasm condenses formaldehyde into sugars and these,
in turn, into more complex carbohydrates, the structure and relationships
to each other of the final products of photosynthesis are well known, and
are discussed at length in the following chapter.
References
BARNES, C. R.--"Physiology" (Part II of Coulter, Barnes and Cowles'
"Textbook of Botany"), 187 pages, 18 figs., Chicago, 1910.
GANONG, W. F.--"Plant Physiology," 265 pages, 65 figs., New York, 1908 (2d
ed.).
JOST, L., trans. by GIBSON, R. J. H.--"Plant Physiology," 564 pages, 172
figs., Oxford, 1907.
MARCHLEWSKI, L.--"Die Chemie des Chlorophylls," 187 pages, 5 figs., 7
plates, Berlin, 1909.
PARKIN, JOHN.--"The Carbohydrates of the Foliage Leaf of the Snowdrop
(_Galanthus nivalis L._) and their Bearing on the First Sugar of
Photosynthesis," in _Biochemical Journal_, Vol. 6, pages 1 to 47, 1912.
PFEFFER, W., trans. by EWART, A. J.--"Physiology of Plants." Vol. I, 632
pages, 70 figs., Oxford, 1900.
CHAPTER IV
CARBOHYDRATES
These substances comprise an exceedingly important group of compounds, the
members of which constitute the major proportion of the dry matter of
plants. The name "carbohydrate" indicates the fact that these compounds
contain only carbon, hydrogen, and oxygen, the last two elements usually
being present in the same proportions as in water. As a rule, natural
carbohydrates contain six, or some multiple of six, carbon atoms and the
same number of oxygen atoms less one for each additional group of six
carbons above the first one; e.g., C_{6}H_{12}O_{6}, C_{12}H_{22}O_{11},
C_{18}H_{32}O_{16}, etc.
Carbohydrates are classed as open-chain compounds, that is, they may be
regarded as derivatives of the aliphatic hydrocarbons. From the standpoint
of the characteristic groups which they contain, they are
aldehyde-alcohols. In common with many other polyatomic open-chain
alcohols, they generally possess a characteristic sweet, or mildly
sweetish, taste. In the case of the more complex and less soluble forms,
this sweetish taste is scarcely noticeable and these compounds are commonly
called the "starches," as contrasted with the more soluble and sweeter
forms, known as "sugars."
The characteristic ending _ose_ is added to the names of the members of
this group. As systematic names, the Latin numeral indicating the number of
carbon atoms in the molecule is combined with this ending; e.g.,
C_{5}H_{10}O_{5}, pentose, C_{6}H_{12}O_{6}, hexose, etc.
In recent years, as a matter of scientific interest, many sugarlike
substances which contain from two to nine carbon atoms combined with the
proper number of hydrogen and oxygen atoms to be equivalent to the same
number of molecules of water in each case, have been artificially prepared
in the laboratory and designated as dioses, trioses, tetroses, pentoses,
hexoses, heptoses, octoses, and nonoses, respectively. Substances
corresponding in composition and properties with the artificial tetroses
and one or two derivatives of heptoses are occasionally found in plant
tissues, and a considerable number of pentoses and their condensation
products are common constituents of plant gums, etc.; but the great
majority of the natural carbohydrates are hexoses and their derivatives.
GROUPS OF CARBOHYDRATES
Since the simpler carbohydrates are sugars, i.e., they possess the
characteristic sweet taste, the name "saccharide" is used as a basis for
the classification of the entire group. The simplest natural sugars, the
hexoses, C_{6}H_{12}O_{6}, are known as _mono-saccharides_. The group of
next greater complexity, those which have the formula C_{12}H_{22}O_{11}
and may be regarded as derived from the combination of two molecules of a
hexose with the dropping out of one molecule of water at the point of
union, are known as _di-saccharides_. Compounds having the formula
C_{18}H_{32}O_{16} (i.e., three molecules of C_{6}H_{12}O_{6} minus two
molecules of H_{2}O) are _tri-saccharides_; and the still more complex
groups, having the general formula (C_{6}H_{10}O_{5})_n_, are called the
_poly-saccharides_. The mono-, di-, and tri-saccharides are generally
easily soluble in water, have a more or less pronouncedly sweet taste, and
are known as the _sugars_; while the polysaccharides are generally
insoluble in water and of a neutral taste, and are called _starches_. As
will be seen later, there are many natural plant carbohydrates belonging to
each of these groups.
In addition to these saccharide groups, there are other types, or groups,
of compounds which resemble the true carbohydrates in their chemical
composition and properties and are often considered as a part of this
general group. These are the pentoses, C_{5}H_{10}O_{5}, and their
condensation products, the pentosans (C_{5}H_{8}O_{4})_n_, and their methyl
derivatives, C_{6}H_{12}O_{5}; certain polyhydric alcohols having the
formula C_{6}H_{8}(OH)_{6}; pectose and its derivatives, pectin and pectic
acid; and lignose substances of complex composition. It is doubtful whether
these compounds are actual products of photosynthesis in plants, or have
the same physiological uses as the carbohydrates and it has seemed wise to
consider them in a separate and later chapter.
ISOMERIC FORMS OF MONOSACCHARIDES
Four sugars having the formula C_{6}H_{12}O_{6}, namely, glucose, fructose,
mannose, and galactose, occur very commonly and widely distributed in
plants. In addition to these, thirteen others having the same percentage
composition have been artificially prepared, while seven additional forms
are theoretically possible. In other words, twenty-four different
compounds, all having the same empirical formula and similar sugar-like
properties are theoretically possible. In order to arrive at a conception
of this multiplicity of isomeric forms, it is necessary to understand the
two types of isomerism which are involved. One of these is _structural_
isomerism, and the other is _space_- or _stereo_-isomerism.
=Structural Isomerism.=--This refers to an actual difference in the
characteristic groups which are present in the molecule. As has been said,
all carbohydrates, from the standpoint of the characteristic groups which
they contain, are aldehyde-alcohols. The hexoses all contain five alcoholic
groups and one primary aldehyde, or one secondary aldehyde (ketone), group.
If the aldehyde oxygen is attached to the carbon atom which is at the end
of the six-membered chain, the structural arrangement is
|
that of an aldehyde, C=O and the sugar is of the type known
|
H
as "aldoses"; whereas, if the oxygen is attached to any other
|
carbon in the chain, the ketone arrangement, C=O results and
|
the sugar is a "ketose." This difference is illustrated in the Fischer
open-chain formulas for glucose (an aldose) and fructose (a ketose) as
follows:
Glucose Fructose
CH_{2}OH CH_{2}OH
| |
CHOH CHOH
| |
CHOH CHOH
| |
CHOH CHOH
| |
CHOH C=O
| |
CHO CH_{2}OH
=Stereo-isomerism=, or space isomerism, as its name indicates, depends upon
the different arrangement of the atoms or groups in the molecule in space,
and not upon any difference in the character of the constituent groups.
This possibility depends upon the existence in the molecule of the
substance in question of one or more _asymmetric carbon atoms_ and
manifests itself in differences in the optical activity of the compound.[1]
Thus, in the formula for glucose shown above there appear four asymmetric
carbon atoms, namely, those of the four secondary alcohol groups (in the
terminal, or primary alcohol, group, carbon is united to hydrogen by two
bonds, and in the aldehyde group it is united to oxygen by two bonds).
Similarly, fructose contains three asymmetric carbon atoms.
As an example of how the presence of these asymmetric carbon atoms results
in the possibility of many different space relationships, the following
graphic illustrations of the supposed differences between dextro-glucose
and levo-glucose, and between dextro- and levo-galactose, may be cited.[2]
_d_-glucose _l_-glucose _d_-galactose _l_-galactose
CH_{2}OH CH_{2}OH CH_{2}OH CH_{2}OH
| | | |
H-C-OH H-C-OH H-C-OH HO-C-H
| | | |
H-C-OH H-C-OH HO-C-H H-C-OH
| | | |
HO-C-H H-C-OH HO-C-H H-C--OH
| | | |
H-C-OH HO-C-H H-C-OH HO-C-H
| | | |
CHO CHO CHO CHO
Comparisons of the above formulas will show that the difference
between the formulas for _d_- and _l_-glucose lies in the arrangement of
the H atoms and the OH groups around the two asymmetric carbon atoms next
the aldehyde end of the chain; while the _d_- and _l_-galactoses differ in
that this arrangement is in the reverse order around all four of the
asymmetric carbons. By similar variations in the grouping around the four
asymmetric atoms, it is possible to produce the sixteen different space
arrangements shown on page 37 for the groups of an aldohexose. Sugars
corresponding to fourteen of these different forms have been discovered,
three of which are of common occurrence in plants, either as single
mono-saccharides or as constituent groups in the more complex
carbohydrates; the remaining two forms have only theoretical interest.
Similarly, for a ketohexose, which contains three asymmetric carbon atoms,
there are eight possible arrangements. Three sugars of this type are known,
only one (fructose) being common in plants; the others are of only
theoretical interest.
FOOTNOTES:
[1] It is assumed that the reader, or student, is familiar with the
theoretical and experimental evidence in support of the existence of the
so-called "asymmetric" carbon atom and its relation to the effect of the
compound which contains it, when in solution, in rotating the plane of
polarized light. For purposes of review, or of study of this most
interesting and important phenomenon, the reader is referred to any
standard text-book on Organic Chemistry.
[2] Attention should be called, at this point, to the fact that such
formulas as these cannot possibly accurately represent the actual
arrangement of the constituent groups of a carbohydrate molecule around an
asymmetric carbon atom. The limitations of a plane-surface formula prevent
any illustration of the three-dimension relationships in space.
Furthermore, there are certain facts in connection with the birotation
phenomenon and the relation of the molecular configuration to biochemical
properties (which see) that cannot be explained on the basis of the
open-chain arrangement represented by the Fischer formulas used here. A
closed-ring arrangement, showing the aldehyde oxygen as linked by its two
bonds to the first and the fourth carbon atoms of the chain, thus forming a
closed-ring of four carbon and one oxygen atoms, instead of being attached
by both bonds to a single carbon atom, as in the above formulas, is
undoubtedly a more nearly accurate representation of the actual linkage in
the molecule than are the open-chain formulas used above.
The differences in conception embodied by these two types of formulas may
be shown by the following formulas for glucose:
CH_{2}OH CH_{2}OH
| |
CHOH CHOH
| |
CHOH CH-CHOH ------O-------
| | | | |
CHOH O | or CH_{2}OH·CHOH·CH·CHOH·CHOH·CHOH
| | |
CHOH CH-CHOH
| |
CHO OH
Fischer's Closed-ring formulas
formula
It will be observed that in the closed-ring formula there are five
asymmetric carbon atoms, and the asymmetry of the terminal one forms the
basis for the explanation of the existence of the so-called [alpha] and
[beta] modification of _d_-glucose (see page 46). However, the ordinary
aldehyde reactions of the sugars are more clearly indicated by the
open-chain formula. Some investigators are inclined to be that sugars
actually exist in the open-chain arrangement when in aqueous solution, and
in the closed-ring arrangement when in alcoholic solution. The closed-ring
formulas will be used in this text in the discussions of the birotation
phenomena and of biochemical properties, but for the explanations of the
stereo-isomeric forms and similar phenomena, the open-chain formulas are
just as useful in conveying an idea of the possibilities of different space
relationships, and are so much simpler in appearance and in mechanical
preparation, that it seems desirable to use these rather than the more
accurate closed-ring formulas.
CHEMICAL CONSTITUTION OF MONOSACCHARIDES
The term "monosaccharides," as commonly used, refers to hexoses. It applies
equally well, however, to any other sugar-like substance which either
occurs naturally or results from the decomposition of more complex
carbohydrates, and which cannot be further broken down without destroying
its characteristic aldehyde-alcohol groups and sugar-like properties.
All such monosaccharides, being alcohol-aldehydes, can easily be reduced to
the corresponding polyatomic alcohols, containing the same number of carbon
atoms as the original monosaccharides, each with one OH group attached to
it. All aldose monosaccharides are converted, by gentle oxidation, into the
corresponding monobasic acid, having a COOH group in the place of the
original CHO group. Further oxidation either changes the alcoholic groups
into COOH groups, producing polybasic acids, or breaks up the chain. When
ketose monosaccharides are submitted to similar oxidation processes, they
are broken down into shorter chain compounds.
The various monosaccharides which have thus far been found as constituents
of plant tissues, or as parts of other more complex compounds which occur
in plants, are shown in the following table:
_Trioses_ (C_{3}H_{6}O_{3}) _Tetroses_ (C_{4}H_{8}O_{4})
Aldose--Glyceric aldehyde, Aldoses--_d_- and _l_-Erythrose,
or glycerose _l_-Threose
Ketose--Dioxyacetone
_Pentoses_ (C_{5}H_{10}O_{5}) _Methyl Pentoses_ (C_{6}H_{12}O_{5})
Aldoses--_d_- and _l_-Arabinose Aldoses--Rhamnose
_d_- and _l_-Xylose Fucose
_l_-Ribose Rhodeose
_l_-Lyxose Chinovose
_Hexoses_ (C_{6}H_{12}O_{6})
Mannitol series Dulcitol series
Aldoses--_d_- and _l_-Glucose _d_- and _l_-Galactose
_d_- and _l_-Mannose _d_- and _l_-Talose
_d_- and _l_-Gulose
_d_- and _l_-Idose
_d_-Altrose
_d_-Allose
Ketoses-- _d_-Fructose _d_-Tagatose
_d_-Sorbose
_Heptoses_ _Octoses_ _Nonoses_
(C_{7}H_{14}O_{7}) (C_{8}H_{16}O_{8}) (C_{9}H_{18}O_{9})
Glucoheptose Gluco-octose Glucononose
Mannoheptose Manno-octose Mannononose
Galactoheptose Galacto-octose
Persuelose
Sedoheptose
The hexoses are by far the most important group of monosaccharides. They
are undoubtedly the first products of photosynthesis, and all the other
carbohydrates may be considered to be derived from them by condensation.
Because of their biochemical significance and their immense importance as
the fundamental substances for all plant and animal energy-producing
materials, the following detailed studies of their chemical composition and
molecular configuration are fully warranted.
That all the hexoses contain five alcoholic groups is proved by the
experimental evidence that each one forms a penta-ester, by uniting with
five acid radicals, when treated with mineral or organic acids under proper
conditions. Thus, glucose penta-acetate, penta-nitrate, penta-benzoate,
etc., have all been prepared. The presence of the aldehyde group is proved
by the fact that all aldohexoses have been converted, by gentle oxidation,
into pentaoxy-monobasic acids, and the ketohexoses broken down into shorter
chain compounds by similar gentle oxidations; these reactions being
characteristic of compounds containing an aldehyde and a ketone group
respectively. This experimental evidence establishes the nature of the
characteristic groups in the molecule, in each case.
The molecular configurations illustrated in the following table are those
suggested by Emil Fischer, as a result of his exhaustive studies of the
chemical constitution of the various carbohydrates. There is, of course, no
thought that the printed formulas here presented accurately represent the
actual relationships in space of the different groups; but there is fairly
conclusive evidence that the variations in special groupings in the
different sugars are properly referable to the particular asymmetric carbon
atoms as indicated in the several formulas as presented.
1. Aldohexoses of the mannitol series:
_d_-Glucose _l_-Glucose _d_-Mannose _l_-Mannose
CH_{2}OH CH_{2}OH CH_{2}OH CH_{2}OH
| | | |
H-C-OH HO-C-H H-C-OH HO-C-H
| | | |
H-C-OH HO-C-H H-C-OH HO-C-H
| | | |
HO-C-H H-C-OH HO-C-H H-C-OH
| | | |
H-C-OH HO-C-H HO-C-H H-C-OH
| | | |
CHO CHO CHO CHO
_d_-Gulose _l_-Gulose _d_-Idose _l_-Idose
CH_{2}OH CH_{2}OH CH_{2}OH CH_{2}OH
| | | |
HO-C-H H-C-OH HO-C-H H-C-OH
| | | |
H-C-OH HO-C-H H-C-OH HO-C-H
| | | |
HO-C-H H-C-OH HO-C-H H-C-OH
| | | |
HO-C-H H-C-OH H-C-OH HO-C-H
| | | |
CHO CHO CHO CHO
_d_-Altrose _l_-Altrose _d_-Allose _l_-Allose
(unknown) (unknown)
CH_{2}OH CH_{2}OH CH_{2}OH CH_{2}OH
| | | |
H-C-OH HO-C-H H-C-OH HO-C-H
| | | |
HO-C-H H-C-OH H-C-OH HO-C-H
| | | |
HO-C-H H-C-OH H-C-OH HO-C-H
| | | |
HO-C-H H-C-OH H-C-OH HO-C-H
| | | |
CHO CHO CHO CHO
2. Aldohexoses of the dulcitol series:
_d_-Galactose _l_-Galactose _d_-Talose _l_-Talose
CH_{2}OH CH_{2}OH CH_{2}OH CH_{2}OH
| | | |
H-C-OH HO-C-H H-C-OH HO-C-H
| | | |
HO-C-H H-C-OH HO-C-H H-C-OH
| | | |
HO-C-H H-C-OH HO-C-H H-C-OH
| | | |
H-C-OH HO-C-H HO-C-H H-C-OH
| | | |
CHO CHO CHO CHO
3. Ketohexoses:
_d_-Fructose _d_-Sorbose _d_-Tagatose
CH_{2}OH CH_{2}OH CH_{2}OH
| | |
H-C-OH HO-C-H H-C-OH
| | |
H-C-OH H-C-OH HO-C-H
| | |
HO-C-H HO-C-H HO-C-H
| | |
C=O C=O C=O
| | |
CH_{2}OH CH_{2}OH CH_{2}OH
Reference will be made in subsequent paragraphs to the probable chemical
constitution of the monosaccharides other than hexoses; but the above
discussion of the structure of the hexoses will serve as a sufficient
introduction to the study of the composition of the common carbohydrates.
CHARACTERISTIC REACTIONS OF HEXOSES
=Specific Rotatory Power.=--All soluble carbohydrates, since they contain
asymmetric carbon atoms, with the consequent larger groups on one side of
the molecule than the other, rotate the plane of polarized light when it
passes through a solution of the carbohydrate in question. The amount of
the rotation depends upon the nature of the carbohydrate, the concentration
of the solution, and the length of the column of solution through which the
ray of polarized light passes. But the same definite amount of the same
sugar, dissolved in the same volume of water, and placed in a tube of the
same length, will always cause the same angular deviation, or rotation, of
the plane in which the polarized light which passes through it is vibrated.
In other words, the same number of molecules of the optically active
substance in solution will always produce the same rotatory effect. This is
called the specific rotatory power of the substance in question. It is
expressed as the number of degrees of angular deviation of the plane of
polarized light caused by a column of the solution exactly 200 mm. in
length, the concentration of the solution being 100 grams of substance in
100 cc. at a temperature of 20° C. Actual determinations of specific
rotatory power are usually made with solutions more dilute than this
standard, and the observed deviation multiplied by the proper factor to
determine the effect which would be produced by the solution of standard
concentration. If the direction of the deviation is to the right (i.e., in
the direction in which the hands of the clock move) it is spoken of as
"dextro" rotation and is indicated by the sign +, or the letter _d_; while
if in the opposite direction, it is called "levo" rotation and indicated by
the sign -, or the letter _l_. For example, the specific rotation of
ordinary glucose is +52.7°; of fructose, -92°; of sucrose, +66.5°.
=Reducing Action.=--All of the hexose sugars are active reducing agents.
This is because of the aldehyde group which they contain. Many of the
common heavy metals, when in alkaline solutions, are strongly reduced when
boiled with solutions of the hexose sugars. Alkaline copper solutions yield
a precipitate of red cuprous oxide; ammoniacal silver solutions give silver
mirrors; alkaline solutions of mercury salts are reduced to metallic
mercury, etc. Any sugar which contains a potentially active aldehyde group
will exhibit this reducing effect and is known as a "reducing sugar." In
some of the di- and tri-saccharides, the linkage of the hexose components
together is through the aldehyde group, in such a way that it loses its
reducing effect; such sugars are known as "non-reducing." Advantage is
taken of this property for both the detection and quantitative
determination of the "reducing sugars." A standard alkaline copper solution
of definite strength, known as "Fehling's solution," is added to the
solution of the sugar to be tested and the mixture boiled, when the
characteristic brick-red precipitate appears. If certain standard
conditions of volume of solutions used, length of time of boiling, etc.,
are observed, the quantity of cuprous oxide precipitated bears a definite
ratio to the amount of sugar which is present, so that if the precipitate
be filtered off and weighed under proper conditions, the weight of sugar
present in the original solution can be calculated. The proper conditions
for carrying on such a determination and tables showing the amounts of the
various "reducing sugars" which correspond to the weight of cuprous oxide
found, are given in all standard text-books dealing with the analysis of
organic compounds.
=Fermentability.=--The common hexoses are all easily fermented by yeast,
forming alcohol and carbon dioxide, according to the equation
C_{6}H_{12}O_{6} = 2C_{2}H_{5}OH + 2CO_{2}.
The importance and biochemical significance of this reaction will be
considered in detail in connection with the discussions of the relation of
molecular configuration to biochemical properties (see page 56) and the
nature of enzyme action (see page 194).
=Formation of Hydrazones and Osazones.=--Another property of the hexoses
which is due to the presence of an aldehyde group in the molecule, is that
of forming addition products with phenyl hydrazine, known as "hydrazones"
and "osazones." For example, glucose reacts with phenyl hydrazine in acetic
acid solution, in two stages. The first, which takes place even in a cold
solution may be represented by the equation
C_{6}H_{12}O_{6}+C_{6}H_{5}·NH·NH_{2} = C_{6}H_{12}O_{5}:N·NH·C_{6}H_{5}
Glucose Phenyl-hydrazine Glucose-hydrazone + H_{2}O.
The structural relationships involved may be represented as follows:
CHO H_{2}N·NH CH=N-------NH
| /\ | /\
(CHOH)_{4} + | | = (CHOH)_{4} | |
| | | | | |
CH_{2}OH \/ CH_{2}OH \/
The hydrazones of the common sugars, with the exception of the one from
mannose, are colorless compounds, easily soluble in water. Hence, they do
not serve for the separation or identification of the individual sugars.
But if the solution in which they are formed contains an excess of phenyl
hydrazine and is heated to the temperature of boiling water for some time,
the alcoholic group next to the aldehyde group (the terminal alcohol group
in ketoses) is first oxidized to an aldehyde and then a second molecule of
phenyl hydrazine is added on, as illustrated above, forming a
di-addition-product, known as an "osazone." The osazones are generally more
or less soluble in hot water, but on cooling they crystallize out in yellow
crystalline masses each with definite melting point and crystalline form.
All sugars which have active aldehyde groups in the molecule form osazones.
These afford excellent means of identification of unknown sugars, or of
distinguishing between sugars of different origin and type.
Glucose, mannose, and fructose all form identical osazones. This is because
the structure of these three sugars is identical except for the arrangement
within the two groups at the aldehyde end of the molecule (see formulas on
page 44). Since it is to these two groups that the phenyl hydrazine residue
attaches itself, it follows that the resulting osazones must be identical
in structure and properties. All other reducing sugars yield osazones of
different physical properties.
When an osazone is decomposed by boiling with strong acids, the phenyl
hydrazine groups break off, leaving a compound containing both an aldehyde
and a ketone group. Such compounds are known as "osones." The osones from
glucose, mannose, and fructose are identical. By carefully controlled
reduction, either one of the C==O groups of the osone may be changed to an
alcoholic group, producing thereby one of the original sugars again. Hence,
it is possible to start with one of these sugars, convert it into the osone
and then reduce this to another sugar, thereby accomplishing the
transformation of one sugar into another isomeric sugar.
=Formation of Glucosides.=--By treatment with a considerable variety of
different types of compounds, under proper conditions, it is possible to
replace one of the hydrogen atoms of the terminal alcoholic group of the
hexose sugars with the characteristic group of the other substance, forming
compounds known, respectively, as glucosides, fructosides, galactosides,
etc. The structural relation of methyl glucoside to glucose, for example,
may be illustrated as follows:
Glucose (C_{6}H_{12}O_{6}) Methyl Glucoside (C_{7}H_{14}O_{6})
CHO CHO
| |
(CHOH)_{4} (CHOH)_{4}
| |
CH_{2}OH CHOH
|
CH_{3}
A general formula for glucosides is R·(CHOH)_{5}·CHO; and the R may
represent a great variety of different organic radicals (see the chapters
dealing with Glucosides and with Tannins). When the glucosides are
hydrolyzed, they yield glucose and the hydroxyl compound of the radical
with which it is united. All the statements which have been made with
reference to glucosides, apply equally well with reference to fructosides,
galactosides, mannosides, etc.
It is possible, by various laboratory processes, to replace additional
hydrogen atoms in the glucose molecule with the same or other organic
radicals, thus producing glucosides containing two or more R groups; but
most of the natural glucosides contain only one other characteristic group.
=Oxidations.=--When the hexoses are oxidized they give rise to three
different types of acids, depending upon the conditions of the oxidation
and the kind of oxidizing agent used. With glucose, for example, the
relationships involved may be illustrated as follows:
CHO COOH CHO COOH
| | | |
(CHOH)_{4} (CHOH)_{4} (CHOH)_{4} (CHOH)_{4}
| | | |
CH_{2}OH CH_{2}OH COOH COOH
Glucose Gluconic acid Glucuronic acid Saccharic acid
An important property of the acids of the _gluconic_ type is that when
heated with pyridine or quinoline to 130°-150° they undergo a molecular
rearrangement whereby the acid corresponding to an isomeric sugar is
produced. For example, gluconic acid, under these conditions, becomes
mannonic acid, which can be reduced to mannose. The process is reversible;
mannose can be converted to mannonic acid, thence to gluconic acid, thence
to glucose. Similarly, galactonic acid can be converted into talonic acid,
and this to talose, and this process is reversible. These facts afford
another means of conversion of one sugar into another.
From the standpoint of physiological processes, _glucuronic acid_ is the
most interesting and important oxidation product of glucose. It is often
found in the urine of animals, as the result of the partial oxidation of
glucose in the animal tissues. Normally, glucose is oxidized in the body to
its final oxidation products, carbon dioxide and water. But when many
difficultly oxidizable substances, such as chloral, camphor, turpentine
oil, aniline, etc., are introduced into the body, the organism has the
power of combining these with glucose to form glucosides. These so-called
"paired" compounds are then oxidized to the corresponding glucuronic acid
derivatives and eliminated from the body in the urine. No phenomenon
similar to this occurs in plants, however, and glucuronic acid has never
been found in plant tissues.
=Synthesis and Degradation of Hexoses.=--Monosaccharides of any desired
number of carbon atoms can be produced from aldoses having one less carbon
atoms, by way of the familiar "nitrile" reaction. Aldoses, like all other
aldehydes, combine directly with hydrocyanic acid, forming compounds known
as nitriles, which contain one more carbon atom than was present in the
original aldehyde; the cyanogen group can easily be converted into a COOH
group; and this, in turn, reduced to an aldehyde, thus producing an aldose
with one more carbon atom than was present in the initial sugar. These
changes may be illustrated by the following equations:
(1) CHO + HCN CHOH·CN CN
| | |
(CHOH)_{3} = (CHOH)_{3} or (CHOH)_{4}
| | |
CH{2}OH CH_{2}OH CH_{2}OH
Aldopentose Nitrile
(2) CN + H_{2}O COOH + NH_{3}
| |
(CHOH)_{4} = (CHOH)_{4}
| |
CH_{2}OH CH_{2}OH
Nitrile Acid
(3) COOH - O CHO
| |
(CHOH)_{4} = (CHOH)_{4}
| |
CH_{2}OH CH_{2}OH
Acid Aldohexose
It is possible, by this process, to advance step by step from formaldehyde
to higher sugars, Emil Fischer and his students having carried the process
as far as the production of glucodecose (C_{10}H_{20}O_{10}). It usually
happens, however, that two stereo-isomers result from the "step-up" by way
of the nitrile reaction; thus, arabinose yields a mixture of glucose and
mannose, glucose yields glucoheptose and mannoheptose, etc.
The reverse process, or the so-called "degradation" of a sugar into another
containing fewer carbon atoms, may be readily accomplished in either one or
two ways. In Wohl's process, the aldehyde group of the sugar is first
converted into an _oxime_, by treatment with hydroxylamine; the oxime, on
being heated with concentrated sodium hydroxide solution, splits off water
and becomes the corresponding _nitrile_; this, on further heating, splits
off HCN and yields an aldose having one less carbon atom than the original
sugar. This process is the exact reverse of the nitrile synthesis,
described above. The second method of degradation, suggested by Ruff, makes
use of Fenton's method of oxidizing aldehyde sugars to the corresponding
monobasic acid, using hydrogen peroxide and ferrous sulfate as the
oxidizing mixture; the _aldonic acid_ thus formed is then converted into
its calcium salt, which, when further oxidized, splits off its carboxyl
group and one of the hydrogens of the adjacent alcoholic group, leaving an
aldose having one less carbon atom than the original aldose sugar.
=Enolic Forms.=--A final avenue for the interconversion of glucose,
mannose, and fructose into one another, is through the spontaneous
transformations which these undergo when dissolved in water containing
sodium hydroxide or potassium hydroxide. This change is due to the
conversion of the sugar, in the alkaline solution, into an _enol_, which is
identical for all three sugars, and which may subsequently be reconverted
into any one of the three isomeric hexoses. The relationships involved are
illustrated in the following formulas:
CHO CHO CH_{2}OH CHOH
| | | ║
H-C-OH HO-C-H C=O C-OH
| | | |
HO-C-H HO-C-H HO-C-H HO-C-H
| | | |
H-C-OH H-C-OH H-C-OH H-C-OH
| | | |
H-C-OH H-C-OH H-C-OH H-C-OH
| | | |
CH_{2}OH CH_{2}OH CH_{2}OH CH_{2}OH
Glucose Mannose Fructose Enolic Form
The preceding technical discussion of the chemical constitution and
reactions of the hexoses has been presented, not because it has any direct
connection with the occurrence or functions of these compounds in plant
tissues, but for the purpose of giving to the student a graphic conception
of the structure and properties of these simple carbohydrates, as a basis
for the understanding of the nature, properties, possible chemical
reactions, syntheses, etc., of the more complex types of carbohydrates,
which, along with these simple monosaccharides, constitute the most
important single group of organic components of plants.
THE OCCURRENCE AND PROPERTIES OF MONOSACCHARIDES
Only two monosaccharides occur as such in plants. These are glucose and
fructose. All the other hexoses, whose structure is shown on pages 37 and
38, occur in plants only as constituents of the more complex saccharides,
in glucoside-formations, or as the corresponding polyatomic alcohols.
The aldo-hexoses which occur most commonly in plants, either free or in
combination, are _d_-glucose, _d_-mannose, and _d_-galactose; while
_d_-fructose and _d_-sorbose are the common keto-hexoses.
=Glucose= (often called also dextrose, fruit sugar, or grape sugar) occurs
widely distributed in plants, most commonly in the juices of ripening
fruits, where it is usually associated with fructose and sucrose, the two
hexoses being easily derived from sucrose by hydrolysis. Glucose is also
produced by the hydrolysis of many of the more complex carbohydrates, by
the action either of enzymes or of dilute acids; lactose, maltose,
raffinose, starch, and cellulose, as well as many glucosides all yielding
glucose as one of the products of their hydrolysis. In all such cases, it
is _d_-glucose which is obtained.
Glucose is a crystalline solid (although it does not form such sharply
defined crystals as does sucrose, or "granulated sugar"), which is easily
soluble in water. It usually appears on the market in the form of thick
syrups, which are produced commercially by the hydrolysis of starch with
dilute sulfuric acid, removal of the acid after the hydrolysis is complete,
and evaporation of the resulting solution to the desired syrupy
consistency. (Since corn starch is commonly used as the raw material for
this process, these syrups are often spoken of as "corn syrup.") The
sweetness of glucose is about three-fifths that of ordinary cane sugar.
Glucose exhibits all the properties of hexoses which have been described in
general terms above. It is a reducing-sugar, and is easily fermented. The
specific rotatory power of _d_-glucose is +52.7°. But when glucose is
dissolved in water, it exhibits in a marked degree the phenomenon known as
"mutarotation"; that is, freshly made solutions exhibit a certain definite
rotatory power, but this changes rapidly until it finally reaches another
definite specific rotation. In other words, glucose is "birotatory," or
possesses two distinct specific rotatory powers, and the changing rotation
effect in aqueous solutions is due to the change from one form to the
other. When dissolved in alcohol, it does not exhibit this change in
rotatory power. In order to explain this phenomenon, it is necessary to
assume that there are two modifications of _d_-glucose, which have been
designated respectively as the [alpha] and [beta] forms. The possibility of
the existence of these two forms is explained by the assumption of the
closed-ring arrangement of the glucose molecule, as indicated in the
following formulas which represent the two possible isomeric arrangements:
HO-C-H H-C-OH
/ \ / \
/ \ / \
H-C-OH \ H-C-OH \
| O | O
HO-C-H / HO-C-H /
\ / \ /
\ / \ /
C-H C-H
| |
H-C-OH H-C-OH
| |
CH_{2}OH CH_{2}OH
[alpha]-Glucose [beta]-Glucose
It is assumed that the [alpha] modification (with its specific rotatory
power of +105°) is the normal form for crystalline glucose, but that when
dissolved in water it is changed into an _aldehydrol_, i.e., a compound
containing two additional OH groups, which later breaks down again, into
the [beta] modification (with its specific rotatory power of +22°). When
dissolved in alcohol, this change does not take place because of the
absence of the excess of water necessary to produce the intermediate
aldehydrol form.
There are other examples of the existence of the [alpha] and [beta]
modification of glucose. For example, [alpha]-methyl-glucoside and
[beta]-methyl-glucoside (specific rotatory powers, +157° and -33°,
respectively) are both known, as well as several other similar glucoside
arrangements.
=Mannose.=--This sugar does not occur as such in plants; but complex
compounds which yield _d_-mannose when hydrolyzed, known as "mannosans,"
are found in a number of tropical plant forms. The mannose which is
obtained from these by hydrolysis is very similar to glucose in its
properties, forms the same osazones as do glucose and fructose, exhibits
mutarotation, etc. Mannose may also be obtained by oxidizing mannitol, a
hexatomic alcohol, known as "mannite," which occurs in many plants,
especially in the manna-ash (_Fraxinus ornus_), the dried sap from which is
known as "manna."
=Galactose= occurs in the animal kingdom as one of the constituents of
lactose, or milk-sugar. It is also one of the constituents of raffinose, a
trisaccharide sugar found in plants, and occurs as "galactans" in many gums
and sea-weeds. The _d_-galactose, obtained by the hydrolysis of any of
these compounds, is a faintly sweet substance which resembles glucose in
many of its properties; having one characteristic difference, however, in
that it forms mucic acid instead of saccharic acid when oxidized by
concentrated nitric acid. These oxidation products are very different in
their physical properties and this difference serves to distinguish between
the two sugars from which they are derived.
=Fructose= (levulose, honey sugar, or "diabetic" sugar) occurs along with
glucose in the juices of many fruits, etc. It is a constituent of sucrose,
of raffinose, and of the polysaccharide inulin, from which it may be
obtained by hydrolysis. It is a ketose sugar, reduces Fehling's solution,
forms the same osazone as glucose, and is easily fermentable by yeast. Its
sweetness is slightly greater than that of ordinary cane sugar.
_d_-fructose (the ordinary form) is easily soluble in water, and is
strongly levorotatory, its specific rotatory power at 20° C. being -92.5°;
it is unique in the very large effect which is produced in its rotatory
power by increasing the temperature of the solution; at 87° its specific
rotatory power is reduced to -52.7°, exactly equal to but in the opposite
direction of the effect of glucose; hence, _invert sugar_, which is a
mixture of an equal number of molecules of glucose and fructose, and which
has a specific rotatory power of -19.4° at 20° C., becomes optically
inactive at 82° C.
=Sorbose= is the only other ketohexose which has any importance in plant
chemistry. It does not occur free in plants, but is the first oxidation
product from the hexatomic alcohol, sorbitol, which is present in the juice
of the berries of the mountain-ash. Sorbose is a crystalline solid, which
is not fermentable by yeast, but which otherwise closely resembles
fructose.
DISACCHARIDES
The disaccharides, having the formula C_{12}H_{22}O_{11}, may be regarded
as derived from the monosaccharides by the linking together of two hexose
groups with the dropping out of a molecule of water, in the same way that
many other organic compounds form such linkages. That this is a perfectly
correct conception, is shown by the fact that, when hydrolyzed, the
disaccharides break down into two hexose sugars, thus
C_{12}H_{22}O_{11} + H_{2}O = C_{6}H_{12}O_{6} + C_{6}H_{12}O_{6}.
With all known disaccharides, at least one of the hexoses obtained by
hydrolysis is glucose; hence all disaccharides may be regarded as
glucosides (C_{6}H_{12}O_{5}·R) in which the R is another hexose group.
Since hexoses have both alcoholic and aldehyde groups, and since either of
these types of groups may function in the linkage of the two hexoses to
form a disaccharide, it is possible for two hexoses, both of which are
reducing sugars to be linked together in three different ways: (1) through
an alcoholic group of each hexose, (2) through an alcoholic group of one
and the aldehyde group of the other, and (3) through the aldehyde group of
each hexose. Disaccharides linked in either of the first two ways will be
reducing sugars, since they still contain a potentially active aldehyde
group; but those of the third type will not be reducing sugars, since the
linkage through the aldehyde groups destroys their power of acting as
reducing agents. Examples of each of these three types of linkage are found
among the common disaccharides, as will be pointed out below.
The following table shows the general characteristics of the common
disaccharides.
_Type 1.--Aldehyde group potentially active, reducing sugars:_
Sugar Components
Maltose Glucose and glucose
Gentiobiose Glucose and glucose
Lactose Glucose and galactose
Melibiose Glucose and galactose
Turanose Glucose and fructose
_Type 2.--Non-reducing sugars:_
Sucrose Glucose and fructose
Trehalose Glucose and glucose
The disaccharides of Type 1 reduce Fehling's solution and form hydrazones
and osazones, although somewhat less readily than do the hexoses. They all
show mutarotation and exist in two modifications, indicating that the
component groups have the closed-ring arrangement.
The disaccharides of Type 2, since they contain no potentially active
aldehyde group, do not reduce Fehling's solution, nor form osazones;
neither do they exhibit mutarotation. The only disaccharides which occur as
such in plants are of this type. Disaccharides of Type 1 may be obtained by
the hydrolysis of other, more complex, carbohydrates.
All disaccharides are easily hydrolyzed into mixtures of their component
hexoses, by boiling with dilute mineral acids, or by treatment with certain
specific enzymes which are adapted to the particular disaccharide in each
case (see page 55, also Chapter XIV).
=Sucrose= (cane sugar, beet sugar, maple sugar) is the ordinary "granulated
sugar" of commerce. It occurs widely distributed in plants, where it serves
as reserve food material. It is found in largest proportions in the stalks
of sugar cane, in the roots of certain varieties of beets, and in the
spring sap of maple trees, all of which serve as industrial sources for the
sugar. In the sugar cane, and beet-roots, it constitutes from 12 to 20 per
cent of the green weight of the tissue and from 75 to 90 per cent of the
soluble solids in the juice which can be expressed from it. Its universal
use as a sweetening agent is due to the combined facts that it crystallizes
readily out of concentrated solutions and, hence, can be easily
manufactured in solid form, and that it is sweeter than any other of the
common sugars except fructose.
Sucrose is a non-reducing sugar, forms no osazone, and is not directly
fermentable by yeast, although most species of yeasts contain an enzyme
which will hydrolyze sucrose into its component hexoses, which then readily
ferment.
When hydrolyzed by acids, or by the enzyme "invertase," it yields a mixture
of equal quantities of glucose and fructose. Sucrose is dextrorotatory, but
since fructose has a greater specific rotatory action to the left than
glucose has to the right, the mixture resulting from the hydrolysis of
sucrose is levorotatory. Since the hydrolysis of sucrose changes the
rotatory effect of the solution from the right to the left, the process is
usually called the "inversion" of sucrose, and the resultant mixture of
equal parts of glucose and fructose is called "invert sugar." As has been
pointed out, solutions of invert sugar become optically inactive when
heated to 82° C., because of the reduction in the rotatory power of
fructose due to the higher temperature.
The probable linkage of the two hexoses to form sucrose, in such a way as
to produce a non-reducing sugar, is illustrated in the following formula:
------O-------
| |
CH_{2}OH·CHOH·CH·CHOH·CHOH·CH
|
O
|
CH_{2}OH·CHOH·CHOH·CH·C·CH_{2}OH
\ /
O
=Trehalose= seems to serve as the reserve food for fungi in much the same
way that sucrose does for higher plants. It is composed of two molecules of
glucose linked together through the aldehyde group of each, as trehalose is
a non-reducing sugar. This linkage is illustrated in the following formula:
------O-------
| |
CH_{2}OH·CHOH·CH·CHOH·CHOH·CH
|
O
|
CH_{2}OH·CHOH·CH·CHOH·CHOH·CH
| |
------O-------
Trehalose may be hydrolyzed into glucose by dilute acids and by the enzyme
"trehalase," which is contained in many yeasts and in several species of
fungi. It is strongly dextrorotatory (specific rotatory power, +199°). It
is not fermentable by yeast.
Trehalose appears to replace sucrose in those plants which contain no
chlorophyll and do not elaborate starch. The quantity of trehalose in such
plants reaches a maximum just before spore formation begins. Since it is
manufactured in the absence of chlorophyll, its formation must be
accomplished by some other means than photosynthesis, yet it is composed
wholly of glucose--a natural photosynthetic product.
=Maltose= rarely occurs as such in plants, although its presence in the
cell-sap of leaves has sometimes been reported. It is produced in large
quantities by the hydrolysis of starch during the germination of barley and
other grains. This hydrolysis is brought about by the enzyme "diastase,"
which is present in the sprouting grain.
Maltose is easily soluble in water, and crystallizes in masses of slender
needles. It is a reducing sugar; readily forms a characteristic osazone; is
strongly dextrorotatory (specific rotatory power +137°); and is readily
fermented by ordinary brewer's yeast, which contains both "maltase" (the
enzyme which hydrolyzes maltose to glucose) and "zymase" (the
alcohol-producing enzyme). When hydrolyzed, either by dilute acids or by
maltase, one molecule of maltose yields two molecules of glucose. Its
component hexoses are, therefore, the same as those of trehalose, a
non-reducing sugar, this difference in properties being due to the
difference in the point of linkage between the two glucose molecules, that
for maltose being such as to leave one of the aldehyde groups potentially
active, as shown in the following formula,
-------O------
| |
CH_{2}OH·CHOH·CH·CHOH·CHOH·CH
|
O
|
CHOH·CHOH·CHOH·CH·CHOH·CH_{2}
| |
----------O-----
=Isomaltose= is a synthetic sugar, obtained by Fischer, by condensing
two molecules of glucose. Its properties are quite similar to those of
maltose, but it yields a slightly different osazone and is not fermentable
by yeast. These differences are explained by the assumption that this
sugar is a glucose-[beta]-glucoside, while normal maltose is a
glucose-[alpha]-glucoside.
=Gentiobiose= is a disaccharide which results from the partial hydrolysis
of the trisaccharide _gentianose_ (see page 53). It is very similar in its
general properties to isomaltose. =Cellobiose= is a disaccharide which
results from the hydrolysis of cellulose. It is a reducing sugar, forms an
osazone, and resembles maltose.
Maltose, isomaltose, gentiobiose, and cellobiose, are all
glucose-glucosides, the difference between them being undoubtedly due to
linkage being between different alcoholic groups in the glucose molecules.
The disaccharide =lactose= is a glucose-galactoside. It is the sugar which
is present in the milk of all mammals. It has never been found in plants.
=Melibiose=, which is the corresponding vegetable glucose-galactoside, may
be obtained by the partial hydrolysis of the trisaccharide _raffinose_ (see
below). It is a reducing sugar; forms a characteristic osazone; and
exhibits mutarotation. It is not fermented by ordinary top-yeasts, but is
first hydrolyzed and then fermented by the enzymes present in
bottom-yeasts.
TRISACCHARIDES
Trisaccharides, as the name indicates, consist of three hexoses (or
monosaccharides) linked together by the dropping out of two molecules of
water. Their formula is C_{18}H_{32}O_{16}. When completely hydrolyzed,
they yield three molecules of monosaccharides; when partially hydrolyzed,
one each of a disaccharide and a monosaccharide.
One trisaccharide of the reducing sugar type, namely _rhamnose_, exists in
plants as a constituent of the glucoside xanthorhamnin. It is composed of
one molecule of glucose united to two molecules of rhamnose (methyl
pentose, C_{6}H_{12}O_{5}). It is of interest only in connection with the
properties of the glucoside in which it is present (see page 84).
Three trisaccharides which are non-reducing sugars are found in plants;
namely, raffinose, gentianose, and melizitose.
=Raffinose= occurs normally in cotton seeds, in barley grains, and in
manna; also, in small quantities in the beet root, associated with sucrose.
It is more soluble in water than is sucrose and hence remains in solution
in the molasses from beet-sugar manufacture, which constitutes the
commercial source for this sugar. Raffinose crystallizes out of
concentrated solutions, with five molecules of water of crystallization, in
clusters of glistening prisms. It is strongly dextrorotatory, the anhydrous
sugar having a specific rotatory power of +185°, and the crystalline form,
C_{18}H_{32}O_{16}, showing a specific rotation of +104.5°. It does not
reduce Fehling's solution, nor form an osazone, and in its other properties
it closely resembles sucrose.
The hydrolysis of raffinose presents several interesting possibilities. If
its structure is represented as follows:
C_{6}H_{11}O_{5}-C_{6}H_{10}O_{4}-C_{6}H_{11}O_{5}
Fructose Glucose Galactose
\______ _____/ \_______ ______/
\/ \/
Sucrose Melibiose
it is apparent that it may break down by hydrolysis in three different
ways: (1) into sucrose and galactose, (2) into fructose and melibiose, and
(3) into fructose, glucose, and galactose. As a matter of fact, it does
actually break down in these three different ways, under the influence of
different catalysts; invertase or dilute acids break it down into fructose
and melibiose, emulsin hydrolyzes it to sucrose and galactose, while strong
acids or the enzymes of bottom-yeasts break it down into the three hexoses.
=Gentianose=, a trisaccharide found in the roots of yellow gentian
(_Gentiana lutea_), is a non-reducing sugar, which when hydrolyzed yields
either fructose and gentiobiose, or fructose and two molecules of glucose.
=Melizitose=, a trisaccharide which, in crystallized form, has the formula,
C_{18}H_{32}O_{16}·2H_{2}O, occurs in the sap of _Larix europea_ and in
Persian manna, and has recently been found in considerable quantities in
the manna which collects on the twigs of Douglas fir and other conifers.
When hydrolyzed, it yields one molecule of fructose and one of turanose, a
disaccharide containing fructose and glucose linked together in a slightly
different way than they are in sucrose. Turanose itself is a reducing
sugar, but when linked with fructose to form melizitose its reducing
properties are destroyed. Melizitose is a very sweet sugar.
TETRASACCHARIDES
A complex saccharide, known as _stachyose_, which is found in the tubers of
_Stachys tuberifera_, is said by some investigators to be a tetrasaccharide
and by others to have the formula C_{36}H_{62}O_{31}·7H_{2}O (i.e., a
hexasaccharide). It is a crystalline solid, with a faintly sweetish taste,
and a specific rotatory power of +148°. When hydrolyzed it yields glucose,
fructose, and two (or more) molecules of galactose.
THE RELATION OF THE MOLECULAR CONFIGURATION OF SUGARS
TO THEIR BIOCHEMICAL PROPERTIES
As will be pointed out later (see Chapter XIV), all chemical reactions
which are involved in vital phenomena, including those of plant growth and
metabolism, are controlled by enzymes. The biochemical reactions which the
soluble carbohydrates undergo afford such excellent illustrations of the
relation of the molecular configuration of an organic compound to the
possibility of the action of an enzyme upon it, that it seems desirable to
discuss this relationship at this point, rather than to postpone it until
after the nature of enzyme action has been considered. Undoubtedly, the
student, after he has studied the nature of enzymes and their mode of
action, as presented in Chapter XIV, will find it profitable to return to
this section and review the facts here presented, as illustrating the
principles and mechanism of enzyme action. But a consideration, at this
time, of the relation of the molecular configuration of the sugars to their
biochemical reactions cannot fail to add interest to the study of these
matters from the chemical and biological standpoints.
It has been known for a long time that the dextro- and levo-isomers of a
compound which contains one or more asymmetric carbon atoms are affected
differently by biological agents, such as yeasts, moulds, bacteria, etc.
Pasteur, as early as 1850, showed that the green mould, _Penicillium
glaucum_, when growing in solutions of racemic acid (a mixture of equal
molecules of _d_- and _l_-tartaric acids) uses up only the _d_-acid,
leaving the _l_-form absolutely untouched. Later, it was found that the
same green mould attacks _l_-mandelic acid in preference to the _d_- form;
whereas the yeast, _Saccharomyces ellipsoideus_, exhibits the opposite
preference for these acids.
These observations upon some of the earlier known forms of optically active
organic acids led the way to a general study of this phenomenon as
exhibited by the optically active soluble carbohydrates. The results of
these studies may be considered in connection with the several different
types of reactions which these sugars undergo, as follows:
=Glucoside Hydrolysis.=--As was pointed out in connection with the
discussion of the mutarotation of glucose, this sugar may exist in either
the [alpha] or the [beta] modification. Glucosides of both [alpha] and
[beta] glucose are of common occurrence. The difference in molecular
configuration, in such cases, may be represented by the following formulas:
R-O-C-H H-C-O-R
/ \ / \
/ \ / \
H-C-OH \ H-C-OH \
| O | O
HO-C-H / HO-C-H /
\ / \ /
\ / \ /
C-H C-H
| |
H-C-OH H-C-OH
| |
CH_{2}OH CH_{2}OH
[alpha]-Glucoside [beta]-Glucoside
The radical represented by the R may be either a common alkyl radical (as
CH_{3}, C_{2}H_{5}, etc.), another saccharide group (as in the case of the
disaccharides, trisaccharides, etc.), or some other complex organic group
(as in the case of the natural glucosides described in Chapter VI). But, in
every case, the glucoside is easily hydrolyzed by the enzyme _maltase_ (or
[alpha]-glucase) if the molecular arrangement is that represented by the
[alpha]-attachment, or by the enzyme _emulsin_ (or [beta]-glucase) if the
glucoside is of the [beta] type; but emulsin is absolutely without effect
upon [alpha]-glucosides, and maltase does not produce the slightest change
in [beta]-glucosides. These statements hold true regardless of the nature
of the group which is represented by the R in the formulas above. Hence,
the biochemical properties of the glucosides, so far as their hydrolysis
by the enzymes which are present in many biological agents is concerned,
depends wholly upon the molecular configuration of the glucose itself.
Furthermore, neither the mannosides, which differ from glucosides only in
the arrangement of the H and OH groups attached to one of the asymmetric
carbon atoms in the hexose, nor galactosides in which two such arrangements
are different (see configuration formulas on page 57), are attacked by
either maltase or emulsin. But other enzymes specifically attack other
disaccharides, or polysaccharides, or glucoside-like complexes. For
example, _lactase_ acts energetically upon ordinary lactose and all
other [beta]-galactosides; but not upon any glucoside, mannoside, etc.
Again, neither [alpha]- nor [beta]-xylosides, which correspond with the
above-described glucosides in every particular except that the HCOH group
next the terminal CH_{2}OH group is missing, are hydrolyzed by either
emulsin or maltase.
These instances, selected from among many similar observations, clearly
prove that not only the number and kind of groups in the molecule, but also
the arrangement of the constituent groups in space, must be identical in
order that the compound may be acted upon by any given enzyme acting as a
biological hydrolytic agent.
=Fermentability.=--The enzyme _zymase_, present in all yeasts, promotes the
fermentation of the natural _d_- forms of the three hexoses, glucose,
mannose, and fructose, but is without effect upon the artificial _l_- forms
of the same sugars. The uniform action of zymase upon these hexoses is
easily explained upon the basis of the same assumption which was used to
account for the formation of identical osazones from these sugars and their
easy transformation into each other; namely, their easy transformation into
an _enolic_ form which is identical for all three.
Further, galactose is fermented by some yeasts (although not by all), but
much less readily than are the other sugars, and the temperature reaction
is quite different with galactose than with the others. Talose and tagatose
are entirely unfermentable. A study of the configuration formulas for these
several sugars shows the explanation for these observed facts. These
formulas are as follows:
CHO CHO CH_{2}OH CHOH
| | | ║
H-C-OH HO-C-H C=O C-OH
| | | |
HO-C-H HO-C-H HO-C-H HO-C-H
| | | |
H-C-OH H-C-OH H-C-OH H-C-OH
| | | |
H-C-OH H-C-OH H-C-OH H-C-OH
| | | |
CH_{2}OH CH_{2}OH CH_{2}OH CH_{2}OH
Glucose Mannose Fructose Enol
CHO CHO CH_{2}OH
| | |
H-C-OH HO-C-H C=O
| | |
HO-C-H HO-C-H HO-C-H
| | |
HO-C-H HO-C-H HO-C-H
| | |
H-C-OH H-C-OH H-C-OH
| | |
CH_{2}OH CH_{2}OH CH_{2}OH
Galactose Talose Tagatose
It will be noted that in the case of glucose, mannose, and fructose, the
configuration is identical at every point except at the aldehyde end of the
chain, and that here the two groups readily arrange themselves into the
same enolic form for the three sugars. Galactose differs from these three
sugars only in the arrangement of the H and OH groups attached to one of
the other carbon atoms (the third from the alcoholic end); the difficulty
of its fermentation indicates that some molecular rearrangement to bring
this group into its proper configuration must precede the fermentation
process. The fact that it is the third HCOH group which thus undergoes
rearrangement is significant because of the participation of these parts of
molecules in groups of threes in many biological processes, as will be
mentioned elsewhere. Talose is unfermentable, even though the arrangement
of its upper three groups is the same as in the galactose and the lower
three the same as in mannose.
If further proof that fermentability depends upon molecular configuration
were needed, it is furnished by the fact that no pentose is fermentible,
even though the stereo-arrangement of each of the four alcoholic groups in
the molecule is identical with the corresponding groups in a fermentible
hexose.
=Oxidation by Bacteria.=--The bacillus _Bacterium xylinum_ contains an
enzyme, or enzymes, which promote the oxidation of the aldehyde group of an
aldose sugar to COOH, or of one alcoholic CHOH group next the terminal
CH_{2}OH group of a hexatomic alcohol to C=O. But these oxidizing enzymes
affect only those compounds in which the OH groups are on the same side of
the two asymmetric carbon atoms next the end of the molecule where the
oxidation takes place, as indicated in the following groupings.
| | | |
H-C-OH H-C-OH H-C-OH H-C-OH
| | | |
H-C-OH or H-C-OH but not HO-C-H or HO-C-H
| | | |
CHO CH_{2}OH CH_{2}OH CHO
The configuration of the remainder of the molecule is immaterial to action
by these oxidizing bacteria; hence, the enzymes in this case are apparently
concerned only with the configuration arrangement of a portion of the
molecule, instead of with the whole hexose grouping, as in the cases of the
other reactions which have been thus far considered.
It is apparent from these illustrations, and from many more which might be
cited, that there is a very definite relation between the molecular
configuration of a carbohydrate and its biochemical properties, as
represented by the possibilities of the action of enzymes upon it. The
probable nature of this relationship will be better understood after the
general questions involved in the mode of enzyme action have been
considered (see chapter XIV). But for the present, it will be sufficient to
note that it seems to be necessary that the enzyme shall actually fit the
molecular arrangement of the compound at all points, in the same way that a
key fits its appropriate lock; or a still better illustration is that of
the fitting of a glove to the hand. On the basis of the latter
illustration, it is just as impossible for a dextro-enzyme to affect a
levo-sugar, or for [alpha]-glucase to affect a [beta]-glucoside, as it is
to fit a right-hand glove upon a left hand. Further attention will be given
to these matters in later chapters.
POLYSACCHARIDES
The polysaccharides which, like the simpler saccharides, or sugars, which
have thus far been studied, undoubtedly serve as reserve food for plants,
are known under the general name of "starches." They are substances of high
molecular weight, whose constitution is represented by the general formula
(C_{6}H_{10}O_{5})_{n}. It should be noted that an exactly accurate formula
should be (C_{6})_{n}(H_{12}O_{6})_{n-1}; but since the value of _n_ is
very high, the simpler formula is approximately correct. The value of _n_
has not been accurately determined for any of the individual members of the
group, but is probably never less than 30 and may often be 200 or more. The
fact that these compounds are insoluble in most of the solvents which can
be used for molecular weight determinations makes it difficult to determine
their actual molecular constitution.
When completely hydrolyzed, the polysaccharides yield only hexoses. They
are, therefore, technically known as "hexosans." Each individual
polysaccharide which has been studied thus far yields only a single hexose,
although the particular hexose obtained varies in different cases. In fact,
the polysaccharides are often classified according to the hexoses which
they yield on hydrolysis, into the following groups: the dextrosans, which
yield glucose, and include starch, dextrin, glycogen, lichenin, etc.; the
levulosans, which yield fructose, and include inulin, graminin, triticin,
etc.; the mannans; and the galactans. The more common representatives of
each of these groups are discussed below.
(A) THE DEXTROSANS
These are by far the most common type of polysaccharides to be found in
plants.
=Starch.=--It is probable that no other single organic compound is so
widely distributed in plants as is ordinary starch. It is produced in large
quantities in green leaves as the temporary storage form of photosynthetic
products. As a permanent reserve food material, it occurs in seeds, in
fruits, in tubers, in the pith, medullary rays and cortex of the stems of
perennials, etc. It constitutes from 50 to 65 per cent of the dry weight of
seeds of cereals, and as high as 80 per cent of the dry matter of potato
tubers.
Starch occurs in plant tissues in the form of microscopic granules,
composed of concentric layers, there being apparently alternate layers of
two types of carbohydrate material, which have been distinguished from each
other by several different pairs of names used by different authors: thus,
Nägeli uses the terms "granulose" and "amylocellulose"; Meyer, "[alpha] and
[beta] amylose"; Wolff, "amylo-cellulose" and "amylo-pectin"; while Kramer
asserts that the layers are alternate lamella of crystalline and colloidal
starch. Many theories as to the nature of these concentric layers and their
mode of deposition have been advanced, but it would not be profitable to
discuss them in detail here.
For purposes of study, starch may be prepared from the ground meal of
cereals, potatoes, etc., by kneading the meal in a bag or sieve of
fine-meshed muslin or silk, under a slow stream of water. The starch
granules, being microscopic in size, readily pass through the cloth with
the water, and may be caught in any suitable container. The starch is then
allowed to settle to the bottom, the water poured off and the starch
collected and dried.
Starch is insoluble in water; but if boiled in water, the granules burst
and a slimy opalescent mass, known as "starch paste," is obtained. This is
undoubtedly a colloidal suspension of the starch in water. By various
processes, such as boiling with very dilute acids, treatment with acetone,
etc., starch is converted into "soluble starch" which dissolves in water to
a clear solution. Soluble starch is precipitated out of solution by
alcohol, or by lead subacetate solution.
Air-dried starch contains from 15 to 20 per cent of water; but this can be
completely removed, without altering the starch in any way, by heating for
some time at 100° C.
The starch granules from different sources vary considerably in size and
shape, and can generally be identified by observation under the microscope.
The most characteristic reaction of starch is the blue color which it gives
with iodine. The reaction is most marked with starch paste or soluble
starch, but even dry starch granules are colored blue when moistened with a
solution of iodine in water containing potassium iodide, or with tincture
of iodine.
When hydrolyzed, either by boiling with dilute acids or under the influence
of enzymes, starch undergoes a series of decompositions, yielding first
dextrins, then maltose, and finally glucose. These transformations can be
traced by the iodine color reaction, as starch will show its characteristic
blue, dextrins purple or rose-red, and maltose and glucose no color with
iodine.
=Dextrins= may occur in plants as transition products in the transformation
of starch into sugars, or _vice versa_. Most commonly, however, they are
artificial products resulting from the partial hydrolysis of starch in the
laboratory or factory. They are amorphous substances, which are readily
soluble in water, forming sticky solutions which are often used as
adhesives ("library paste" is a common example of a very concentrated
preparation of this kind). They are precipitated from solution by alcohol,
but not by lead subacetate (distinction from starch). They are strongly
dextrorotatory (specific rotatory power +192° to +196°); are not fermented
by yeast alone, but readily undergo hydrolysis to glucose which does
ferment. There are several different modifications, or forms, of dextrins,
depending upon the extent to which the simplification of the starch
molecule by hydrolysis is carried. Three fairly definite forms are
generally recognized, as follows: _amylo-dextrin_, or soluble starch,
slightly soluble in cold water, readily so in hot water, giving a blue
color with iodine; _erythro-dextrin_ easily soluble in water, neutral
taste, red color with iodine; and _achroo-dextrin_, easily soluble in
water, sweetish taste, no color with iodine.
Commercial dextrin, which is much used in the preparation of mucilages and
adhesive pastes, is prepared by heating dry starch to about 250° C. It is
composed chiefly of achroo-dextrin, mixed with varying quantities of
erythro-dextrin and glucose.
=Glycogen=, or "animal starch," is one of the most widely distributed
reserve foods of the animal body; in fact, it is the only known form of
carbohydrate-reserve in animal tissues. But it is present only rarely in
plants. It occurs in certain fungi, particularly in yeasts. In the animal
body, glycogen is found in all growing cells; also in the muscles and
blood; but most largely in the liver, where it is stored in large
quantities. The glycogen found in yeasts is identical with that found in
animal tissues. The quantity of glycogen in a yeast cell increases rapidly
as the yeast grows during the fermentation process.
Glycogen is a white, amorphous compound, readily soluble in hot water,
forming an opalescent solution similar in appearance to the solutions of
soluble starch. It is strongly dextrorotatory (specific rotatory power
+190°), is colored brown by iodine, and is hydrolyzed to dextrin and
maltose, and finally to glucose.
=Lichenin=, =para dextran=, and =para isodextran= are dextrosans which have
been isolated from various lower plants. They all yield glucose when
completely hydrolyzed. They resemble starch in chemical properties, but
differ from it in physical form, etc.
(B) LEVULOSANS
=Inulin= replaces starch as the reserve food carbohydrate in a considerable
number of natural orders of plants, particularly in the Compositae. It is
the carbohydrate of the tubers of the dahlia and artichoke and of the
fleshy roots of chicory. It is often found associated with starch in
monocotyledonous plants, such as many species of _Iris_, _Hyacinthus_, and
_Muscari_. Among the monocotyledons, starch seems to be the characteristic
carbohydrate reserve of aquatic, or moisture-loving, species, while inulin
is more common among those which prefer dry situations.
Inulin may be prepared from the tubers of dahlias or artichokes, by boiling
the crushed tubers with water containing a little chalk (to precipitate
mineral salts, albumins, etc.) filtering and cooling the filtrate
practically to the freezing point, which precipitates the inulin.
Inulin is a white, tasteless, semi-crystalline powder, which is soluble in
hot water, from which it may be precipitated by alcohol or by freezing. It
forms no paste like that of starch or dextrin, and gives no color with
iodine. It is levorotatory, and when hydrolyzed by acids or by the enzyme
_inulinase_ yields fructose; in fact, inulin bears the same relation to
fructose that starch does to glucose.
=Graminin, irisin, phlein, sinistrin, and triticin= are all inulin-like
polysaccharides, which have been found in the plants after which they are
named. Their solutions are, as a rule, sticky or gummy in consistency,
which suggests that these compounds bear the same relation to inulin that
dextrins do to starch.
(C) MANNOSANS, OR MANNANS
=Mannan= bears the same relation to mannose that starch does to glucose and
inulin to fructose. It occurs as a reserve food substance in many plants.
It has been reported as present in moulds, and in ergot; in the roots of
asparagus, chicory, etc.; in the leaves and wood of many trees, such as the
chestnut, apple, mulberry, and many conifers; also as a part of the
so-called "hemi-celluloses" which are present in the seeds of many plants,
notably the palms, the elders, cedar, larch, etc.
It is a white, amorphous powder, which is difficultly soluble in water, is
strongly dextrorotatory (specific rotatory power +285°), and when
hydrolyzed yields mannose.
=Secalin= (or carubin) is a substance which is found in the seeds of
barley, rye, etc., which is similar to mannan, but is optically inactive.
(D) GALACTANS
These bear the same relation to galactose that the preceding dextrosans do
to their constituent hexoses. Four different galactans have been isolated
from plant tissues; they are all white, amorphous solids which dissolve
with difficulty in water, forming gummy solutions.
Both galactans and mannans commonly occur associated with cellulose and
hemi-celluloses in the seeds or other storage organs of plants. They are
practically indigestible by animals, as the proper enzymes to hydrolyze
them are not present in the digestive tract; hence, they are commonly
classed with the indigestible cellulose as the "crude fiber" of plants
which are to be used as food by animals.
PHYSIOLOGICAL USE AND BIOLOGICAL SIGNIFICANCE OF CARBOHYDRATES
If the organic compounds produced by plants be classified with reference to
their uses in metabolism into the three groups known, respectively, as
temporary foods, storage products, and permanent structures, it is clear
that the carbohydrates which have been discussed in this chapter may fall
into either one of the first two of these classes. There can be no doubt
that the first products of photosynthesis, whichever ones they may be in
different plants, may be directly used as temporary foods, to furnish the
energy and material for the building up of permanent structures. Also,
there can be no doubt that these same carbohydrates are translocated to
the storage organs and accumulated for later use by the same plant (as, for
example, in the case of the perennials), or by the next generation of the
plant (when the storage is in the endosperm adjoining the embryo of the
seed).
There is no known explanation as to why different species of plants make
use of different carbohydrates for these purposes; or why certain species
elaborate starch out of the same raw materials from which other species
produce sugars, inulin, or glycogen, etc.
In general, starch is the final product of photosynthesis in most green
plants; but there are many exceptions to this. The polysaccharides, which
are generally insoluble, must be broken down into the simpler soluble
sugars before they can be translocated to other organs of the plant for
immediate, or future, use. When they reach the storage organs, they may be
recondensed into insoluble polysaccharides, or stored as soluble sugars.
Examples of the latter type of storage are, sucrose in beet roots, glucose
in onion bulbs, etc. Sometimes, this habit of storage seems to be a species
characteristic; as potatoes store starch, while beets, growing in the same
soil and under exactly the same environment, store sugar. But in other
cases, the nature of the carbohydrate stored undoubtedly is correlated with
the external temperatures at the time of storage. It has been shown that
cold, which tends to physiological dryness, very frequently favors the
storage of sugars instead of starches. Thus, in temperate zones, among
aquatic, or moisture-loving plants, those species which hibernate during
the winter at the bottom of lakes or ponds and are killed by temperatures
below freezing, store starch and no sugar; while in the same ponds, the
species whose storage organs pass the winter above the level of the water
and can withstand temperatures as low as -7° C. contain sugar during the
winter months, even if they contain starch during warmer periods.
Similarly, sugars often appear in the leaves and stems of conifers during
the winter months, only to disappear, or be replaced by starch, when spring
approaches. This same phenomenon is noticeable in arctic plants, which
generally contain but small proportions of starch and relatively large
amounts of sugars.
Similarly, the phenomenon of the turning sweet of potatoes when exposed to
low temperatures has often been noted. The change of the starch in potato
tubers to sugar is most rapid at the temperature of 0° C., and ceases at
7°, or above. Also, if potatoes in which the maximum amount of sugar is
present (not over one-sixth of the total starch can be converted into
sugar) are exposed to a higher temperature the sugar soon disappears.
In general, however, it may be said that each particular species of plant
has its own particular preference for a specific carbohydrate as its
reserve food material, and elaborates the proper enzymes to make it
possible to utilize this particular carbohydrate for its metabolic needs.
Again, the question as to whether the storage of energy-producing materials
for the use of the next generation shall be in the form of carbohydrates or
of fats seems to be definitely connected with the size of the seed, and the
consequent available storage space (see page 138). Animals habitually use
the space-conserving form of fats for their energy-storage, while plants
more commonly use carbohydrates for this purpose, except in the case of
those small seeds in which sufficient energy cannot be stored in
carbohydrate form to develop the young seedling to the point where it can
manufacture its own food. As a general rule, nuts, which contain the embryo
of slow-growing seedlings, and need large proportions of energy reserve,
are characteristically _oily_ instead of _starchy_ in type.
But, aside from temperature reactions and space requirements, there is no
law which has yet been discovered which determines the character of the
energy-storage compound which any given species of plant will elaborate.
The process of photosynthesis would seem to be identical in all cases, at
least up to the point of the production of the first hexose sugar; but the
transformation of glucose into other monosaccharides, disaccharides, and
polysaccharides seems to be a matter which obeys no rule or law.
Finally, there remains to be considered the occurrence and uses of sugars
in the fleshy tissues of fruits. These tissues have, of course, no direct
function in the life history of the plant. They surround the seed, but they
must decay or be destroyed before the seed can come into the proper
environment for germination and growth. In most fruits, starch is the form
in which the carbohydrate material is first deposited in the green tissue,
but as the fruit ripens the starch rapidly changes into sugars, with the
result that the fruit takes on a flavor which makes it much more attractive
as a food for men and animals. This purely biological significance of the
presence of sugars (and of the other substances which give desirable
flavors to fruits, vegetables, etc.), can have no possible relation to the
physiological needs of the individual plant, however.
It is apparent that the production of these immense stores of reserve food
by plants makes them useful as food for animals, and it is, of course, the
storage parts of the plants which are most useful for this purpose. This
biological relationship needs no further emphasis.
REFERENCES
ABDERHALDEN, E.--"Biochemisches Handlexikon, Band 2 ... Die Einfachen
Zuckerarten, Inuline, Cellulosen, ...," 729 pages, Berlin, 1911, and "Band
8--1 Ergänzungsband (same title as Band 2)--" 507 pages; Berlin, 1914.
ARMSTRONG, E. F.--"The Simple Carbohydrates and Glucosides," 233 pages.
_Monographs_ on Biochemistry, London, 1919 (3d ed.).
FISCHER, E.--"Untersuchung ueber Kohlenhydrate und Fermente, 1884-1908,"
912 pages, Berlin, 1909.
MACKENSIE, J. E.--"The Sugars and their Simple Derivatives," 242 pages, 17
figs., London, 1913.
TOLLENS, B.--"Kurzes Handbuch der Kohlenhydrate," 816 pages, 29 figs.,
Leipzig, 1914 (3d ed.).
CHAPTER V
GUMS, PECTINS, AND CELLULOSES
These substances constitute a group of compounds which are very similar to
the polysaccharide carbohydrates in composition and constitution, but which
serve entirely different purposes in the plant. As a class, they are
condensation products of pentoses, known as pentosans and having the
formula (C_{5}H_{8}O_{4})_{n}, or hexosans having the formula
(C_{6}H_{10}O_{5})_{n}, or combined pentosan-hexosans.
In general, these compounds make up the skeleton, or structural framework
material, of the plant, in contrast with the protoplasmic materials or food
substances for which most of the other types of organic compounds
(discussed in other chapters of this book) serve. They are the principal
constituents of "woody fiber," of cell-walls, and of the "middle lamella"
which fills up the spaces between the plant cells. They are, therefore,
found in largest proportions in the stems of woody plants; but they are
also present in every other organ of plants, as the cell-wall or other
structural material.
For purposes of study, these compounds may conveniently be divided into
three groups; namely, the natural gums and pentosans, the pectins and
mucilages, and the celluloses. The segregation into these three groups is
not sharply defined. The distinction between the groups is based upon the
solubility of the compounds in water. The gums and pentosans readily
dissolve in water; the pectins form colloidal solutions which are easily
converted into "jellies"; the mucilages do not dissolve but form slimy
masses; while the celluloses are insoluble in and unaltered by water. Some
authors add a fourth group, known as "humins"; but as these are the
products of decay (usually in the soil) of these structural compounds,
rather than of growth and development, they need not be taken into
consideration in a study of the chemistry of plant growth.
THE NATURAL GUMS AND PENTOSANS
The natural gums, when hydrolyzed, yield large proportions of sugars, but
most of them also contain a complex organic acid nucleus, by means of which
they form salts with calcium, magnesium, etc. Some of them, such as cherry
gum and those which are found in the woody stems of plants (wood gum, and
those found in corn stalks, the straw of cereals, etc.) yield practically
pure pentoses. These are known as pentosans. They bear the same relation to
the pentose sugars as do the dextrosans to glucose, etc. The wound gums,
for example, yield arabinose, and the wood gums yield xylose. But most of
the natural gums yield a mixture of galactose, some pentose, and some
complex organic acid.
The gums are translucent, amorphous substances, whose solutions in water
are levorotatory. They are precipitated out of solution by alcohol and by
lead subacetate solution.
Gums are extremely difficult to hydrolyze, the laboratory process of
hydrolysis usually requiring from eighteen to twenty-four hours of
continuous boiling with acids for its completion. Because of this
difficulty of hydrolysis, gums are practically indigestible by animals and
of little use as food.
The following common examples will serve to illustrate the general nature
of these compounds.
=Gum arabic=, found in the exudate from the stems of various species of
Acacia, is a mixture of the calcium, magnesium, and potassium salts of a
diaraban-tetragalactan-arabic acid. Arabic acid has the formula
C_{23}H_{38}O_{22}, and one molecule of this acid serves as the nucleus for
the union of eight galactose and four arabinose groups, linked together in
some unknown way. The formula for the compound, exclusive of the metallic
elements with which it is loosely united is C_{91}H_{150}O_{78}. This gives
some idea of its complexity.
When boiled with nitric acid, it is oxidized to mucic, saccharic, and
oxalic acids. It gives characteristic reactions with alum, basic lead
acetate, and other common reagents.
Gum arabic comes on the market as a brittle, glassy mass, which is used in
the preparation of mucilages, and as a carrier for essential oils, etc., in
certain toilet preparations.
Recent investigations have shown that the so-called "meta-pectic acid,"
which is often found in sugar beets and interferes with the process of
sugar manufacture, is identical with gum arabic in composition and
properties.
=Gum tragacanth= is the soluble portion of the natural gum which is found
in several species of _Astragalus_. It constitutes only 8 to 10 per cent of
the total gum-like material which is present, the remainder being composed
of insoluble gummy substances of unknown composition. The soluble gum
consists of calcium, potassium, and magnesium salts of an acid which, when
hydrolyzed, yields several molecules of arabinose, six of galactose, and
one of geddic acid (an isomer of arabic acid). It is said to be produced by
the metamorphosis of the medullary rays under unfavorable conditions of
growth. It comes on the market in globular masses of amorphous material,
and is used in the manufacture of cosmetics, etc.
=Wound gum= is frequently found in the tracheæ of plants, and near surface
wounds, which it stanches. It is secreted by the cells surrounding the
injured part. It responds to the reactions of other gums and to some of
those of woody fiber. Its exact composition is not known, but probably lies
between that of the true gums and that of cellulose.
These gums are generally considered to be decomposition products of
celluloses, resulting from the action of some hydrolytic ferment, usually
stimulated by some unfavorable condition of growth, some injury, or some
morbid condition.
The =pentosans=, araban and xylan, occur normally in the stems and outer
seed coats of many common plants. They constitute a considerable proportion
of these tissues, as indicated by the following results of typical
analyses: Wheat bran, 22 to 25 per cent; clover hay, 8 to 10 per cent; oat
straw, 16 to 20 per cent; wheat straw, 26 to 27 per cent; corn bran, 38 to
43 per cent; jute fiber, 13 to 15 per cent; various wood gums, 60 to 92 per
cent.
They are white, fluffy solids, which are difficultly soluble in cold water,
more readily in hot water. They are very difficult to hydrolyze, and
indigestible by animals. When finally hydrolyzed, they yield arabinose and
xylose, respectively. The pith of dry corn stalks is a good illustration of
their general character.
MUCILAGES
These are characterized by forming slimy masses when moistened with water.
They are secreted by hairs on the skin of many plants, so that the external
walls of the leaves, fruit, and seeds are often mucilaginous when damp.
This is particularly true of aquatic plants. The chemical composition of
the mucilages is unknown. When hydrolyzed, they yield arabinose and a
hexose; the latter is sometimes galactose and sometimes mannose.
When present on the surface of plant tissues, the mucilages probably serve
to prevent the too rapid diffusion of materials through the skin, in the
case of the aquatic plants, and too rapid transpiration, in the case of
young vegetative tissues or in other plants when growing under extremely
dry conditions. When found in tubers, or other storage organs, it has been
supposed that they may serve as reserve food materials, but it seems that
such difficultly hydrolyzable compounds as these can hardly function as
normal reserve foods.
PECTINS
Many fruits, such as currants, gooseberries, apples, pears, etc., and many
fleshy roots of vegetables, such as carrots, parsnips, etc., contain
substances known as _pectins_. These are readily soluble in water, and when
dissolved in concentrated solutions in hot water, they set into "jellies"
when the solution is cooled. These jellies carry with them the soluble
sugars and flavors which are present in the fruits, and constitute a
familiar article of diet.
There are undoubtedly several different modifications of the pectins, to
which the names "meta-pectin," "para-pectin," "pectic acid," "meta-pectic
acid," and "para-pectic acid," have been applied. These all seem to be
products of hydrolysis of a mother substance known as "pectose," which
constitutes the middle lamella of unripe fruit, etc. As the fruit ripens,
the pectose is hydrolyzed into the various semi-acid, or acid, bodies
mentioned above. The intermediate products of the hydrolysis are the
pectins, which swell up in water and readily form jellies; while the final
meta-pectic acid is easily soluble in water and resembles the true gums in
its properties. When the middle lamella reaches the pectic acid stage, the
fruit becomes soft and "mushy" in texture.
The pectins more nearly approach to the composition, properties, and
functions of the celluloses than do any of the other groups of organic
compounds. They have been extensively studied in connection with the
parasitism of certain fungous diseases which cause the soft rots of fruits
and vegetables. These parasites usually penetrate the tissues of the host
plant by dissolving out the middle lamella material, which may sometimes
serve as food material for the fungus; but more often the parasite secures
its food supply from the protoplasm of the cell contents. In such cases,
the parasite secretes both a pectose-dissolving enzyme, known as "pectase"
and a "cellulase" which attacks the cell-wall material in order to provide
for the entry of the fungus into the cells. Other enzymes, known as
"pectinases," which coagulate the soluble pectins or pectic acids into
insoluble jellies in the tissues of the plants seem to aid the plant in
resisting the penetration by the parasite.
CELLULOSES
Used in its general sense, this term includes all those substances which
are elaborated by protoplasm to constitute the cell-wall material.
Cellulose proper is a definite chemical compound, whose properties are well
established. In plants, however, this true cellulose is nearly always
contaminated by various encrusting materials; and in the process of
wood-formation, the cell-wall material continually thickens by the
conversion of the cellulose into ligno-cellulose and the protoplasm of the
cell as continuously diminishes in volume. Thus the protoplasm of the cell
produces a number of different kinds of material which are deposited in the
walls of the cell. All of these, taken together, constitute the general
group known as the celluloses.
These may be divided into three classes: namely, (1) the hemi-celluloses,
(2) the normal celluloses, and (3) the compound celluloses.
The =hemi-celluloses= (pseudo-, or reserve celluloses) include a series of
complex polysaccharides which occur in the cell-walls of the seeds of
various plants. They are found in the shells of nuts, rinds of cocoanuts,
shells of stony fruits, etc., and in the seedcoats of beans, peas and other
legumes. They are much more easily hydrolyzed than the other members of
this group, and when hydrolyzed yield various sugars, chiefly galactose,
mannose, and the pentoses. They bear the same relation to these sugars that
starch does to glucose, and are generally supposed to serve as reserve food
material, although it is difficult to conceive how the shells, etc., in
which they appear can be utilized by a growing seedling. They differ in
structure from the fibrous celluloses and are probably not cell-wall
building material. They appear to be a form of reserve carbohydrates, which
differ from the glucose-polysaccharides in being condensed in, or as a part
of, the external structural material rather than in the internal storage
organs. They are soluble in water and exhibit the properties of gums, and
are often classified with the gums and described under the names
"galactans," "mannosans," "pentosans," etc.
The =normal celluloses=, of which the fibers obtained from cotton, flax,
hemp, etc., are typical examples, are widely distributed in plants and form
the commercial sources for all textile fibers of vegetable origin. Ordinary
cotton fiber contains 91 per cent of cellulose, about 7.5 per cent of
water, 0.4 per cent of wax and fat, 0.55 per cent of pectose derivatives,
and 0.25 per cent of mineral matter; or a total of only 1.2 per cent of
non-cellulose solids. Filter paper is practically pure cellulose.
Pure cellulose is a white, hygroscopic substance, which is insoluble in
water and in most other solvents. If heated with water under pressure to
about 260° C., it dissolves completely without decomposition. If boiled
with a strong solution of zinc chloride, or treated in the cold with zinc
chloride and concentrated hydrochloric acid, or with an ammoniacal solution
of copper hydroxide (Schweitzer's reagent), it dissolves to a clear
solution from which it may be reprecipitated without chemical change by
neutralizing or diluting the solution.
Cellulose has the formula (C_{6}H_{12}O_{5})_{n}. When hydrolyzed under the
influence of the enzyme _cytase_, it breaks down, first into cellobiose, an
isomer of maltose, and then into glucose. It is, therefore, chemically
like, but not identical with, starch; and structurally it is arranged in
fibrous form instead of in granules. Under the action of fermentative
enzymes, as when vegetable matter decays under stagnant water, in swamps,
etc., cellulose breaks down into carbon dioxide and marsh gas, according to
the equation
(C_{6}H_{12}O_{5})_{n} + {n}_H_{2}O = 3_{n}CO_{2}+3_{n}CH_{4}.
Cellulose is acted upon by caustic alkalies in a variety of ways. When
fused with a mixture of dry sodium and potassium hydroxides, it is
decomposed into oxalic and acetic acids. When heated with a 10 to 15 per
cent solution of caustic soda, cellulose fibers thicken and become
translucent, thus resembling silk fibers. This process, known as
"Mercerizing," is largely used for the production of commercial fabrics.
Acids also act on cellulose in a variety of ways. When heated with nitric
acid (sp. gr. 1.25), it is converted into _oxycellulose_; while dilute
sulfuric acid, under similar conditions, yields _hydro-cellulose_, a
substance having the formula C_{12}H_{22}O_{11}, which retains the fibrous
structure of the original cellulose but which, when dry, may be rubbed up
into a fine powder. Concentrated nitric acid, or better, a mixture of
concentrated nitric and sulfuric acids, acts upon cellulose, converting it
into various nitro-derivatives, several of which have great industrial
value. The number of NO_{3} groups which unite with the cellulose molecule
under these conditions depends upon the temperature, pressure, etc.,
employed during the nitration process; di-, tri-, tetra-, penta-, and
hexanitrates are all known. _Pyroxylin_, or _collodion_, is a mixture
of the tetra- and penta-nitrates, which is soluble in alcohol and is
used in surgery, in photography, and in the manufacture of celluloid,
which is a mixture of collodion and camphor. The hexanitrate,
C_{12}H_{14}(NO_{3})_{6}O_{4}, is the violent explosive known as
_gun-cotton_.
Gentler oxidizing agents, such as "bleaching powder," etc., have no effect
upon cellulose, and hence are extensively used in the treatment of cotton
and other vegetable fibers, in preparation for their use in the manufacture
of textiles, paper, etc.
Cellulose is indigestible in the alimentary tract of animals, but the
putrefactive bacteria which are generally present there ferment it, with
the production of acids of the "fatty acid" series, carbon dioxide,
methane, and hydrogen. Excessive fermentations of this kind are responsible
for the distressing phenomenon known as "bloat."
The =compound celluloses= comprise the larger proportion of the material of
the woody stems of plants. They consist of a base of true cellulose, which
is either encrusted with or chemically combined with some non-cellulose
constituent. Depending upon the nature of the non-cellulose component, the
compound celluloses are divided into three main groups, known respectively
as (1) ligno-celluloses, (2) pecto-celluloses, and (3) adipo-, or
cuto-celluloses. As the names indicate, the non-cellulose component in the
first group is lignin; in the second, pectic substances; and in the third,
fats or waxes.
=Ligno-celluloses.=--In the young plant cell, the cell-walls consist of
practically pure cellulose; but as the plant grows older, this becomes
permeated with lignin, or woody fiber, until in the stem of a tree, for
example, the proportion of cellulose in the tissue is only 50 to 60 per
cent. In the preparation of wood pulp for the manufacture of paper, the
lignin materials are dissolved off by means of various chemical reagents,
leaving the cellulose fibers in nearly pure form for use as paper. The
lignin material generally consists of two types of substances, one of which
contains a closed-ring nucleus of unknown composition and the other is
probably a pentosan. These materials are so extremely difficult to
hydrolyze that their composition has not yet been definitely determined.
=Pecto-celluloses= are found in various species of flowering plants; those
which are present in the stems and roots being true pecto-celluloses, while
those which are found in fruits and seeds contain mucilages rather than
pectose derivatives, and are generally designated as "muco-celluloses." The
exceedingly inert character of these compounds makes their study difficult
and their functions uncertain.
The term =cuto-celluloses= is applied to the group of substances, including
suberin and cutin, which constitute waterproof cell-walls. These were
formerly supposed to consist of true cellulose impregnated with fatty or
wax-like materials. Recent investigations seem to indicate, however, that
there is really no cellulose nucleus in such walls as these, but that they
are compound glyceryl esters resembling the true fats (see chapter X) in
composition. If this view should finally be established as a fact, this
sub-group of supposed compound celluloses should be dropped from
consideration as such.
PHYSIOLOGICAL USE OF CELLULOSES
There seems to be no question that the sole use of celluloses is to serve
as structure-building materials. They are undoubtedly elaborated from the
carbohydrates as the cell grows. In only rare cases, however, is there any
evidence that they can be reconverted into carbohydrates to serve as food
material. Certain bacteria can make use of cellulose as food, and secrete
an enzyme, cytase, which aids in the hydrolysis of cellulose to sugars for
this purpose. But this enzyme seems rarely, if at all, to be present in the
tissues of higher plants. It has been reported that some cellulose is
hydrolyzed during the malting of barley, indicating that this might have
some food use for the growing seedling; but this observation has not been
confirmed and later investigations seem to throw doubt upon its accuracy.
Bacteria of decay also act upon cellulose materials, converting them
chiefly into gaseous products; but this seems to be a provision of nature
for the destruction of the cell-wall material of dead plants, rather than
an arrangement for the constructive use of it as food for the bacterium.
When fibrous plant residues decay in the soil, the cellulose compounds are
first converted into a series of complex organic acids, known as "humins,"
which undoubtedly have a significant effect upon the chemical and physical
properties of the soil, but these have little interest or significance in a
study of the chemistry of plant growth.
REFERENCES
ABDERHALDEN, E.--"Biochemisches Handlexikon, Band 2, Gummisubstanzen,
Hemicellulosen, Pflanzenschleimen ..." 729 pages, Berlin, 1911; and "Band
8--1 Ergänzungsband (same title as Band 2)--," 507 pages, Berlin, 1914.
SCHWALBE, C. G.--"Die Chemie der Cellulose," 665 pages, Berlin, 1911.
CHAPTER VI
GLUCOSIDES
Strictly speaking, the term _glucoside_ should be applied only to such
compounds as contain glucose as the characteristic basic group. But in
common usage, it refers to any compound which, when hydrolyzed, yields a
sugar as one of the products of the hydrolysis. In all the natural
glucosides which occur in plant tissues, the other organic constituent,
which is represented by the R in the formula for glucosides
(R·C_{6}H_{11}O_{5}, or R·(CHOH)_{5}CHO) is some aromatic group, or
closed-ring benzene derivative.[3] The different organic constituents of
glucosides are of a great variety of types, such as phenols, alcohols,
aldehydes, acids, oxyflavone derivatives, mustard oils, etc. It is
noteworthy, however, that no nitrogenous groups of the protein type have
been found combined with sugars in glucosides.
Some glucosides contain more than one saccharide group, possibly as di- or
trisaccharides. Under proper conditions of hydrolysis, one or more of the
saccharide groups can be removed from such compounds, resulting in
glucosides of simpler structure.
Most of the common glucosides are derived from _d_-glucose. Some are known,
however, which are derivatives of galactose or rhamnose; while in some
cases the exact nature of the sugar which is present has not yet been
determined.
FOOTNOTES:
[3]
HC
// \
HC CH
The structural formula for benzene, C_{6}H_{6}, | ║ is one which it is
HC CH
\\ /
CH
difficult and inconvenient to reproduce in type. On that account, it is
/\
customary to indicate this formula by a plane hexagon, thus | |.
\/
It is understood, in all such cases, that the figure represents six carbon
atoms arranged in a closed ring, with alternate double and single bonds,
and with a hydrogen atom attached to each carbon. The printing of some
other group as OH, CH_{3}, adjacent to an angle of the hexagon means that
this group replaces the H atom in the compound which is being illustrated.
HYDROLYSIS OF THE NATURAL GLUCOSIDES
All natural glucosides are hydrolyzed into a sugar and another organic
residue by boiling with mineral acids; although they vary widely in the
ease with which this hydrolysis is brought about.
In most cases, the glucoside is easily hydrolyzed by an enzyme which occurs
in the same plant tissue, but in different cells than those which contain
the glucoside. Injury to the tissues, germination processes, and perhaps
other physiological activities of the cells, result in bringing the enzyme
in contact with the glucoside and the hydrolysis of the latter takes place.
A large number of such enzymes have been found in plants, many of which
hydrolyze only a single glucoside. However, two enzymes, namely, the
emulsin of almond kernels, and _myrosin_ of black mustard seeds, each
hydrolyze a considerable number of glucosides. In general, _emulsin_ will
aid in the hydrolysis of any glucoside which is a derivative of
[beta]-glucose, and myrosin will help to split up any sulfur-containing
glucoside. Glucosides which are derivatives of rhamnose require a special
enzyme, known as _rhamnase_, for their hydrolysis.
The following reactions for the hydrolysis of arbutin and of amygdalin are
typical of this action, and will serve to illustrate the general structure
of these compounds:
------O-------
| |
CH_{2}OH·CHOH·CH·CHOH·CHOH·CH·O·C_{6}H_{4}OH + H_{2}O
Arbutin
= C_{6}H_{12}O_{6} + HOC_{6}H_{4}OH
Glucose Hydroquinone
C_{6}H_{5}
|
(_a_) C_{6}H_{11}O_{5}·O·C_{6}H_{10}O_{4}·O·CH + H_{2}O
Amygdalin |
CN
C_{6}H_{5}
|
= C_{6}H_{11}O_{5}·O·CH + C_{6}H_{12}O_{6}
Mandelo-nitrile | Glucose
glucoside CN
C_{6}H_{5}
|
(_b_) C_{6}H_{11}O_{5}·O·CH+H_{2}O = C_{6}H_{5}·CHOH·CN+C_{6}H_{12}O_{6}
Mandelo-nitrile | Mandelo-nitrile Glucose
glucoside CN
(_c_) C_{6}H_{5}·CHOH·CN + H_{2}O = C_{6}H_{5}·CHO + HCN
Mandelo-nitrile Benzaldehyde Hydrocyanic
acid
GENERAL PROPERTIES OF GLUCOSIDES
As a rule, glucosides are easily soluble in water. They are generally
extracted from plant tissues by digestion with water or alcohol. In most
cases, the enzyme which is present in other cells of the same tissue must
be killed by heating the material, in a moist condition, to the temperature
of boiling water, before the extraction is begun, as otherwise the
glucoside will be hydrolyzed as rapidly as it is extracted from its parent
cell. Maceration or otherwise bruising the tissue, after the enzyme has
been destroyed, facilitates the extraction. The glucosides, after
extraction and purification by recrystallization, are generally colorless,
crystalline solids, having a bitter taste and levorotatory optical
activity. This latter property is remarkable, as most of them are compounds
of the strongly dextrorotatory _d_-glucose.
Many of the natural glucosides have marked therapeutic properties and are
largely used as medicines; others are the mother-substances for brilliant
dyes; for example, indican, from which indigo is obtained, and the alizarin
glucosides.
Several hundred different glucosides have been isolated from plant tissues,
and their properties described, and this number is being added to
constantly, as the methods of isolation and study are improved. They may be
classified into groups, according to the nature of the organic compound
other than sugars which they yield when hydrolyzed. The following
descriptions of the occurrence, constitution, products of hydrolysis, and
special properties of typical members of each of the several different
classes of glucosides will serve to illustrate their general relationship
to plant growth.
THE PHENOL GLUCOSIDES
=Arbutin=, C_{12}H_{16}O_{7}, is obtained from the leaves of the bear berry
(_Arctostaphylos uva-ursi_), a small evergreen shrub. When hydrolyzed by
mineral acids or emulsin, it yields glucose and hydroquinone.
C_{12}H_{16}O_{7}+H_{2}O = C_{6}H_{12}O_{6}+C_{6}H_{4}(OH)_{2}.
Hydroquinone has strongly antiseptic properties. Arbutin is both an
antiseptic and a diuretic, and is used in medicine.
=Phloridzin=, C_{21}H_{24}O_{10}, is found in the bark of apple, pear,
cherry, plum, and similar trees. Mineral acids (but not emulsin) hydrolyze
it to glucose and _phloretin_ (C_{15}H_{14}O_{5}), according to the
equation
CH_{3}
C_{21}H_{24}O_{10} + H_{2}O = C_{6}H_{12}O_{6} + |
(OH)_{3}C_{6}H_{2}·CO·CH·C_{6}H_{4}OH.
It is used in medicine as a remedy for malaria, having marked anti-periodic
properties.
=Glycyphyllin=, C_{21}H_{24}O_{9}, found in leaves of Smilax, yields
rhamnose and phloretin, when hydrolyzed.
=Iridin=, C_{24}H_{26}O_{13} (glucose and irigenin), found in rootstocks of
Iris, is used in medicine as a cathartic and diuretic.
=Baptisin=, C_{26}H_{32}O_{14}·9H_{2}O (two rhamnose and baptigenin), found
in roots of wild indigo (_Baptisia_), has strong purgative properties.
=Hesperidin=, C_{50}H_{60}O_{27} (one rhamnose+two glucose+hesperitin), is
found in the pulp of lemons and oranges.
The characteristic phenol group which is present in these glucosides has
the following structural formula, in each case, the X indicating the H atom
which is replaced by the sugar molecule to form the glucoside:
Phloretin
/\ /\
/ \----C----CH------/ \
| | ║ | | |OX
HO| |OH O CH_{3} | |OH
\ / \ /
\/ \/
Irigenin
O
/ \
-CH_{2}-C-O--/\
/\ \ / \O·CH_{3}
/ \ O-| |
| | | |OX
CH_{3}O-| | \ /
\ / \/
O
|
CH_{3}
Hesperitin
/\ /\
/ \ / \
HO| |-CH=CH-C-| |OX
CH_{3}O| | ║ | |
\ / O \ /
\/ \/
THE ALCOHOL GLUCOSIDES
=Salicin=, C_{13}H_{18}O_{7} (glucose+saligenin, or _o_-oxy benzyl alcohol)
is found in the bark, leaves, and flowers of most species of willow, the
proportion present depending upon the season of the year, and the sex of
the tree. It is used as a remedy against fevers and rheumatism, causing
less digestive disturbances than the salicylic acid which is the oxidation
product of saligenin and which is sometimes used as a remedy for
rheumatism.
=Coniferin=, C_{16}H_{22}O_{8} (glucose and coniferyl alcohol), is found in
the bark of fir trees. The coniferyl alcohol obtained from coniferin by
hydrolysis can be easily oxidized to _vanillin_, and is, therefore, the
source for the artificial flavoring extract used as a substitute for the
true extract of the vanilla bean.
=Populin=, C_{20}H_{22}O_{8} (glucose+saligenin+benzoic acid), found in the
bark of poplar trees, is used in medicine as an antipyretic. It can be
hydrolyzed, by a special enzyme, into salicin and benzoic acid.
The structure of the two typical closed-ring alcohols which are present in
these glucosides is indicated by the following formulas;
Coniferyl alcohol
Saligenin
CH=CH·CH_{2}OH
/\ /\
/ \ / \
| |CH_{2}OH | |
| |OX | |OCH_{3}
\ / \ /
\/ \/
OX
THE ALDEHYDE GLUCOSIDES
=Salinigrin=, C_{13}H_{16}O_{7} (glucose and _m_-oxy benzaldehyde), is
found in the bark of one species of willow (_Salix discolor_). Its isomer,
known as _helicin_ (glucose and _o_-oxy benzaldehyde, or salicylic
aldehyde), does not occur naturally in any plant, but is easily produced
artificially by the gentle oxidation of salicin. Their relationships are
shown on the following formulas;
Salicin Helicin Salinigrin
/\ /\ /\
/ \ / \ / \
| |CH_{2}OH | |CHO | |CHO
| |OX | |OX | |
\ / \ / \ /
\/ \/ \/
OX
=Amygdalin=, also contains a benzaldehyde group, but there is linked with
it a hydrocyanic acid group; hence, this glucoside is usually classed with
the cyanophoric glucosides (see page 86).
THE ACID GLUCOSIDES
The most common example of this group is =gaultherin=, C_{14}H_{18}O_{8},
which is found in the bark of the black birch and is a combination of
glucose with methyl salicylate. Both the glucoside itself and the methyl
salicylate ("oil of wintergreen") which is derived from it are used as
remedies for rheumatism.
=Jalapin=, C_{44}H_{56}O_{16} (glucose and jalapinic acid), and
=convolvulin=, C_{54}H_{96}O_{27} (glucose+rhodeose+convolvulinic acid),
are glucosides of very complex organic acids, found in jalap resin, which
are used in medicine as cathartics or purgatives.
THE OXY-CUMARIN GLUCOSIDES
Cumarin itself is widely distributed in plants. No glucoside containing
cumarin as such has yet been isolated; but several glucosides of its
oxy-derivatives are known. The following are common ones:
=Skimmin=, C_{15}H_{16}O_{8} (glucose and skimmetin), is found in _Skimmia
japonica_; =æsculin=, C_{15}H_{16}O_{9} (glucose and æsculetin), is found
in the bark of the horse-chestnut, _Æsculus hippocastanum_, and its isomer,
=daphnin= (glucose and daphnetin), in several species of _Daphne_; and
fraxin, C_{16}H_{18}O_{10} (glucose and fraxetin), is found in the bark of
several species of ash.
The structural arrangement of the oxy-cumarin groups which are found in
these glucosides is shown in the following formulas. It is not known to
which OH group the sugar is attached, in each case.
Skimmetin Æsculetin
CH=CH·CO CH=CH·CO
/\ | /\ |
/ \_O_| / \_O_|
| | | |
| | HO| |
\ / \ /
\/ \/
OH OH
Daphnetin Fraxetin
CH=CH·CO CH=CH·CO
/\ | /\ |
/ \_O_| / \_O_|
| | HO| |
| |OH HO| |
\ / \ /
\/ \/
OH OCH_{3}
=Scopolin=, C_{22}H_{28}O_{14}, found in _Scopolia japonica_, contains two
glucose molecules united to a monomethyl ether of æsculin; while
=limettin=, found in certain citrus trees, is the dimethyl ether of
æsculin.
THE PIGMENT GLUCOSIDES
Many, if not all, of the red, yellow, violet, and blue pigments of plants
either exist as, or are derived from, glucosides. These are of three types:
the madder, or alizarin, reds are derivatives of various
oxy-anthraquinones; most of the soluble yellow pigments are glucosides
derived from flavones or xanthones; and the soluble red, blue, and violet
pigments of the cell-sap of plants are mostly anthocyan derivatives. The
four basic groups, or nuclei, which are present in these different types of
compounds are complex groups consisting essentially of two benzene rings
linked together through a third ring in which there are either two oxygen
atoms in the ring, or one oxygen in the ring and a second attached to the
opposite carbon in the (C=O) arrangement, as shown by the following
diagrammatic formulas:
Xanthone Anthraquinone
O O
║ 1 1 ║ 1'
/\ C /\ /\ C /\
/ \ / \ / \ / \ / \ / \
| | | |2 2| | | |2'
| | | |3 3| | | |3'
\ / \ / \ / \ / \ / \ /
\/ O \/ \/ C \/
4 4 ║ 4'
O
Flavone Anthocyan
1 1
/\ O 5'____4' /\ O 5'____4'
/ \ / \____/ \ / \ / \____/ \
2| | | \____/3' 2| | | \____/3'
3| | |5 1' 2' 3| | |6 1' 2'
\ / \ / \ / \ /
\/ C \/ 5
4 ║ 4
O
The red dyes which were formerly obtained from madder, the powdered roots
of _Rubia tinctoria_, but are now almost wholly artificially synthetized,
consist of at least four different glucosides, the organic group of which,
in each case, is an hydroxy-derivative of anthraquinone. The most important
of these is _ruberythric acid_, composed of two molecules of glucose linked
with one of alizarin (1,2, dioxyanthraquinone). _Xanthopurpurin_ contains
1,3, dioxyanthraquinone, which is isomeric with alizarin; and _rubiadin_ is
a monomethyl (the CH_{3} being in the 4 position), derivative of this
compound. _Purpurin_ is a glucoside of 1,2,4, trioxyanthraquinone.
The soluble yellow pigments are generally glucosides of hydroxy-derivatives
of xanthone or flavone, known as oxyxanthones or oxyflavones. The sugars
which are united to these nuclei vary greatly, so that there are a great
variety of yellow, white, or colorless flavone or xanthone pigment
compounds. These compounds are almost universally present in plants. For
example, one typical set of examinations of the wood, bark, leaves, and
flowers of over 240 different species of tropical plants showed that
flavone derivatives were present in every sample which was tested, the
pigments being usually located in the powdery coating of the epidermis of
the tissues.
The following typical examples will serve to illustrate the composition and
properties of the glucosides of this type.
=Quercitrin=, C_{21}H_{20}O_{11}, is found in oak bark, in the leaves of
horse-chestnut, and in many other plants, often associated with other
pigments. It is a brilliant yellow crystalline powder. Industrially, it
ranks next to indigo and alizarin in importance as a natural dye stuff. It
is a glucoside of rhamnose with 1,3,3',4', tetraoxyflavonol (i.e., the
flavone nucleus with five OH groups replacing the hydrogens in the 1, 3, 5,
3', and 4' positions). =Quercetin=, C_{15}H_{10}O_{7}, which is the
tetraoxyflavonol itself, without any sugar in combination with it, is found
in the leaves of several species of tropical plants and in the bark of
others. =Isoquercitrin=, C_{21}H_{20}O_{12}, is derived from the same
flavone, but contains glucose instead of rhamnose, as the sugar constituent
of the glucoside.
=Apiin=, C_{26}H_{20}O_{9}, the yellow glucoside found in the leaves of
parsley, celery, etc., contains apiose (a pentose sugar of very unusual
structure, represented by the formula,
CH_{2}OH
\
COH·CHOH·CHO), and apigenin, which is a 1,3,4',trioxyflavone.
/
CH_{2}OH
=Xanthorhamnin=, C_{34}H_{42}O_{20}, is a very complex glucoside containing
two rhamnose and one galactose groups, united with rhamnetin, which is
quercitin with the H of the OH in either the 1, or 3, position replaced by
a methyl group. There are several similar pigments which differ from
xanthorhamnin only in the number or position of the methoxy groups (i.e.,
the OH groups with a CH_{3} replacing the H), or in the nature of the sugar
which is present in the compound. Rhamnetin itself is found in the fruits
of certain species of _Rhamnus_, and is used in dyeing cotton.
The structural arrangement of the characteristic groups of these flavone
pigments will be dealt with more in detail in the chapter dealing with
Pigments (Chapter VIII).
The best-known yellow pigment which is a _xanthone_ derivative is
=euxanthic acid=, known as "Indian yellow," which is a "paired" compound of
glucuronic acid (see page 42) and euxanthone. The latter is a 2, 3',
dioxyxanthone. The pigment is found in the urine of cattle which have been
fed on mango leaves.
The soluble red, blue, and violet pigments are glucosides of various
hydroxy-derivatives of the anthocyan nucleus. Their constitution and
properties will be discussed in detail in the chapter dealing with the
Pigments. These compounds are isomeric with similar flavone and xanthone
derivatives, and the transition from one color to the other in plants takes
place very easily under the action of oxidizing or reducing enzymes. This
accounts for the change of reds and blues to yellows and browns, and vice
versa, under changing temperature conditions.
The following red or blue plant pigments, which are anthocyan
glucosides, have been isolated and studied (for the structural
arrangement of the characteristic groups, see pages 116): from
cornflower and roses, _cyanin_, C_{28}H_{31}O_{16}Cl (2 molecules
glucose + cyanidin); from cranberries, _idain_, C_{21}H_{21}O_{10}Cl
(galactose + cyanidin); from geranium, _pelargonin_, C_{27}H_{30}O_{15}Cl
(2 molecules glucose + pelargonidin); from pæony, _pæonin_,
C_{28}H_{33}O_{16}Cl (2 molecules glucose + pæonidin, a monomethyl
cyanidin); from blue grapes, _[oe]nin_, C_{23}H_{25}O_{12}Cl (glucose +
[oe]nidin); from whortle berry, _myrtillin_, C_{22}H_{23}O_{12}Cl
(glucose + myrtillidin); from larkspur, _delphinin_, C_{41}H_{39}O_{21}Cl
(2 molecules glucose + 2 molecules _p_-oxybenzoic acid + delphinidin);
and from mallow, _malvin_, C_{29}H_{35}O_{17}Cl (2 molecules glucose +
malvidin).
The blue dye, indigo, is derived from a glucoside of an entirely different
type, known as _indican_. Indican is readily extracted from the leaves of
various species of indigo plants. When hydrolyzed, it yields glucose and
_indoxyl_ (colorless). Indoxyl is easily oxidized to _indigotin_ (the deep
blue dye known as "indigo"). The equations illustrating these changes are
as follows:
(_a_) C_{14}H_{17}O_{6}N + H_{2}O = C_{6}H_{12}O_{6} + C_{8}H_{7}ON
Indican Glucose Indoxyl
(_b_) 2C_{8}H_{7}ON + O_{2} = C_{16}H_{10}O_{2}N + 2H_{2}O
Indoxyl Indigotin
The structural relationships of indoxyl and indigotin may be illustrated by
the following formulas:
O O
/\ /\ ║ ║ / \
/ \___COH / \___C C____/ \
| | ║ | | | | | |
| | C-H | | C=C | |
\ /\ / \ /\ / \ / \ /
\/ N \/ N N \/
| | |
H H H
Indoxyl Indigotin
Natural indigo dye is prepared by fermentation of indigo leaves, the decay
of the cell-walls liberating the enzymes in the tissues, which bring about
the chemical changes illustrated in the above equations.
THE CYANOPHORE GLUCOSIDES
Several glucosides which yield hydrocyanic acid as one of the products of
their hydrolysis are of common occurrence in plants. These are generally
spoken of as the "cyanogenetic" glucosides; but as they do not actually
produce cyanogen compounds, but only liberate them when hydrolyzed, the
recently suggested term "cyanophore" undoubtedly more correctly indicates
their properties.
The best known and most widely distributed of these is =amygdalin=.
Amygdalin was first discovered in 1830, and was one of the first substances
to be recognized as a glucoside. It is found in large quantities in bitter
almonds and in the kernels of apricots, peaches, and plums; also in the
seeds of apples, etc., in fact in practically all the seeds of plants of
the Rose family. It is the mother substance for "oil of bitter almonds,"
which is widely used as a flavoring extract.
Amygdalin has been the object of very extensive studies, and even yet the
exact nature of the linkage between its constituent groups is not certainly
known. When completely hydrolyzed, it yields two molecules of glucose and
one each of benzaldehyde and hydrocyanic acid. Recent studies indicate that
the two sugar molecules are separately united to the other constituents,
rather than united with each other in the disaccharide relationship. In
other words, amygdalin is a true _glucoside_ rather than a _maltoside_.
This is indicated by the fact that when submitted to the action of all
known hydrolyzing agents which affect it, it has never been found to yield
maltose as one of the products of hydrolysis. Furthermore, the rate of
hydrolysis of amygdalin is not affected by the presence of maltose; and the
segregation of the two glucose molecules is accomplished by enzymes other
than maltase, which is the only enzyme which is known to break up a maltose
molecule. Since the exact nature of the linkage is not known, it is
customary and convenient to indicate the unit groups as linked together in
the following order:
C_{6}H_{11}O_{5}-O-C_{6}H_{10}O_{4}-O-C_{6}H_{5}·CH-C[trb]N
(1) (2) (3)(4)
A study of the hydrolysis reactions of amygdalin shows that there are three
different linkages in the molecule which may be broken by the simple
interpolation of a single molecule of water and a fourth which may be split
by a different type of hydrolysis, namely, the C[trb]N linkage. These are
indicated by the numbers below the corresponding portion of the formula
above. Most hydrolyzing agents break the molecule first at (1), yielding
one molecule of glucose and one of mandelo nitrile glucoside (see page 77).
The next step usually breaks the latter at the point indicated by (2),
yielding glucose and benzaldehyde cyanhydrin, or mandelo nitrile. The
latter in turn breaks down at (3) into benzaldehyde and HCN. But when
amygdalin is boiled with concentrated hydrochloric acid, the first change
is the splitting off at (4) of the nitrogen in the form of ammonia and the
consequent conversion of the CN group into a COOH group, producing
amygdalinic acid. On further hydrolysis, this breaks up in the same order
as before. Similarly, it is possible to convert mandelo nitrile into
mandelic acid by splitting off the nitrogen to form a COOH group, instead
of splitting off the HCN group leaving benzaldehyde.
The mandelo nitrile glucoside contains an asymmetric carbon atom which is
wholly outside its glucose group, thus C_{6}H_{10}O_{5}-O-C_{6}H_{5}·CH·CN.
Hence, it may exist in dextro, levo, and racemic forms. In the amygdalin
molecule, it exists in the dextro form, which has been named "prunasin."
The levo form, known as "sambunigrin," has been obtained by hydrolysis
of a compound isomeric with amygdalin, whose composition has not been
definitely worked out; while the racemic form, known as "prulaurasin,"
has been prepared from isoamygdalin, by the action of alkalies. Hence,
all the possible compounds indicated by the presence of the asymmetric
carbon have been found and identified.
The crude enzyme preparation which is obtained from almond seeds, known as
"emulsin," contains two enzymes, _amygdalase_, which breaks the amygdalin
molecule at linkage (1), and _prunase_, which breaks it at (2). The action
of amygdalase must always precede that of prunase. In other words, it is
never possible to break off a disaccharide sugar from the molecule, either
by the action of prunase alone, or by means of any other hydrolytic agent.
=Dhurrin=, C_{14}H_{17}O_{7}N, is another glucoside of fairly general
occurrence in plants, which yields HCN as one of the products of its
hydrolysis. It is found in the leaves and stems of several species of
millets and sorghums. Frequent cases of poisoning of cattle from eating of
these plants as forage have been reported. On hydrolysis, dhurrin first
yields glucose and paraoxy-mandelo nitrile; the latter then breaks down
into paraoxy-benzaldehyde and HCN.
=Vicianin=, C_{19}H_{25}O_{10}N, is a cyanophoric glucoside, found in the
seeds of wild vetch, etc. On hydrolysis, it yields glucose, arabinose, and
_d_-mandelo nitrile. It is, therefore, similar to amygdalin, except that
one glucose molecule is replaced by arabinose.
THE MUSTARD OIL GLUCOSIDES
The seeds of several species of plants of the Cruciferæ or mustard family
contain glucosides in which the other characteristic group is a
sulfur-containing compound. These glucosides yield "mustard oils" when they
are hydrolyzed by the enzyme _myrosin_, which accompanies them in the
plant. The following glucosides, found in the seeds of white and black
mustard, are the best-known representatives of this class.
=Sinigrin=, C_{10}H_{16}O_{9}NS_{2}K, found in black mustard seeds, when
hydrolyzed yields glucose, acid potassium sulfate, and allyl
isosulfocyanide (mustard oil), as indicated by the equation.
C_{10}H_{16}O_{9}NS_{2}K+H_{2}O = C_{6}H_{12}O_{6} +
C_{3}H_{5}N[trb]C=S+KHSO_{4}.
The acid potassium sulfate group separates first and most readily, leaving
a compound known as _merosinigrin_, for which the following formula has
been suggested:
-----O------
| |
CH_{2}OH·CHOH·CH·CHOH·CH·CH
| |
O S
| /
|/
C=N[trb]C_{3}H_{5}
This compound usually breaks down into glucose and mustard oil; but by
special treatment it is possible to obtain from it thioglucose,
C_{6}H_{11}O_{5}·SH. This indicates that in the original glucoside the
glucose is linked with the mustard oil through the sulfur atom.
=Sinalbin=, C_{30}H_{42}O_{15}N_{2}S_{2}, from white mustard seeds, when
hydrolyzed by myrosin, yields glucose, sinalbin mustard oil (a
paraoxybenzyl derivative of allyl isosulfocyanide) and sinapin acid
sulfate; according to the equation
C_{30}H_{42}O_{15}N_{2}S_{2}+H_{2}O = C_{6}H_{12}O_{6}+C_{7}H_{7}O·NCS
Sinalbin Glucose Sinalbin mustard oil
+ C_{16}H_{24}O_{5}N·HSO_{4}.
Sinapin acid sulfate
The sinalbin mustard oil may be represented by the formula
____
/ \
HO-CH CH-CH_{2}NCS. Hydrolysis of the sinapin acid sulfate converts
\____/
it into sinapinic acid, C_{6}H_{2}OH·(OCH_{3})_{2}·CH=CH·COOH, choline,
N(CH_{3})_{4}C_{2}H_{4}OH (see page 152), and H_{2}SO_{4}. It is,
therefore, a very complex glucoside.
TEE DIGITALIS GLUCOSIDES
The five, or more, glucosides which are present in the leaves and seeds of
the foxglove (_Digitalis purpurea_) have been extensively studied, as they
are the active principles in the various digitalis extracts which are used
in medicine as a heart stimulant.
=Digitoxin=, C_{34}H_{54}O_{11}, which is the most active of these
glucosides in its physiological effects, when hydrolyzed, yields
digitoxigenin, C_{22}H_{32}O_{4}, and a sugar having the formula
C_{6}H_{12}O_{4}, which is known as "digitoxose" and is supposed to be a
dimethyl tetrose.
=Digitalin=, C_{35}H_{56}O_{14}, is also strongly active. When hydrolyzed,
it yields digitaligenin, C_{22}H_{10}O_{3}, glucose, and digitoxose.
=Digitonin=, C_{54}H_{92}O_{28}, constitutes about one-half of the total
glucosides in the extract which is obtained from most species of the
digitalis plants. It is much less active than the others. It is a saponin
(see page 90) in type. On hydrolysis, it yields 2 molecules of glucose, 2
of galactose, and one of digitogenin.
=Gitonin=, C_{49}H_{80}O_{23}, containing 3 molecules of galactose, one of
a pentose sugar, and one of gitogenin; and =gitalin=, C_{28}H_{48}O_{10},
containing digitoxose and gitaligenin, have also been isolated from
digitalis extracts.
The structural arrangement of the characteristic groups in these glucosides
has not yet been definitely worked out.
=Cymarin=, the active principle of Indian hemp (_Apocynum cannabinum_), is
similar in type to the digitalis glucosides. When hydrolyzed, it yields a
sugar known as "cymarose," C_{7}H_{14}O_{7}, which seems to be a monomethyl
derivative of digitoxose, and cymarigenin, C_{23}H_{30}O_{5}, a compound
which is either identical or isomeric with the organic residue obtained
from other members of this group.
THE SAPONINS
The saponins constitute a group of glucosides which are widely distributed
in plants, whose properties have been known since early Grecian times. They
have been found in over four hundred different species of plants, belonging
to more than forty different orders.
The most characteristic property of saponins is that they form colloidal
solutions in water which produce a soapy foam when agitated, and are
peculiarly toxic, especially to frogs and fishes. In dry form, they have a
very bitter, acrid taste, and their dust is very irritating to the mucous
membranes of the eye, nose, and throat.
On hydrolysis, the saponins yield a variety of sugars,--glucose, galactose,
arabinose, and sometimes fructose, and even other pentoses--and a group of
physiologically active substances, known as "sapogenins."
The more toxic forms of these glucosides are known as "sapotoxins."
The chemical composition of the saponins varies so widely that it is
scarcely possible to cite typical individuals. Sarsaparilla, the dried root
of smilax plants, contains a mixture of non-poisonous saponins, from which
at least four individual glucosides have been isolated and studied. Corn
cockle contains a highly poisonous sapotoxin which, on hydrolysis, yields
four molecules of a sugar and one of sapogenin, C_{10}H_{16}O_{2}. Other
sapotoxins are obtained from the roots of soapwort and from several species
of _Gypsophila_. Digitonin and digito-saponin are glucosides of this type
which are found in the extracts from various species of _Digitalis_.
THE PHYSIOLOGICAL USES OF GLUCOSIDES
It is scarcely conceivable that substances which vary so widely in
composition as do the different types of glucosides can possibly all have
similar physiological uses in plants. The cyanophoric glucosides, the
pigment glucosides, the mustard oil glucosides, and the saponins, for
example, can hardly be assumed to have the same definite relationships to
the metabolism and growth of the plant. To be sure, they are alike in that
they all contain one or more sugar molecules, and it is probable that the
carbohydrates which are held in this form may serve as reserve food
material, especially when the glucoside is stored in the seeds; but it is
obvious that the simpler and more normal form of such stored food is that
of the polysaccharides which contain no other groups than those of the
carbohydrates. It seems much more probable that the physiological uses of
glucosides depend upon their ability to form temporarily inactive "pairs"
with a great variety of different types of organic compounds which are
elaborated by plants for a variety of purposes.
It has been noted that in most, if not all, instances, the glucosides are
accompanied in the same plant tissue (although in separate cells) by the
appropriate enzyme to bring about their hydrolysis and so set free both the
sugar and the other characteristic component whenever the conditions are
such as to permit the enzyme to come in contact with the glucoside. This
occurs whenever the tissue is injured by wound or disease, and also during
the germination process.
Injury to the plant tissue seems to be a necessary preliminary to the
functioning of the active components of the glucoside, except in the case
of the seeds. This leads naturally to the supposition that at least some of
these glucosides are protective or curative agents in the plant tissues.
This conception is further supported by the facts that many of the
non-sugar components of glucosides are bactericidal in character and that
the glucosides commonly occur in parts of the plant organism which are
otherwise best suited to serve as media for the growth of bacteria. Thus,
it is known that in the almond, as soon as the tissue is punctured,
amygdalin is hydrolyzed and all bacterial action is inhibited. Similarly,
the almost universal presence of glucosides containing bactericidal
constituents in the bark of trees insures natural antiseptic conditions
for all wounds of the outer surfaces of the stem of the plant. In fact, it
is easily conceivable that at least one of the reasons for the failure of
the processes of decay of plant tissues to set in until after the death of
the cells, is that during living, respiratory activity these antiseptic
glucosides are so generally present in the tissues.
Further, it has been fairly well established that the "chromogens," or
mother-substances of the pigments, which, under the influence of oxidase
enzymes, serve to regulate the respiratory activities of the plant are
essentially glucosidic in character. This, and other, functions of the
pigments, most of which are glucosides, will be discussed at some length in
the chapter dealing with the Pigments (Chapter VIII).
Many gaseous anæsthetics are known to have a marked effect in stimulating
plant growth. In a number of cases, it has been shown that the contact of
plant tissues with these anæsthetics brings about an interaction of the
enzyme and glucoside which are present in the tissue, with the consequent
hydrolysis of the latter, setting free its characteristic components. This
observation has led to the supposition that many of the organic
constituents of glucosides are definite plant stimulants, to which the name
"hormones" has been applied. There is considerable experimental evidence to
support this conception that glucosides may be the source of stimulating
hormone substances, which will be discussed more in detail in the chapter
dealing with these plant stimulants (Chapter XVII).
Glucosides may also serve as the mechanism for putting out of action of
harmful products which may appear in the tissues as the result of abnormal
conditions. These harmful substances may be rendered soluble by combination
with sugars and so transposed by osmosis to some other part of the plant.
The abnormally large percentages of glucosides which are present in certain
species of plants during unfavorable climatic conditions lends some support
to this view.
Finally, it may be assumed that easily oxidizable substances, such as
aldehydes and acids, are possibly protected against too rapid, or
premature, oxidation by being transformed into glucosides.
In general, it may be said that the glucosides seem to serve as the
regulatory, protective, and sanatory agencies of the plant mechanism.
BIOLOGICAL SIGNIFICANCE OF GLUCOSIDES
The bitter taste of glucosides and their almost universal presence in the
bark of plants undoubtedly helps to prevent the destructive gnawing of the
bark by animals.
Glucosides having either a strong bitter taste, or pronouncedly poisonous
properties, likewise undoubtedly serve to protect such important organs of
plants as the seeds and fruits from being prematurely eaten by birds and
animals. The common disappearance of these bitter substances as the seed or
fruit ripens adds to the attractiveness of the material for food for
animals at the proper stage of ripeness to provide for wider distribution
of the seeds for further propagation. Further, the very general occurrence
of these protective glucosides in many of the vegetative parts of plants
during the early stages of growth, followed by their disappearance after
the seeds of the plant have been formed, certainly serves to protect these
plants from consumption as forage by animals before they have been able to
develop their reproductive bodies. The lack of palatability, and even the
production of digestive disorders resulting from the eating of unripe fruit
may be due, in part at least, to the presence of protective glucosides in
unripe fruits and vegetables.
On the other hand, the almost universal presence of the brilliant pigment
glucosides in the external parts of flowers undoubtedly serves to attract
the insects which are biologically adapted to provide for the
transportation of pollen from one blossom to another and so to insure the
cross-fertilization which is so important in maintaining the vigor of many
species of plants.
It is apparent that this important group of compounds, with its exceedingly
varied and complex constituent groups, may play a variety of significant
rôles in plant growth.
REFERENCES.
ARMSTRONG, E. F.--"The Simple Carbohydrates and Glucosides," 239 pages,
_Monographs_ on Biochemistry, London, 1919 (3d ed.).
VAN RIJN, J. J. L.--"Die Glykoside," 511 pages, Berlin, 1900.
CHAPTER VII
TANNINS
Using the term in its general application to a group of substances having
similar chemical and physical properties, rather than in its limited
application to a single definite chemical compound known commercially as
"tannin," the _tannins_ are a special group of plant substances, mostly
glucosides, which have the following characteristic properties. First, they
are non-crystalline[4] substances, which form colloidal solutions with
water, which have an acid reaction and a sharp astringent taste. Second,
they form insoluble compounds with gelatine-containing tissues, as shown by
the conversion of hide into leather. Third, they form soluble, dark-blue or
greenish-black compounds with ferric salts, the common inks. Fourth, they
are precipitated from their solutions by many metallic salts, such as lead
acetate, stannous chloride, potassium bichromate, etc. Fifth, they
precipitate out of solution albumins, alkaloids, and basic organic coloring
matters. Finally, most tannins, in alkaline solutions, absorb oxygen from
the air and become dark brown or black in color.
FOOTNOTES:
[4] The needle-like forms, in which commercial "tannin" comes on the
market, are not true crystals, but are broken fragments of the threads into
which the colloidal tannin is "spun-out" from the syrupy extracts of
nutgalls, etc.
OCCURRENCE
Tannins occur widely distributed in plants. Practically every group of
plants, from the fungi up to the flowering plants, contains many species of
plants which show tannin in some of their tissues. Among the higher plants,
tannins occur in a great variety of organs. Thus, they are found in the
roots of several species of tropical plants; in the sterns, both bark and
wood, of oaks, pines, hemlock, etc.; in the leaves of sumac, rhododendron,
etc.; in many fruits, especially in the green, or immature, stages; and in
the seeds of several species, either before or after germination. Tannins
are also found in certain special structures, such as gland cells, cells of
the pulvini, laticiferous tissues, etc. Further, they are especially
abundant in the pathological growths known as galls, which often contain
from 40 to 75 per cent of tannin and constitute the most important
commercial source for these materials.
The principal commercial sources of tannin, which is used in the
manufacture of inks, in the tanning of leather, in certain dyeing
operations, etc., are oak-galls, the bark and wood of oak, hemlock, acacia,
and eucalyptus, the bark of the mangrove, the roots of canaigre, and the
leaves of several species of sumac.
CHEMICAL CONSTITUTION
Tannins are either free phenol-acids or, more commonly, glucosides of these
acids. Common "tannin," when hydrolyzed, yields from 7 to 8 per cent of
glucose, which indicates that it is a penta-acid ester of glucose, i.e.,
each glucose molecule has five acid groups attached to it. The formula for
such a tannin is, therefore, as follows,
------O-------
| |
CH_{2}OR·CHOR·CH·CHOR·CHOR·CHOR
in which the R represents a complex phenol-acid like tannic acid, or
digallic acid. These acids are derivatives of the common phenols, whose
constitution will be brought to mind by the following series of formulas:
Ordinary phenol Pyrocatechol Resorcinol Hydroquinone
/\ /\ /\ /\
/ \ / \ / \ / \
| |OH | |OH | |OH | |OH
| | | |OH | | HO| |
\ / \ / \ / \ /
\/ \/ \/ \/
OH
Pyrogallol Oxyhydroquinone Phloroglucinol
OH OH OH
/\ /\ /\
/ \ / \ / \
| |OH | |OH | |
| |OH | | HO| |OH
\ / \ / \ /
\/ \/ \/
OH
These phenols themselves do not occur as constituents of tannins, although
they are often found in other glucosides, gums, etc. The following
mono-carboxyl acid derivatives of these phenols are, however, found both
free and in glucoside formation as constituents of many of the common
tannins.
_Pyrocatechuic acid_, derived from pyrocatechol, represented by the
formula,
OH
/\
/ \
| |OH
| |
\ /
\/
COOH
_Gallic acid_, derived from pyrogallol, and represented by the formula,
OH
/\
/ \
HO| |
HO| |COOH
\ /
\/
COOH
In most of the common tannins, however, the characteristic acids are
oxy-derivatives of the so-called "tannon" group, represented by the
formula, C_{6}H_{5}·CO·O·C_{6}H_{5}. For example, _digallic acid_, which is
a constituent of many common tannins, is a tetra-oxy, mono-carboxyl
derivative of this group, having the structural formula,
/\ /\
/ \ / \
HO| |--CO·O--| |COOH
HO| | HO| |
\ / \ /
\/ \/
OH OH
_Ellagic acid_, which is an hydrolysis product of many of the pyrogallol
tannins (see below) and which produces the characteristic "bloom" on
leather tanned by this type of tannins, has the following formula,
-----CO·O-----
/\ /\
/ \__________/ \
HO| | | |OH
HO| |___CO·O___| |OH
\ / \ /
\/ \/
CLASSES OF TANNINS
The tannins are divided into two general classes, known respectively as the
_pyrogallol tannins_ and the _catechol tannins_. These differ in their
characteristic reactions as follows:
Pyrogallol variety Catechol variety
Ferric salts Dark blue Greenish black
Bromine water No precipitate Yellow or brown precipitate
Leather Produce a "bloom" No "bloom"
Conc. sulfuric acid Yellow or brown Red or pink
Lime water Gray or blue ppte. Pink to brown ppte.
Pyrogallol tannins contain approximately 52 per cent of carbon; while the
catechol tannins usually contain 59 per cent to 60 per cent, the difference
being due to the absence of glucose from the molecule in the latter types.
The two types are distributed in plants as follows: pyrogallol tannins in
oak-galls, oak wood, sumac, chestnut, divi-divi, and algaro billa; catechol
tannins in the barks of pines, hemlocks, oaks, acacias, mimosas, cassia,
and mangrove, in quebracho wood, canaigre roots, cutch and gambier. The
so-called "pseudo-tannins" (i.e., compounds which do not tan leather but
possess other properties like tannins) are found in hops, tea, wine,
fruits, etc.
SOME COMMON TANNINS
Ordinary commercial "_tannin_," or "_tannic acid_," is a compound of one
molecule of glucose with five of digallic acid. It is found in many plants,
and is prepared commercially from the Turkish oak-galls and the Chinese
sumac-galls. It exhibits all the characteristic properties which have been
listed above for tannins in general and responds to all the characteristic
reactions of a pyrogallol tannin. It is extensively used for the
manufacture of blue-black ink, and in many technical processes.
=Catechu tannin= and =catechin= are compounds of the catechol tannin type.
The latter is obtained from acacia wood, mahogany wood, mimosa wood, etc.
It is not a true tannin, since it does not convert hide into leather; but
when heated to 120° or above, it is easily dehydrated, forming catechu
tannin which is identical with that which is obtained directly from gambier
and Bombay cutch (products made by evaporating water extracts from the bark
of various tropical trees). This latter is a true tannin, which is much
used in dyeing and other technical processes.
"=Quercitannic acid=," obtained from oak bark, etc., is likewise a catechol
tannin. It yields no glucose on hydrolysis.
A great many other tannins are known, and their possibilities for technical
use in tanning, dyeing, etc., have generally been investigated; but so
little has been learned about their composition and relation to the plant's
own needs, that it seems unnecessary to discuss them in detail here.
PHYSIOLOGICAL USES OF TANNINS
Tannins are probably not direct products of photosynthesis. They are,
however, elaborated in the green leaves of plants and translocated from
there to the stems, roots, etc. Their close association with the
photosynthetic carbohydrates has led many investigators to seek to
establish for them some significant function as food materials, or as
plastic substances in cell metabolism. Many conflicting views have been
advanced, but a careful review of these leads inevitably to the conclusion
that tannins probably do not serve in any significant way as food material.
The glucose which is generally present in the tannin molecule may, of
course, serve as reserve food material, but it seems probable that it
functions as a constituent of the tannins only to assist in making them
more soluble and hence more easily translocated through the plant tissues.
Some fungi, and perhaps other plants as well, can actually utilize tannins
as food material under suitable conditions and in the absence of a proper
supply of carbohydrates. But this does not prove that tannins can normally
replace carbohydrates as food material for these species of plants.
There seems to be ample evidence that tannins are elaborated where intense
metabolism is in progress, such as occurs in green leaves during the early
growing season; in the rapid tissue formation which takes place after the
stings of certain insects, producing galls, etc.; during germination, and
as a result of any other unusual stimulation of metabolism. It may be,
therefore, that tannins serve as safety accumulations of excessive
condensations of formaldehyde, or other photosynthetic products, under such
conditions. It seems certain that in all such cases tannins are the result
of, and not (as some investigators have supposed) the causative agents
for, the abnormally rapid metabolism.
It seems to be fairly well demonstrated that tannins are intermediate
products for the formation of cork tissue. This may account for their
common occurrence in the wood and bark of trees. Indeed, it has been shown
that gallic and tannic acids are present in considerable proportions in
those parts of the plant where cork is being formed. Further, that they
bear direct relation to cork-formation has been demonstrated in two
different ways. First, cork-like substances have been artificially produced
by passing a stream of carbon dioxide through mixtures of formaldehyde with
various tannic acids. Second, by various treatments of cork, decomposition
compounds showing tannin-like properties may be obtained.
Some investigators have held that not only cork tissue but also other
lignose, or cell-wall material, may be developed from tannins. Certain
observations with _Spirogyra_ seem to indicate that tannin may play an
important part in the formation of new cell walls during conjugation, as
cells which are ready to conjugate are rich in tannin, which gradually
diminishes in quantity until it is practically absent at the time of
spore-formation. There seems to be no evidence that tannins perform any
such function as this in higher plants, however.
Again, tannins may play a very important part in pigment-formation. They
are very similar in structure to the anthocyanin pigments, both being made
up of practically identical decomposition units, the phenolic bodies. The
disappearance of tannins during the process of ripening of fruits may be
connected, in part at least, with the development of the brilliant red,
blue, and yellow pigments which give such rich colors to the thoroughly
ripe fruits.
Finally, certain of the tannins undoubtedly serve as protective agents to
prevent the growth of parasitic fungi in fruits, etc. Recent investigations
show that at least some of the varieties of fruits which are resistant to
the attacks of certain parasitic diseases utilize tannins for this purpose.
This protective effect may be accomplished in two different ways. Either
the tannin actually serves as an antiseptic to prevent the growth of the
parasitic fungus within the tissues of the host plant, or it assists in the
development of a corky layer which "walls-off" the infected area and so
prevents further spread of the disease through the tissue. Examples of
both types of protective action have recently been reported.
It is obvious that the different forms of tannins may play different rôles
in plant life, and the same tannin substance may possibly serve different
purposes under different conditions.
BIOLOGICAL SIGNIFICANCE OF TANNINS IN FRUITS
The presence of tannins in fruits and the changes which they undergo during
the ripening process cannot fail to attract attention to their biological
significance in serving to protect the fruit from premature consumption as
food by animals.
Tannins are of frequent occurrence in green fruits, imparting to them their
characteristic astringent taste. They nearly always disappear as the fruit
ripens. The fact that during the ripening process both sugars and fruit
esters, as well as attractive surface pigments, are developed has led
certain investigators to the conclusion that tannins serve as
mother-substances for these materials in the green fruits and are converted
into these attractive agencies during ripening. There is nothing in the
chemical composition of tannins which indicates, however, that they are
precursors of sugars or fruit esters, although (as has been pointed out)
they may give rise to anthocyan pigments.
Further, recent researches concerning the tannin of persimmons (the
best-known and most striking example of the phenomena under discussion)
clearly show that the tannin is not actually used up during the ripening
process; that instead it remains in the ripe fruit in practically
undiminished quantity; but that when the fruit is ripe, the tannin is
enclosed in certain special large cells or sacs, which are surrounded by an
insoluble membrane, so that when the fruit is eaten by animals the
astringent tannin, enveloped in these insoluble sacs, passes by the organs
of taste of the animal without causing any disagreeable effects. This
walling-off of the astringent tannins can be stimulated in partially ripe
fruits by treating them with several different chemical agents, the
simplest method being that of placing the unripe fruit in an atmosphere of
carbon dioxide gas for a short period. The artificial "processing" of
persimmons to render them edible for a longer period before they become
naturally fully ripe and subject to decay is now a commercial enterprise.
This process is of interest because of its possible connection with the
conversion of tannins into cork, under the influence of carbon dioxide gas,
as mentioned in a preceding paragraph.
From these facts, it is apparent that in persimmons, and probably in other
tannin-containing fruits, the process of natural selection has developed a
mechanism for the secretion of tannin in green fruits, followed by a
process for walling it off in harmless condition when the fruit is ripe,
which serves most admirably to protect the fruit from consumption by
animals before the enclosed seeds have fully developed their reproductive
powers.
REFERENCES.
ABDERHALDEN, E.--"Biochemisches Handlexikon, Band 7, Gerbstoffe,
Flechtenstoffe, Saponine, Bitterstoffe, Terpene, Aetherische Oele, Harze,
Kautschuk," 822 pages, Berlin, 1912.
ALLEN'S Commercial Organic Analysis, Vol. 5, "Tannins, Dyes and Coloring
Matters, Inks," 704 pages, 6 figs., Philadelphia, 1911 (4th ed.).
COOK, M. T. and TAUBENHAUS, J. J.--"The Toxicity of Tannin," Delaware
College Agricultural Experiment Station _Bulletin_ No. 91, 77 pages, 43
figs., Newark, Del., 1911.
DEKKER, J.--"Die Gerbstoffe," 636 pages, 3 figs., Berlin, 1913.
GORE, H. C.--"Experiments on the Processing of Persimmons to Render them
Nonastringent," U. S. Department of Agriculture, Bureau of Chemistry
_Bulletin_ No. 141, 31 pages, 3 plates, 1911; and No. 155, 20 pages, 1912.
LLOYD, F. E.--"The Tannin-Colloid Complexes in the Fruit of the Persimmon,
_Diospyros_," in _Biochemical Bulletin_, Vol. 1, No. 1, pages 7 to 41, 34
figs., New York, 1911.
CHAPTER VIII
PIGMENTS
Practically all plant structures contain pigments. These may be considered
as of two types: (_a_) the vegetative pigments, which have a definite
energy-absorbing rôle in the metabolic processes of the tissues which
contain them, and (_b_) the ornamental pigments. It is probable that the
same chemical compound may serve in either one of these capacities under
different conditions, but, in general, it is possible to assign either a
definite vegetative, or physiological, use, or else a simple ornamental, or
biological, significance to each of the common pigments. The first type is
found widely distributed through the protoplasm, or cell-sap, of the plant
structures; while the ornamental pigments are located chiefly in the
epidermal cells, especially of flowers.
With respect to their colors, the plant pigments may be grouped as follows:
Green--the chlorophylls.
Yellow--the carotinoids, flavones, and xanthones.
Red--phycoerythrin, lycopersicin, anthocyanin.
Blue--anthocyan derivatives.
Brown--phycophæin, fucoxanthin.
Of these, the chlorophylls, the carotinoids, phycoerythrin (in red
sea-weeds) and phycophæin (in brown sea-weeds) are generally vegetative
pigments; while the others form the basis for most of the ornamental
pigments, although they may have a definite energy-absorbing effect, in
some cases.
THE CHLOROPHYLLS
The importance of the green coloring matter in plants has been understood
for more than a century, its connection with photosynthesis having been
known as far back as 1819. But definite knowledge as to its chemical
constitution is of very recent origin. As recently as 1908, it was asserted
that chlorophyll is a lecithin-like body, yielding choline and
glycero-phosphoric acid on hydrolysis. It is now known, however, that
chlorophyll contains neither choline nor phosphorus, the earlier
observations being due to mixtures of various other materials with the true
chlorophyll in the extracts which were examined. Beginning with 1912,
Willstätter and his collaborators, in a series of classic papers which were
finally collected in book form, clearly demonstrated the chemical
constitution of the green pigments of plants, which had been previously
designated under the single name "chlorophyll." In 1912, Willstätter and
Isler first showed that the green coloring matter which is extracted from
plants by alcohol, ether, etc., is made up of two definite chemical
compounds, to which they assigned the names "chlorophyll _a_" and
"chlorophyll _b_," associated with two yellow pigments, carotin and
xanthophyll, and, in some cases, with the reddish-brown fucoxanthin. The
percentages of total pigment materials, and the relative proportions of the
five different pigments, in several types of plants, are as follows:
-------------------------------------------------------------------------
| Land | Brown | Green
| Plants, | Seaweeds, | Algæ,
| Per Cent. | Per Cent. | Per Cent.
-------------------------------------------------------------------------
Total pigment in the dry matter | 0.99 | 0.29 | 0.21
Proportion of: | | |
Chlorophyll _a_ | 63 | 55 | 44
Chlorophyll _b_ | 22 | 4 | 31
Carotin | 6 | 11 | 7
Xanthophyll | 9 | 10 | 18
Fucoxanthin | | 20 |
The two chlorophylls have the following formulas: chlorophyll _a_,
C_{55}H_{72}O_{5}N_{4}Mg, and chlorophyll _b_, C_{55}H_{70}O_{6}N_{4}Mg.
Hence, they differ only in having two hydrogen atoms in the one replaced by
one oxygen atom in the other. Both are amorphous powders, from which
crystalline chlorophyll (see below) can be obtained by hydrolysis.
Chlorophyll _a_ is blue-black, is easily soluble in most organic solvents,
and when saponified by alcoholic potash gives a transient pure yellow
color. Chlorophyll _b_ is dark green, is somewhat less soluble than the
other form, and when saponified by potash gives a transient brilliant red.
=Amorphous and Crystalline Chlorophyll.=--When the chlorophyll of plants is
extracted by alcohol and the alcoholic extract evaporated nearly to
dryness, beautiful dark green crystals are obtained. Willstätter has shown,
however, that in these crystallized forms the ethyl group (from the ethyl
alcohol used) has replaced the phytyl group (see below) which is present in
the pigments as they exist in the plant tissues; and that, when extracted
by other solvents than alcohol, the pigments may be obtained in the
amorphous forms in which they exist in the plant.
This change from amorphous to crystalline compounds may be understood from
the preliminary statement that the chlorophylls are esters of tri-basic
acids, in which one acid hydrogen is replaced by the methyl (CH_{3}) group
and a second by the phytyl (C_{20}H_{39}, from phytol, or phytyl alcohol,
C_{20}H_{39}OH) group. When treated with ethyl alcohol (C_{2}H_{5}OH) for
the purpose of extracting the pigments, the ethyl (C_{2}H_{5}) group
replaces the phytyl group, thus yielding a methyl-ethyl ester, and these
esters are the crystalline forms of the chlorophylls. This replacement is
made possible through the action on the original pigment in the tissues of
an enzyme, _chlorophyllase_, which is also present in the tissues, which
splits off the phytyl group, forming phytyl alcohol, and leaving a free
COOH group in the pigment, with which the alcohol used in the extraction
forms the ethyl ester (see Chapter IX for a discussion of the formation and
hydrolysis of esters).
While the chlorophylls are tri-basic acids, only two of the acid COOH
groups actually function in ester-formation. The third acid group seems not
to exist as a free acid group; but in chlorophyll _a_, it is in what is
known as the "lactam" arrangement, represented by the --CONH-- group, and
in chlorophyll _b_, it is probably in the "lactone" arrangement,
represented by the --COO-- group; the two bonds in each case being attached
to different structural units in the molecule (see page 106).
The change from amorphous to crystalline forms may be represented by the
following formulas, in which the R represents the whole of the complex
group to which the acid ester groups are united:
COO·CH_{3} COO·CH_{3} COOH
/ / /
R R R
\ \ \
COO·C_{20}H_{39} COO·C_{2}H_{5} COOH
Amorphous chlorophyll Crystalline chlorophyll Chlorophyllin
or or
methyl-phytyl methyl-ethyl
chlorophyllide chlorophyllide
"Chlorophyllin," the compound in which the ester groups have been converted
into free acid groups, as indicated above, may be obtained from either
amorphous or crystalline chlorophyll by treatment with caustic potash
dissolved in methyl alcohol.
=Phytol.=--This alcohol, which furnishes the characteristic ester group in
the chlorophyll of plants, is a compound of very unusual composition, which
has never been found in any other form or in any other type of compound
which is present in either plant or animal tissues. Careful studies of its
addition and oxidation products prove that it has the following structural
arrangement:
H H H H H H H H H H
| | | | | | | | | |
H-C-----C-----C-----C-----C-----C-----C-----C-----C=====C-----C--CH_{2}OH
| | | | | | | | | | |
CH_{3}CH_{3}CH_{3}CH_{3}CH_{3}CH_{3}CH_{3}CH_{3}CH_{3}CH_{3}CH_{3}
As this formula indicates, the compound contains one unsaturated,
double-bond linkage, one primary alcohol group, and eleven methyl groups.
As has been said, this alcohol occurs nowhere else in nature, and its
presence and function in the chlorophyll molecule are, as yet, wholly
unexplainable. Phytol itself is a colorless, oily liquid, with a high
boiling point (145° in vacuo, 204° at 10 mm. pressure).
THE CONSTITUTION OF THE CHLOROPHYLLS
As has been mentioned, chlorophyll _a_ differs from chlorophyll _b_ by
having one more oxygen and two less hydrogen atoms in the molecule, and in
having one of its nitrogen atoms in the "lactam" arrangement. These
differences in structure are represented by the following formulas which
are commonly used to represent the two compounds, but which do not show the
arrangements of the major groups of the complex molecules:
COO·C_{20}H_{39} COO·C_{20}H_{39}
/ /
MgC_{31}H_{29}N_{3}-COO·CH_{3} MgC_{32}H_{28}O_{2}N_{4}-COO·CH_{3}
|\ Chlorophyll _b_
NH-CO
Chlorophyll _a_
The chlorophylls are unstable compounds, readily acted upon by acids or
alkalies, and by the enzyme chlorophyllase, which splits off the phytyl
alcohol group. The progressive action of acids and of alkalies in breaking
down the molecule, and the products of its oxidation and reduction, have
served to establish the chemical composition of the compound in each case.
Because of the importance of these pigments in the whole metabolic
processes of the plant, it seems to be desirable to consider the nature of
these reactions in some detail, as follows:
=Decomposition of the Chlorophylls by Alkalies.=--The first action of
dilute alkalies on the chlorophylls is to split off, by hydrolysis, the
alcoholic groups of the esters, producing the crystalline tri-basic acids,
or _chlorophyllins a_ and _b_. Each of these chlorophyllins exists in two
forms, the normal and the iso, in which the attachment of the COOH groups
to the other groups in the molecule is in different positions. Hence,
chlorophyll _a_ yields chlorophyllin _a_ and isochlorophyllin _a_, and
chlorophyll _b_ yields chlorophyllin _b_ and isochlorophyllin _b_, all four
of which are tri-basic acids.
These compounds, when heated with alkalies, split off carbon dioxide in
successive stages, losing one COOH group at each step, thus yielding a
series of simpler compounds of the following types: First, di-basic acids;
second, monobasic acids; and finally, _ætiophyllin_, a compound in which no
COOH group is present. In all of these compounds, derived from chlorophylls
by the action of alkalies, the Mg remains in the molecule, and all the
Mg-containing derivatives from the chlorophylls are known as "phyllins." At
the stage at which only one COOH group remains in the molecule, only one
group arrangement is possible, and the derivatives from chlorophyllin _a_
and isochlorophyllin _b_, and those from chlorophyllin _b_ and
isochlorophyllin _a_, are identical. At the final stage, the derivatives
from all four forms are identical. This may be graphically illustrated by
the following diagram indicating the progressive decomposition of the two
chlorophylls under the action of alkalies:
[Illustration: Decomposition of Chlorophyll a and Chlorophyll b]
=Decomposition of Chlorophylls by Acids.=--The first action of dilute acids
upon chlorophylls is to remove the magnesium, without otherwise changing
the molecule. Two hydrogens go in in the place of the magnesium. Dilute
acids act in precisely the same way upon each of the "phyllins" shown in
the above scheme. In this way, a whole series of compounds, corresponding
to each of the chlorophylls and their alkali-decomposition products, but
with the magnesium lacking in each case, has been prepared. Thus,
COO·C_{20}H_{39}
/
Chlorophyll _a_, MgC_{31}H_{29}N_{3}-COO·CH_{3},
\\
(NHCO)
COO·C_{20}H_{39}
/
becomes Phæophytin _a_, C_{31}H_{31}N_{3}-COO·CH_{3},
\\
(NHCO)
COO·C_{20}H_{39}
/
Chlorophyll _b_, MgC_{31}H_{29}O_{2}N_{4}
\
COO·CH_{3}
COO·C_{20}H_{39}
/
becomes Phæophytin _b_, C_{32}H_{30}O_{2}N_{4}
\
COO·CH_{3}
Similarly,
Isochlorophyllin _a_, becomes Phytochlorin _e_,
Chlorophyllin _a_, becomes Phytochlorin _f_, and _g_,
COOH
/
C_{32}H_{32}ON_{4}
\
COOH
Isochlorophyllin _b_, becomes Phytorhodin _g_
Chlorophyllin _b_, becomes Phytorhodin _i_ and _k_,
COOH
/
C_{32}H_{30}O_{2}N_{4}
\
COOH
And bodies known as "porphyrins" are similarly derived from all the other
known phyllins.
For example: cyanophyllin, MgC_{31}H_{32}N_{4}(COOH)_{2}, becomes
cyanoporphyrin, C_{31}H_{34}N_{4}(COOH)_{2}; ætiophyllin,
MgC_{31}H_{34}N_{4}, becomes ætioporphyrin, C_{31}H_{36}N_{4}, etc.
Phytochlorin _e_ and phytorhodin _g_ are the chief products of the
decomposition by acids of the chlorophylls. Indeed, it was the production
of these compounds which led to the discovery of the existence of the two
chlorophylls. When treated with alkalies, they lose their carboxyl groups
and become ætioporphyrin.
=Decomposition of the Chlorophylls by Oxidation and Reduction.=--When acted
upon by oxidizing agents, such as chromic acid, the porphyrins yield two
chief oxidation products, which are pyrrole derivatives having the
following formulas,
CH_{3}-C-CO CH_{3}-C-CO
║ \ ║ \
║ NH ║ NH
║ / ║ /
CH_{3}-CH_{2}-C-CO HOOC-CH_{2}-CH_{2}-C-CO
Methylethylmalein imide Hæmatinic acid imide
By reduction, there have been obtained from the chlorophylls and the
various porphyrins, three isomeric pyrrole derivatives having the following
formulas,
CH_{3} H CH_{3}
| | |
C_{2}H_{5}-C=C C_{2}H_{5}-C=C C_{2}H_{5}-C=C
| \ | \ | \
| NH | NH | NH
| / | / | /
CH_{3}-C=C CH_{3}-C=C CH_{3}-C=C
| | |
CH_{3} CH_{3} H
Phyllopyrrole Hæmopyrrole Isohæmopyrrole
As a result of the study of these decomposition units, Willstätter has
suggested the following formulas for the structural arrangement of
ætiophyllin and ætioporphyrin, the compounds which result from the removal
of all of the acid groups and finally of the magnesium from the
chlorophylls,
H HC===CH
| | |
CH_{3}-C--C C---C
║ \\ // ║
║ N N ║
║ / · · \ ║
C_{2}H_{5}-C--C · · C---CH
\ · · //
C------------C
/ · · \
C_{2}H_{5}-C==C · · C==C-C_{2}H_{5}
| \ ·· / |
| N-----Mg-----N |
| / \ |
CH_{3}-C==C C==C-C_{2}H_{5}
| |
CH_{3} CH_{3}
Ætiophyllin
H HC===CH
| | |
CH_{3}-C--C C---C
║ \\ // ║
║ N N ║
║ / \ ║
C_{2}H_{5}-C--C C---CH
\ //
C------------C
/ \
C_{2}H_{5}-C==C C==C-C_{2}H_{5}
| \ / |
| N N |
| / \ |
CH_{3}-C==C C==C-CH_{3}
| |
CH_{3} CH_{3}
Ætioporphyrin
The COOH groups which are attached to these compounds to form the various
phyllins and porphyrins, as well as the original chlorophylls, are supposed
to be attached to the C_{2}H_{5} groups in the above formulas, the
different modifications, or compounds, depending upon the position in which
one or more of these attachments are made.
SIMILARITY OF CHLOROPHYLL AND HÆMOGLOBIN
It seems to be desirable, at this point, to call attention to the
remarkable similarity in the chemical composition of chlorophyll, the most
important pigment of plants, and hæmoglobin, the all-important
respiration-regulating pigment in the blood of animals. Hæmoglobin is a
complex compound, consisting of about 96 per cent of albumin (a protein,
see Chapter XIII) united with about 4 per cent of _hæmatin_, a brilliant
red pigment which has the formula FeClC_{32}H_{32}O_{4}N_{4}. When treated
with acids, the iron (and its accompanying Cl) is removed, and
hæmatoporphyrin, C_{32}H_{36}O_{4}N_{4}, is obtained. When either hæmatin,
or hæmatoporphyrin is oxidized, hæmatinic acid imide identical with that
obtained from ætioporphyrin is obtained. Also, when hæmatoporphyrin is
reduced, hæmopyrrole identical with that from ætioporphyrin is obtained.
Thus, it would appear that the unit structural groups in hæmatin and in
chlorophyll are identical; although chlorophyll may exhibit more variations
in isomeric arrangement of these structural units than have been found in
hæmatin. Hence, it is apparent that the only essential difference in
composition between chlorophyll and hæmatin is that in the former the
structural units are linked together by iron, while in the latter, the same
units are united through magnesium as the linking element. Further, it is
known that while iron is not a constituent element in the chlorophyll
molecule, it is, in some unknown way, absolutely essential to the
production of chlorophyll in plants; plants furnished with an iron-free
nutrient solution rapidly become etiolated and photosynthesis stops.
The following skeleton formulas have been suggested to indicate the way in
which these elements are linked between the structural units in their
respective compounds.
-C C- -C C-
\ / \ /
N N N N
/ \ / \ / \ / \
-C \ / C- -C \ / C-
M g F e
-C / \ C- -C / | \ C-
\ / \ / \ / | \ /
N N N C l N
/ \ / \
-C C- -C C-
Chlorophyll Hæmatin
It is understood, of course, that the mineral element does not furnish the
definite means of holding the structural units together as otherwise it
would not be possible to remove the iron, or magnesium, without breaking
down the molecule, as is done in the case of the porphyrins. The actual
binding linkage is undoubtedly between carbon atoms, as indicated in
Willstätter's formulas for ætiophyllin and ætioporphyrin (see page 109).
The attachment of the magnesium to each one of the four nitrogen atoms in
the skeleton formula assumes the existence of subsidiary valences of 2-4
for magnesium (and of 3-5 for iron), or of possible _oscillating_ valences
similar to those supposed to be exhibited by carbon in its closed-ring
arrangements.
PROPERTIES OF THE CHLOROPHYLLS
The phytyl esters, or natural chlorophylls, are amorphous solids; while the
methylethyl esters (chlorophyllins) and the free acids (phyllins) are
crystalline compounds. All of these compounds are easily soluble in ether
and alcohol, but insoluble in water. The chlorophylls and chlorophyllins
are practically insoluble in petroleum ether and chloroform; but the
monobasic acids (pyrrophyllin and phyllophyllin) and the neutral
ætiophyllin dissolve easily in chloroform.
Solutions of the chlorophylls are fluorescent, being green by transmitted,
and red by reflected light.
Chlorophyll _a_ is a blue-black solid, which gives dark green solutions in
all of its solvents. Chlorophyll _b_ is a dark-green solid, which yields
brilliant green solutions. Solutions in ether of glaucophyllin and of
cyanophyllin are blue; of rhodophyllin, deep violet; of rubiphyllin, light
violet; of erythrophyllin, red; and of pyrrophyllin and phyllophyllin,
bluish-red. Solutions of the porphyrins are all red, the di-basic ones
being usually a bluish-red, and the simpler ones a brilliant red to deep
brownish-red in color.
The several chlorophyll derivatives are further distinguished by
characteristic differences in their absorption spectra. These differences
have been pictured by Willstätter in his book dealing with the results of
his investigations concerning the chlorophylls, and reproduced in one or
two other texts which treat in detail with the physical-chemical properties
of these pigments, but need not be presented in such detail here.
THE CAROTINOIDS
The characteristic brilliant green of healthy plant tissues is due to the
fact that there are always associated with the dark bluish-green
chlorophylls two (or more) yellow pigments. These are known as the
"carotinoids." This group includes the two brilliant yellow pigments,
carotin and xanthophyll, and the reddish brown fucoxanthin and the
brilliant red lycopersicin, which are similar in their chemical
composition. The first two are found universally distributed in plants,
associated with the chlorophylls, and may be regarded as vegetative
pigments, although the characteristic ornamental yellow and orange colors
of many flowers and fruits, as well as that of the roots of carrots, etc.,
due to these pigments.
=Carotin.=--This pigment occurs in various forms in plants, both amorphous
and crystalline. It crystallizes out of solution in flat plates, which are
orange-red by transmitted light, and greenish-blue by reflected light, and
have a melting point of 168°. Carotin is insoluble in water, only very
slightly soluble in acetone or cold alcohol, readily soluble in petroleum
ether, ether, chloroform, and carbon disulfide. Its solutions are strongly
fluorescent.
Its molecular formula is C_{40}H_{56}. It is, therefore, a hydrocarbon of a
very high degree of unsaturation. On exposure to dry air, it absorbs 34.3
per cent of its own weight of oxygen, which corresponds to 11½ atoms of
oxygen, computed on the basis of the molecular formula C_{40}H_{56}, and
would indicate a formula of (C_{40}H_{56})_{2}O_{23} for the oxygenated
compound; this being three oxygen atoms less than would be required to
bring the compound to the theoretical stage of saturation represented by
the unimolecular formula C_{_n_}H_{_2n+2_}. In moist air, two more oxygen
atoms are absorbed, probably forming two OH groups in the molecule.
Moreover, carotin absorbs iodine. When the calculated amount of iodine is
used, a definite compound having the formula C_{40}H_{56}I_{2} is produced;
but in the presence of an excess of iodine another compound having the
apparent formula C_{40}H_{56}I_{3} (or 2C_{40}H_{56}I_{2}+I_{2}) is
obtained. (Note that 2 atoms of iodine plus 12 atoms of oxygen, or 3 of
iodine plus 11 of oxygen, produce the degree of saturation required by
the formula C_{_n_}H_{_2n+2_}.) It is evident from these experimental data,
that a part of the unsaturated linkage in the carotin molecule is of a type
which can easily be saturated by direct addition of oxygen, while the
remainder may be saturated by iodine.
The reaction of carotin toward bromine is peculiar. With this element, it
forms a compound having the formula C_{40}H_{36}Br_{22}, indicating the
direct addition of two atoms of bromine and the substitution of twenty
atoms of this element for the same number of hydrogen atoms.
The oxygenated carotins are colorless substances, while the iodide
crystallizes in beautiful dark-violet prisms, having a coppery red
fluorescence.
=Xanthophyll= is closely related to carotin. It has the molecular formula
C_{40}H_{56}O_{2}. It absorbs 36.55 per cent of oxygen (corresponding to 13
atoms, which would indicate the formation of two OH groups in addition to
the saturation required by the C_{_n_}H_{_2n+2_} formula); and an iodine
addition product having the formula C_{40}H_{56}O_{2}I_{2}, which
crystallizes in dark-violet needles.
Xanthophyll differs markedly from carotin in its solubilities, being
insoluble in petroleum ether and only sparingly soluble in carbon
disulfide. It may be fairly easily reduced to carotin. This transformation
is reversible, and suggests a similarity to the change from hæmoglobin to
oxyhæmoglobin, and the reverse, in the blood of animals, as a part of their
respiration process.
=Separation of the Chlorophylls, Carotin, and Xanthophyll.=--These
pigments, which exist together in most plant tissues, may easily be
separated from each other by taking advantage of the differences in their
solubilities, according to the following procedure. Grind up a small
quantity of the fresh tissue (leaves of the stinging nettle furnish a
conveniently large supply of each of these pigments) with fine sand in a
mortar. Cover with acetone, let stand a few moments and then filter on a
Büchner funnel. Pour the filtrate into a separatory funnel, add an equal
volume of ether and two volumes of water. Shake up once and then allow the
ether layer to separate; the pigments will be in this layer. Drain off the
water-acetone layer. Now to the etherial solution, add about half its
volume of a concentrated solution of potassium hydroxide in methyl alcohol.
Shake well and allow to stand until the mixture becomes permanently green.
Now add an equal volume of water and a little more ether, until the mixture
separates sharply into two layers. The chlorophylls will now be in the
lower dilute alcohol layer, and the carotinoids in the upper ether, and may
be separated by draining of each layer separately. To separate the carotin
from xanthophyll place the ether solution in a small open dish and
evaporate to a small volume. Now add about ten volumes of petroleum spirit
and an equal volume of methyl alcohol, stir up well, transfer to a
separatory funnel and allow the two layers to separate. The carotin will
now be in the upper layer of petroleum ether, and the xanthophyll in the
lower alcohol layer; these layers may be drained off separately and the
solvents evaporated in order to recover the pigments in dry form.
=Lycopersicin= (or lycopin) is a hydrocarbon pigment having the same
formula as carotin. It is, however, brilliantly red in color, and
crystallizes in a different form and has a different adsorption spectrum
from carotin. It is the characteristic pigment of red tomatoes, and is
found also in red peppers. Yellow tomatoes have only carotin as their
skin-pigment, while lycopersicin is usually present in the flesh of the
ripe fruits of all varieties and in the skin of red ones. It has been
shown, however, that if varieties of tomatoes which are normally red when
ripe, are ripened at high temperatures, 90° F. or above, their skins will
be yellow instead of red when fully ripe. Hence, the occurrence of carotin,
or of lycopersicin, as the skin pigment is determined in part by the
varietal character (being different in different varieties when ripened at
normal temperatures) and in part by the temperature at which the fruit
ripens. The two pigments are, of course, isomers; but the difference in
their structural arrangement is not known.
=Fucoxanthin=, C_{40}H_{54}O_{6}, is a brownish-red pigment, found in fresh
brown algæ, and in some brown sea-weeds. Its formula indicates that it is
an oxidized carotin. With iodine, it forms a compound having the formula
C_{40}H_{54}O_{6}I_{4}. It is unlike carotin and xanthophyll in that it has
basic properties, forming salts with acids, which are blue in color.
PHYCOERYTHRIN AND PHYCOPHÆIN
These are the principal pigments of red and brown seaweeds, respectively.
Their most characteristic difference from the pigments of non-aquatic
plants is that they are easily soluble in water, and insoluble in most
organic solvents, such as alcohol, ether, etc. At first thought, this would
appear to be impossible, since the plants grow in water and it would seem
that their water-soluble pigments would be continuously dissolved out of
the tissues. The reason why this does not occur lies in the fact that these
pigments exist in the cells of the seaweeds in colloidal form (see Chapter
XV), and, hence, cannot diffuse out through the cell-walls. The only way in
which they can be extracted from the tissues is by rupturing the cells, by
grinding with sharp sand, etc., after which the pigments can readily be
dissolved out by water.
=Phycoerythrin= is the red pigment. It is a colloidal, nitrogenous
substance, allied to the proteins (see Chapter XIII) but not a true protein
compound. Hydrolysis by acids indicates that it contains leucine and
tyrosine, two amino-acids which are constituents of proteins, along with
other bodies of unknown composition.
The colloidal solution of phycoerythrin in water has a brilliant rose-red
color, with an orange fluorescence. It readily sets to a gel (see Chapter
XV), so that the solution is almost impossible to filter. On this account,
purified solutions of this pigment are very difficult to secure, and no
satisfactory analysis to indicate its composition has yet been obtained.
Actinically, it is a complementary pigment to chlorophyll, that is, it
absorbs the blue and green rays and permits the passage of light which is
of the wave length that is absorbed by chlorophyll.
=Phycophæin.=--Still less is known of the composition of this pigment than
of that of phycoerythrin. It is the characteristic pigment of brown
seaweeds. It is supposed to exist in the cells of algæ, chiefly as a
colorless chromogen, which becomes first yellow and then brown on exposure
to air. Associated with it are other pigments, which have been variously
reported as carotin, phycoxanthin, etc.
THE ANTHOCYANS
These are a group of pigments of red, blue, or violet color, which occur in
the flowers, fruits, or leaves of many species of plants. They are
essentially ornamental pigments, and constitute a large proportion of the
brilliant colors of flowers, etc. They occur not only dissolved in the
cell-sap, but also as deposits of definite crystals or amorphous compounds
in the cell protoplasm.
They are all glucosides. When the anthocyans are hydrolyzed, the sugar
molecules are split off and the characteristic hydroxy-derivatives of the
three-ring anthocyan nucleus (figured on page 83), known as
"anthocyanidins," remain. These anthocyanidins are themselves pigments.
They have been shown to be all derivatives of the anthocyan nucleus. The
oxygen atom in this nucleus is very strongly basic and exhibits its
quadrivalent property by forming stable salts by direct addition of acid
radicles. The variation of color of the anthocyanins has been explained by
Willstätter, as follows; the red is the acid salt, the blue is a neutral
metallic salt, and the violet is the anhydride of the anthocyanidin in
question, thus
Cl Cl
| |
O _____ O _____
HO__ / \ / \ __/ \ KO__ / \ / \ __/ \
| | | \_____/ | | | \_____/
| | | | | |
\ / \ / \ / \ /
C O C
| \
Red | \ Blue
| O _____
|/ \ / \ __/ \
| | | \_____/
| | |
\ / \ /
C
Violet
All of the natural anthocyanin pigments appear to contain a chlorine atom
attached directly to the ring oxygen, as shown in the above partial
formulas. In addition, they have four, five, or six hydroxyl (OH), or
methoxy (OCH_{3}), groups attached at various points around the three
rings. The following formula for _[oe]nidin_, one of the most complex of
these anthocyanidins, will illustrate their structural arrangement.
Cl
| _____OCH_{3}
O / \
HO__ / \ / \ _____/ \OH
| | | \ /
| | | \_____/
\ / \ / OCH_{3} OH
OH C
_Delphinidin_ is the corresponding compound without the two CH_{3} groups;
while _cyanidin_ contains only five OH groups; and _pelargonidin_, only
four OH groups.
The anthocyanin pigments are soluble in water, alcohol, and ether, the
solutions being red or blue in color according to the acidity or alkalinity
of the medium. Their presence in many species of plants is hereditable, as
these plants come true to color from seed, as in the case of red beets, red
cabbage, several species of blue berries, etc. In other cases, the
anthocyanin development depends largely upon the conditions of growth,
particularly those which prevail during the later stages of development: as
in the case of apples, where the amount of red color in the skin depends to
a large extent upon the conditions under which the fruit ripens.
Anthocyanin pigments often make their appearance late in the season; in
fruits, etc., as the result of the normal ripening process but in leaves as
the result of shorter daylight illumination accentuated also by sharp
frosts.
THE ANTHOXANTHINS
The yellow plant pigments, other than the carotinoids, are almost without
exception glucosides having a xanthone or flavone nucleus. These typical
nuclei are illustrated on page 83. In these nuclei, as in the anthocyan
one, the oxygen atom is strongly basic and combines with mineral acids to
form salts (the oxygen becoming quadrivalent) and the color of the pigment
depending upon the nature of the combination formed in this way.
The anthoxanthin pigments are yellow, crystalline solids, which are only
slightly soluble in water. They dissolve readily in dilute acids and
alkalies, giving yellow or red solutions which are of the same color in
either acid or alkaline media. They are extensively used as yellow dyes.
Many of the common members of this group have been mentioned in the chapter
dealing with the glucosides. The characteristic pigment nucleus of several
of these is as follows:
_Chrysin_, found in various species of poplar and mallows,
O _____
HO / \ / \ __/ \
| | | \_____/
| | |
\ / \ /
HO C
║
O
_Apigenin_, found in parsley and celery, as the glucoside apiin,
O _____
HO / \ / \ __/ \OH
| | | \_____/
| | |
\ / \ /
HO C
║
O
_Campferol_, found in Java indigo, as the glucoside campferitrin,
O _____
HO / \ / \ __/ \OH
| | | \_____/
| | |OH
\ / \ /
HO C
║
O
_Fisetin_, found in quebracho wood and fiset wood,
O ____OH
HO / \ / \ __/ \OH
| | | \_____/
| | |
\ / \ /
C
║
O
_Quercitrin_, found in oak bark, horse-chestnut flowers, and in the skin of
onions,
O ____OH
HO / \ / \ __/ \OH
| | | \_____/
| | |OH
\ / \ /
HO C
║
O
_Morin_, found in yellow wood (_Morus tinctoria_).
O HO____
HO / \ / \ __/ \OH
| | | \_____/
| | |OH
\ / \ /
HO C
║
O
_Gentisin_, found in yellow gentian (_Gentiana lutea_),
O
CH_{3}O / \ / \ / \
| | | |
| | | |
\ / \ / \ / OH
HO C
║
O
As a rule, the most brilliant of these yellow pigments are found in the
largest quantities in the bark and wood of various species of tropical
plants; although they are also present, in smaller amounts, in the blossoms
of species growing in temperate zones.
The anthoxanthins are easily converted into anthocyanins, and _vice versa_,
by the action of oxidizing and reducing enzymes which are commonly present
in the tissues of the plants which develop the pigments.
THE PRODUCTION OF ORNAMENTAL PIGMENTS IN FLOWERS, ETC.
The breeding of flowering plants having blossoms of almost any desired
color has become a commercial enterprise of large importance. The results
which have been obtained, in many cases, have been made the object of
scientific study of the genetics of color inheritance. These studies have
developed certain interesting facts with reference to the chemistry of the
development of these ornamental pigments, which may be briefly mentioned
here.
In many of the plants which have been studied, the color of the flowers
depends upon several different factors, as follows:
_C_, a chromogen (or color-producing substance) which is generally a
flavone or xanthone glucoside, and which may be either yellow or
colorless.
_E_, an enzyme which acts upon _C_, to produce a red pigment.
_e_, another enzyme which acts upon the red pigment, changing it to some
other anthocyanin color.
_A_, an antioxidase, or antienzyme, which prevents the action of _E_.
_R_, an enzyme which changes reds to yellows.
Thus, if a plant whose flower contains only the factor _C_ be crossed with
one which contains the factor _E_, a red blossom will result, or if it
contains the factor _e_ more intense pigments are developed. But if either
_A_ or _R_ are present, no change in the color of the original parents will
result from the crossing.
THE PHYSIOLOGICAL USES OF PIGMENTS
The vegetative pigments undoubtedly serve as agencies for regulating the
rate of metabolic processes. At the same time, it is extremely difficult to
determine whether the presence of a pigment in any given case is the cause
or the effect of the changes in the plant's activities which result from
changes in its external environment.
The chlorophylls are, of course, the regulator of photosynthesis, absorbing
solar energy with which the photosynthetic process may be brought about.
The simultaneous presence of carotinoids in varying amounts undoubtedly
serves to modify the amount and character of the radiant energy absorbed,
as these pigments absorb a different part of the spectrum of light and
hence undoubtedly produce a different chemical activity or "actinic effect"
of the absorbed energy. The variations in depth of color of foliage during
different growing conditions, from a pale yellow when conditions are
unfavorable and growth is slow to the rich dark green of more favorable
conditions, is a familiar phenomenon. Whether this change in pigmentation
is the result of an adjustment of the plant protoplasm, so that it can
absorb a more highly actinic portion of the light, or is a direct effect of
the lack of conditions favorable to chlorophyll-production and active
photosynthesis, has not yet been determined.
But there must be some influence other than response to environmental
conditions which controls the vegetative color in plants, since shrubs, or
trees, which have green, yellow, red, and purple leaves, respectively, will
grow normally, side by side, under identical external conditions of
sunlight, moisture supply, etc. The hereditary influence must completely
overshadow the apparent normal self-adjustment of pigment to
energy-absorbing needs, in all such cases.
Again, it appears that there is some definite connection between pigment
content and respiration. It is known, of course, that the gaseous exchanges
involved in animal respiration are accomplished through the reversible
change of hæmoglobin to oxyhæmoglobin, these being the characteristic blood
pigments. The easy change of carotin, C_{40}H_{56}, to xanthophyll,
C_{40}H_{56}O_{2}, and _vice versa_, and the reversible changes of the
yellow anthoxanthins to the red anthocyanins, under the influence of the
oxidizing and reducing enzymes which are universally present in plants,
would indicate the possibility of the service of these pigments as carriers
of oxygen for respiratory activities in plants in a way similar to that in
which the blood pigments serve this purpose in the animal body. The fact,
which has been observed in connection with the experimental studies of the
development of the lycopersicin, that tomatoes which normally would become
red remain yellow in the absence of oxygen, indicates that this
pigmentation, at least, is definitely connected with oxygen supply; and the
further fact that the development of lycopersicin in red tomatoes, red
peppers, etc., is dependent upon the temperature at which the fruit ripens,
may indicate a definite connection of this pigment with the need for more
oxygen (or for more heat, as suggested in the following paragraph) at these
lower temperatures.
Again, many investigators have concluded that at least one function of the
anthocyanin pigments is to absorb heat rays and so to increase
transpiration and other chemical changes. In support of this view, there
may be cited the general presence of such pigments in arctic plants, their
appearance in the leaves of many deciduous trees after a frost in the fall,
etc. Indeed, there is much to support the view that the autumnal changes in
foliage pigments have the physiological function of absorbing heat in order
to hasten the metabolic processes of ripening and preparation for winter
defoliation. The rapid and brilliant changes in foliage coloring after a
sharp frost which kills the tissues and makes rapid translocation of the
food material of the leaves to the storage organs immediately necessary,
have been explained as the response of the pigmentation of the leaves to
the need for increased heat-absorption. On the other hand, the red pigments
of the beet-root, etc., which seem to be identical in composition with the
other anthocyanin pigments, can have no such function as those which have
just been described. Furthermore, the fact that the pigment often varies in
color from red to yellow or brown, depending upon the temperature under
which the tissue is ripening, makes it an open question whether the pigment
is the regulating agency or whether its nature is the result of the
environmental conditions. Or, in other words, it is a question whether
these changes in color are a mechanism by which the plant cell adjusts its
absorptive powers, or whether they are only the inevitable result of the
changes in temperature upon a pigment material which is present in the cell
for an entirely different use.
A very interesting side-light upon the color changes which many species of
plants undergo when the external temperature falls has been shown by the
investigations of the relation of the sugar content of the plant tissues to
their pigmentation. It is a well-known fact that not only do many species
of deciduous plants show the characteristic reddening of their leaves after
frost in the autumn but also many evergreens (_Ligustrum_, _Hedera_,
_Mahonia_, etc.) exhibit a marked reddening, or purpling, of their foliage
during the winter months, with a return to the normal green color in the
spring. Earlier investigations, which have been confirmed by several
repetitions, showed that the red or purple leaves always contain higher
percentages of sugar than do green ones of similar types. More recent
studies have shown that artificial feeding of some species of plants with
abnormally large portions of soluble sugars produces a reddening of the
foliage tissues which is apparently identical with that which these tissues
undergo as the result of low temperatures. Thus, the connection between the
natural winter reddening of foliage and the development of sugar in the
tissues during periods of low temperatures (see page 64) seems to be
clearly demonstrated. It appears that at least a part of the seasonal
changes in color of plants is either the cause of, or the effect of,
variations in sugar content of the tissues of the plants, accompanying the
changes in external temperatures.
Oftentimes, the anthocyanin pigments seem to be associated with sugar
production, as contrasted with the chlorophylls, which seem to be more
favorable to the production of starch. But in this case also, it is
impossible to say whether the pigment is the direct causative agent in the
type of carbohydrate production or whether it is the effect of the same
external factors which determine, or modify, the character of the
carbohydrate condensation.
BIOLOGICAL SIGNIFICANCE OF ORNAMENTAL PIGMENTS
The ornamental pigments undoubtedly have definite biological significance.
When present in the storage roots, such as beet-roots, carrots, etc., or in
the above-ground parts of plants, they may have served to protect these
organs against herbivorous animals which were accustomed to consume green
foods.
In flowers, the brilliant ornamental pigments undoubtedly serve to attract
the insects which visit these blossoms in search of nectar, and in so doing
promote cross-fertilization. Recent experiments have demonstrated that
colors are much more efficient than odors in attracting insects.
Taken altogether, it is apparent that the pigments may have a variety of
important rôles in plants. At the same time, some of them may be waste
products, with no definite use in the plant economy.
REFERENCES
ABDERHALDEN, E.--"Biochemisches Handlexikon, Band 6, Farbstoffe der
Pflanzen- und der Tierwelt," 390 pages, Berlin, 1911.
PERKIN, A. G. AND EVEREST, A. E.--"The Natural Organic Colouring Matters,"
655 pages, London, 1918.
WAKEMEN, NELLIE A.--"Pigments of Flowering Plants," in _Transactions_ of
the Wisconsin Academy of Sciences, Arts, and Letters, Vol. XIX, Part II,
pages 767-906, Madison, Wisc., 1919.
WATSON, E. R.--"Colour in Relation to Chemical Constitution," 197 pages, 65
figs., 4 plates, London, 1918.
WHELDALE, M.--"The Anthocyan Pigments of Plants," 304 pages, Cambridge,
1916.
WILLSTÄTTER, R. AND STOLL, A.--"Untersuchung über Chlorophyllen, Methoden
und Ergebnisse," 432 pages, 16 figs., Berlin, 1913.
CHAPTER IX
ORGANIC ACIDS, ACID SALTS, AND ESTERS
Organic acids, either in free form, or partially neutralized with calcium,
potassium, or sodium, forming acid salts, or combined with various alcohols
in the form of esters, are widely distributed in plants. They occur in
largest proportions in the fleshy tissues of fruits and vegetables, where
they are largely responsible for the flavors which make these products
attractive as food for men and animals. But organic acids and their salts
are also found in the sap of all plants, and undoubtedly play an important
and definite part in the vital processes of metabolism and growth.
CHEMICAL CONSTITUTION
All organic acids contain one (or more) of the characteristic
O
//
acid group, --COOH, or --C, known as "carboxyl." This
\
OH
group is monovalent, and in the simplest organic acid, formic acid
(H_{2}CO_{2}), it is attached to a single hydrogen atom, thus, H·COOH. In
all other monobasic acids, it is attached to some other monovalent group,
usually an alkyl radical, i.e., a radical derived from an alcohol and
containing only carbon and hydrogen (as methyl, CH_{3}, ethyl, C_{2}H_{5},
butyl, C_{4}H_{9}, acryl, C_{2}H_{3}, etc.). Hence, the general formula for
all monobasic organic acids is R·COOH, the R representing any monovalent
radical. In the simplest dibasic acid, oxalic (H_{2}C_{2}O_{4}), two
carboxyl groups are united to each other, thus, HOOC·COOH; but in the
higher members of the series, the two characteristic acid groups are united
through one or more --CH_{2}-- groups, or their oxy-derivatives (as
HOOC·CH_{2}·COOH, malonic acid; HOOC·CH_{2}·CH_{2}·CH_{2}·COOH, glutaric
acid; HOOC·CHOH·CH_{2}·COOH, malic acid, etc.). Polybasic acids, containing
three or more carboxyl groups, linked together through one or more alkyl
carbon atoms, are also possible, and a few typical ones (as
COOH
|
HOOC·CH_{2}·COH·CH_{2}·COOH, citric acid)
are found in fruits and other plant tissues.
The H atom of the COOH group may be replaced by metals, in exactly the same
way as it is replaceable in inorganic acids, producing either neutral or
acid salts, depending upon whether all or only a part of the acid H atoms
are replaced by the basic element.
Thus, with sulfuric acid:
OH ONa
/ /
SO_{2} (H_{2}SO_{4}) + NaOH = SO_{2} (NaH_{2}SO_{4}) + H_{2}O
\ \
OH OH
Sulfuric Acid Acid sodium sulfate
OH ONa
/ /
or, SO_{2} (H_{2}SO_{4}) + 2NaOH = SO_{2} (Na_{2}SO_{4}) + 2H_{2}O
\ \
OH ONa
Sulfuric Acid Neutral sodium sulfate
Similarly, with oxalic acid:
COOH COOK
| (H_{2}C_{2}O_{4}) + KOH = | + H_{2}O
COOH COOH
Oxalic acid Acid potassium oxalate
or, COOH COOK
| (H_{2}C_{2}O_{4}) + 2KOH = | + 2H_{2}O
COOH COOK
Oxalic acid Neutral potassium oxalate
Similarly, the acid H atom of either an organic or an inorganic acid may be
replaced by the alkyl group of an alcohol, producing "ethereal salts," or
"esters."
Thus, with nitric acid;
NO_{2}OH(HNO_{3})+C_{2}H_{5}OH = NO_{2}OC_{2}H_5(C_{2}H_{5}NO_{3})+H_{2}O
Nitric acid Ethyl alcohol Ethyl nitrate
And, with acetic acid;
CH_{3}·COOH(H_{4}C_{2}O_{2})+C_{2}H_{5}OH = CH_{3}·COOC_{2}H_{5}+H_{2}O
Acetic acid Ethyl acetate
With dibasic or polybasic acids, either one or more of the carboxyl H atoms
may be replaced with an alcohol radical, so that both acid and neutral
esters of all such acids are possible. Examples of all of these different
types of derivatives of organic acids are frequently found in plant
tissues.
The occurrence, properties, and functions of a particular type of glycerol,
and other esters of organic acids, which are known as fats and waxes, are
not taken into consideration in the following discussions, but reserved for
a subsequent chapter dealing specially with them.
SOME COMMON ORGANIC ACIDS
Free organic acids, or their mineral salts or volatile esters, sometimes
occur as separate and characteristic individual compounds in particular
species of plants, or fruits; but much more commonly, two, three, or even
more acids or their derivatives, are associated together.
=Formic acid=, H·COOH (H_{2}CO_{2}), occurs in free form and in
considerable proportions in the leaves of several species of nettle, where
it is responsible for the unpleasant effects of the "sting." It may be
detected in small amounts in the vegetative parts of many, if not all,
plants, especially during periods of rapid growth, and is probably one of
the intermediate products in the photosynthesis of carbohydrates (see
Chapter III).
Higher members of the formic acid series (as acetic, CH_{3}·COOH;
propionic, C_{2}H_{5}·COOH; butyric, C_{3}H_{7}·COOH; etc.) are often found
in small quantities in the leaves of many plants and seem to be
characteristically present in certain species. They are easily produced
from carbohydrates by bacterial action and, hence, are always present in
fermenting tissues, such as silage, sauerkraut, etc. Furthermore, the
glycerol esters of higher members of this and other monobasic acid series
are constituents of all natural fats and oils (see Chapter X).
=Oxalic acid=, HOOC·COOH (H_{2}C_{2}O_{4}), is found in small amounts in
nearly all plants and in relatively large proportions in those of _Oxalis_,
rhubarb, etc. It occurs both as the free acid and as neutral, or acid,
oxalates of calcium, potassium, and, perhaps, of magnesium and sodium.
Solid crystals of insoluble calcium oxalate are often found in plant cells,
and it has been shown that when so deposited the calcium cannot become
again available for metabolic uses. It is stated, further, that such
crystals form only when calcium is in excess in the plant sap; hence, the
deposition of crystallized calcium oxalate seems to be a device for the
avoidance of excessive calcium rather than excessive oxalic acid, in the
plant juices.
=Succinic acid=, HOOC·CH_{2}·CH_{2}·COOH (H_{6}C_{4}O_{4}), occurs in many
fruits and vegetables, and is also found in some animal tissues. In fruits,
it is usually associated with its derivatives, malic and tartaric acids.
=Malic acid=, HOOC·CH_{2}·CHOH·COOH (H_{6}C_{4}O_{5}), occurs in apples and
in many small fruits, and in many vegetables. Acid calcium malate is now
produced commercially as a bye-product from the manufacture of syrups from
fruit juices, and is used as a substitute for "cream of tartar" in the
manufacture of baking powders.
=Tartaric acid=, HOOC·CHOH·CHOH·COOH (H_{6}C_{4}O_{6}), is found in many
fruits, but most characteristically in the grape, where it occurs as the
mono-potassium salt. During the fermentation of grape juice into wine, this
salt is deposited in considerable quantities in the bottom of the
wine-casks. This crude product is collected and sold under the name
"argols." From these argols, pure acid potassium tartrate is obtained by
decolorization and recrystallization, and constitutes the "cream of tartar"
of commerce.
COOH
|
=Citric acid=, HOOC·CH_{2}·COH·CH{2}·COOH (H_{8}C_{6}O_{7}), occurs
in large proportions in lemons, and associated with malic acid in
strawberries, cherries, currants, etc. It is also found in small quantities
in the seeds of the common leguminous vegetables, beans, peas, etc.
=Tannic acid= occurs widely distributed in the plant kingdom as a
constituent of the special type of glucosides known as _tannins_, whose
properties and functions have already been discussed (see Chapter VII).
PHYSIOLOGICAL USES OF ORGANIC ACIDS
No conclusive evidence concerning the rôle of organic acids in plant, or
animal, growth, has yet been produced. There can be no doubt that the
hypothetical _carbonic acid_ and its acid and normal salts have a
significant effect in regulating the acidity or alkalinity of plant juices,
or body fluids, and so determining the nature of the enzymic activities and
colloidal conditions of the biological systems (see Chapters XIV and XV).
It is probable that other organic acids, such as formic, acetic, oxalic,
and succinic acids, in plants and sarco-lactic acid, in animal tissues,
perform similar regulatory rôles; but there seems as yet to be no
indication as to why different acids should be used for this purpose by
different species, or organisms; or as to the methods by which they perform
their specific functions, whatever these may be.
In plants, the organic acids are usually in solution in the sap. When the
plant ripens, they generally disappear, either being neutralized by
calcium, or other bases, and deposited as crystals in the leaves or stems,
or else used up in the synthesis of other organic compounds. Small
proportions of these acids are usually present in mature seeds, and the
percentage increases materially during germination, indicating that they
play an important rôle in insuring the proper conditions for the conversion
of the reserve food of the seed into soluble materials available for the
nutrition of the young growing plant.
BIOLOGICAL SIGNIFICANCE OF FRUIT ACIDS, ETC.
The occurrence of organic acids, or their derivatives, which have
pronounced odors or flavors, in the flesh surrounding the seeds of fruits,
in the endosperm of vegetable seeds, or in the tubers, etc., of perennial
plants, thus making them attractive as food for animals and men,
undoubtedly serves to insure a wider distribution of the reproductive
organs of these plants; a fact which has unquestionably had a marked
influence upon the survival of species in the competitive struggle for
existence during past eras and in the development and cultivation of
different species by man. Indirect evidence that the proportion of these
attractive compounds present in certain species may have been considerably
increased by the processes of "natural selection" in the past is furnished
by the many successful attempts to increase the percentage of such
desirable constituents in fruits or vegetables by means of artificial
selection of parent stocks by skillful plant breeders.
CHAPTER X
FATS AND OILS, WAXES, AND LIPOIDS
Included in this group are several different kinds of compounds which have
similar physical properties, and which, in general, belong to the type of
organic compounds known as esters, i.e., alcoholic salts of organic acids.
The terms "oil," "fat," and "wax," are generally applied more or less
indiscriminately to any substance which has a greasy feeling to the touch
and which does not mix with, but floats on, water. There are many oils
which are of mineral origin which are entirely different in composition
from natural fats. These have no relation to plant life and will not be
considered here.
The natural fats, vegetable oils, and plant waxes are all esters. There is
no essential difference between a fat and an oil, the latter term being
usually applied to a fat which is liquid at ordinary temperatures. The
waxes, however, are different in chemical composition from the fats and
oils, being esters of monohydric alcohols of high molecular weight, such as
cetyl alcohol, C_{16}H_{33}OH, myristic alcohol, C_{30}H_{61}OH, and
cholesterol, C_{27}H_{45}OH; whereas the fats and oils are all esters of
the trihydric alcohol glycerol, C_{3}H_{5}(OH)_{3}. Lipoids are much more
complex esters, having some nitrogenous, or phosphorus-containing, group
and sometimes a sugar in combination with the fatty acids and glycerol
which make up the characteristic part of their structure.
In general, waxes and lipoids have a harder consistency than fats: but this
is not always the case, since "wool-fat" and spermaceti, both of which are
true waxes in composition, are so nearly liquid in form as to be commonly
called fats; while certain true fats, like "Japan wax," are so hard as to
be commonly designated as waxes. It is plain that physical properties alone
cannot be relied upon in the classification of these bodies. In fact, there
is no single definite property by which members of this group can be
accurately identified. There are many other types of substances belonging
to entirely different chemical groups, which have oily, or fat-like,
properties.
A. FATS AND OILS
OCCURRENCE
Fats and oils are widely distributed in plants. They occur very commonly in
the reproductive organs, both spores and seeds, as reserve food material.
In fungi, oils are often found in the spores, but sometimes also in
sclerotia, mycelia, or filaments. For example, the sclerotia of ergot have
been found to contain as much as 60 per cent of oil. In higher plants, many
seeds contain high percentages of oil, so as to make them commercial
sources for edible or lubricating oils, such as olive oil, rape-seed oil,
cottonseed oil, castor oil, corn oil, sunflower-seed oil, etc., etc. Nuts
often contain large proportions of oil, the kernel of the Brazil nut, for
example, sometimes contains as high as 70 per cent of oil, while an oil
content of 50 per cent, or more, is common in almonds, walnuts, etc.
Oils also occur as reserve food material in other storage organs of plants,
such as the tubers of certain flowering plants, and the roots of many
species of orchids. Sometimes the appearance of oils in the stems of trees,
or the winter leaves of evergreens, seems to be only temporary and to occur
only during periods of very low temperatures.
Much less frequently, fats or oils are found in the vegetative organs of
plants, as in the leaves of evergreens. Their appearance and functions in
these organs seem to be much less certain than in the other cases cited
above; although in rare cases a considerable proportion of oily material
has been found to exist in definite association with the chloroplasts.
The vegetable fats and oils have many important industrial uses. Some of
them, such as olive oil, cottonseed oil, cocoanut oil, etc., are largely
used as human food. Others, as castor oil, are used as lubricants. The
so-called "drying oils" (see page 132), such as linseed oil, etc., are used
in the manufacture of paints and varnishes. Some cheap vegetable oils are
used as the basis for the manufacture of soaps, etc. Hence, industrial
plants and processes for the extraction of oils from plant tissues are of
very great economic importance.
CHEMICAL CONSTITUTION
The fats (of either plant or animal origin) are glycerides, i.e., glycerol
esters of organic acids. As has been pointed out, esters are derived from
organic acids and alcohols in exactly the same way that mineral salts are
derived from inorganic acids and metallic bases.
--------
Thus, Na|OH + H|ONO_2(HNO_{3}) = NaNO_{3} + H_{2}O
--------
Base Acid Salt
-----------
and, C_{2}H_{5}|OH + H|OOC·H = C_{2}H_{5}OOC·H + H_{2}O
-----------
Alcohol Acid Ester
---------------
or, R·|OH + H|OOC·R = R·OOC·R + H_{2}O
---------------
Any alcohol Any acid Any ester
Glycerol is, however, a trihydric alcohol, i.e., it contains three
replaceable (OH) groups. Its formula is C_{3}H_{5}(OH)_{3}, or
CH_{2}OH·CHOH·CH_{2}OH. Hence, three molecules of a monobasic acid are
required to replace all of its (OH) groups.
For example,
C_{2}H_{5}OH + HOOC·C_{17}H_{35} = CH_{2}OOC·C_{17}H_{35}
| |
CHOH + HOOC·C_{17}H_{35} = CHOOC·C_{17}H_{35} + 3H_{2}O
| |
C_{2}H_{5}OH + HOOC·C_{17}H_{35} = CH_{2}OOC·C_{17}H_{35}
It is theoretically possible, of course, to replace either one, two, or
three of the (OH) groups in the glycerol with acid radicals, thus
producing either mono-, di-, or triglycerides. If the primary alcohol
groups in the glycerine molecule are designated by (1) and the secondary
one by (2), thus, CH_{2}OH(1)·CHOH(2)·CH_{2}OH(1), it is conceivable
that there may be either (1) or (2) monoglycerides, either (1, 1) or (1,
2) diglycerides, or a triglyceride, depending upon which of the (OH)
groups are replaced. Compounds of all of these types have been produced
by combinations of glycerol with varying proportions of organic acids
under carefully controlled conditions; and all of them found to possess
fat-like properties. All natural fats are triglycerides, however. Most
natural fats are mixtures of several different triglycerides in each of
which the three (OH) groups of the glycerol has been replaced by the
same organic acid radical, as in the example of stearin shown above. But
recent investigations have shown that some of the common animal fats,
and perhaps some plant oils, may be made up of mixed glycerides, i.e.,
those in which the different (OH) groups have been replaced by different
acid groups, as oleo-stearin, oleo-stearo-palmitin, etc.
THE ACIDS WHICH OCCUR IN NATURAL FATS
The acids which, when combined with glycerol, produce fats are of two
general types. The first of these are the so-called "fatty acids" having
the general formula C_{_n_}H_{2_n_+1}·COOH. These are the "saturated"
acids, i.e., they contain only single-bond linkages in the radical which is
united to the ·COOH group; hence, they cannot take up hydrogen, oxygen,
etc., by direct addition. The second type are the "unsaturated" acids
belonging to several different groups, as discussed below, but all having
one or more double-linkages between the carbon atoms of the alkyl radical
which they contain. Because of these double linkages, they are all able to
take on oxygen, hydrogen, or the halogen elements, by direct addition. When
exposed to the air, for example, these "unsaturated" acids, or the oils
derived from them, take up oxygen, increasing in weight, and becoming solid
or hard and stiff. Hence, natural oils which contain considerable
proportions of glycerides of these "unsaturated" acids are known as "drying
oils" and are largely used in the manufacture of paints, varnishes,
linoleums, etc.; while oils which contain little of these glycerides are
known as "non-drying," and are used for food, for lubrication, or for other
technical purposes in which it is essential that they remain in unchanged
fluid condition when exposed to the air.
The following are some of the more important of the acids which occur as
glycerides in natural fats: Saturated Acids:
(_a_) Acetic, or stearic, acid series--general formula,
C_{_n_}H_{2_n_+1}·COOH.
(1) Formic acid, H·COOH, occurs free in nettles, ants, etc.
(2) Acetic acid, CH_{3}·COOH, occurs free in vinegar.
(3) Butyric acid, C_{3}H_{7}·COOH, in butter fat.
(4) Capric acid, C_{9}H_{19}·COOH, in butter fat and cocoanut
oil.
(5) Myristic acid, C_{13}H_{27}·COOH, in cocoanut oil and
spermaceti.
(6) Palmitic acid, C_{15}H_{31}·COOH, in palm oil and many
fats.
(7) Stearic acid, C_{17}H_{35}·COOH, in most fats and oils.
Intervening members of this series, such as caprylic acid,
C_{7}H_{15}·COOH, and lauric acid, C_{11}H_{23}·COOH, are also found in
smaller quantities in cocoanut and palm nut oils, in butter fat, and in
spermaceti; while higher members of the series, as arachidic acid,
C_{19}H_{39}·COOH, and lignoceric acid, C_{23}H_{47}·COOH, are found in
peanut oil; and cerotic acid, C_{25}H_{51}·COOH, and melissic acid,
C_{29}H_{59}·COOH, in beeswax and carnauba wax.
Unsaturated Acids:
(_b_) Oleic acid series--general formula, C_{_n_}H_{2_n_-1}·COOH.
(1) Crotonic acid, C_{3}H_{5}·COOH, occurs in croton oil.
(2) Oleic acid, C_{17}H_{33}·COOH, occurs in many fats and
oils.
(3) Brassic acid, C_{21}H_{41}·COOH, occurs in rape-seed oil.
(4) Ricinoleic acid, C_{17}H_{32}OH·COOH, occurs in castor oil.
(_c_) Linoleic acid series--general formula, C_{_n_}H_{2_n_-3}·COOH.
(1) Linoleic acid, C_{17}H_{31}·COOH, occurs in linseed and
other drying oils.
(_d_) Linolenic acid series--general formula, C_{_n_}H_{2_n_-5}·COOH.
(1) Linolenic acid, C_{17}H_{29}·COOH, occurs in many drying
oils.
It will be observed that all of these acids contain a multiple of two total
carbon atoms. No acid containing an uneven number of carbon atoms has been
found in a natural fat. Furthermore, the acids which occur most commonly in
natural fats are those which contain eighteen carbon atoms; in fact, more
than 80 per cent of the glycerides which compose all animal and vegetable
fats are those of the C_{18} acids. This fact, in addition to the one that
the sugars and starches all contain multiples of six carbon atoms in their
molecules, indicates a very great biological significance of the chain of
six carbon atoms. This has been alluded to in connection with the
discussion of the biological significance of molecular configuration (see
page 57) and will be mentioned again in other connections.
THE ALCOHOLS WHICH OCCUR IN NATURAL FATS
=Glycerol=, as has been pointed out, is by far the most common alcoholic
constituent of natural fats and oils. This substance, which is familiar to
everyone under its common name "glycerine," is a colorless, viscid liquid
having a sweetish taste. It is a very heavy liquid (specific gravity 1.27)
which mixes with water in all proportions and when in concentrated form is
very hygroscopic.
Glycerine is made from fats and oils by commercial processes which clearly
prove that the constitution of fats is as described above. The fat is
boiled with a solution of caustic soda and is decomposed, the sodium of the
alkali taking the place of the glyceryl (C_{3}H_{5}) group, the latter
combining with three (OH) groups from the three molecules of alkali
necessary to decompose the fat. A sodium salt of the organic acid, or soap,
and glycerol are thus produced, and are separated by saturating the hot
solution with common salt, which causes the soap to separate out as a layer
on the surface of the liquid, which, on cooling, solidifies into a solid
cake, which is then cut and pressed into the familiar bars of commercial
soap. From the remaining solution, the glycerine is recovered by
evaporation and distillation under reduced pressure. Taking stearin, a
common fat, as the example, the reaction which takes place in the above
process may be expressed by the following equation:
C_{3}H_{5}(C_{17}H_{35}·COO)_{3} + 3NaOH = 3C_{17}H_{35}COONa +
Stearin Sodium stearate--a soap
C_{3}H_{5}(OH)_{3}.
Glycerol
This process, since it yields soap as one of its products, is called
"saponification." All fats, when saponified, yield soaps and either
glycerol or (more rarely) some of the other alcohols which are described
below.
Glycerine is also prepared from fats by hydrolysis with superheated steam.
Using olein, a glyceride which is present in olive oil and many common
fats, as the example in this case, the equation for the reaction is:
C_{3}H_{5}(C_{17}H_{33}·COO)_{3} + 3H_{2}O = 3C_{17}H_{33}·COOH +
Olein Steam Oleic acid
C_{3}H_{5}(OH)_{3}
Glycerol
In this case the free fatty acid, instead of a soap, is the product which
is obtained in addition to glycerol.
In the equations presented above, a single glyceride has been used as the
example in each case. In the saponification, or hydrolysis, of natural fats
and oils which, as has been shown, are mixtures of many glycerides, the
resultant soaps, or fatty acids, are mixtures of as many compounds as there
were individual glycerides of the original fat, but the glycerol is
identical in every case.
When glycerol is heated with dehydrating agents, it is easily converted
into _acrolein_, an unsaturated aldehyde having a peculiar characteristic
pungent odor. Hence, the presence of glycerol, or glycerides, in any
substance may usually be detected by mixing the material with anhydrous
acid potassium sulfate and heating the mixture in a test tube, when the
characteristic odor of acrolein will appear.
Glycerol possesses all the characteristic properties of an alcohol, forming
alcoholates with alkalies, esters with acids, etc. It is an active reducing
agent, being itself easily oxidized to a variety of different products
depending upon the strength of the oxidizing agent used and the conditions
of the experiment. Microorganisms affect it in a variety of ways, either
converting it into simple fatty acids, or condensing it into longer-chain
compounds.
=Open-chain monohydric alcohols=, higher members of the ethyl alcohol
series, such as cetyl, C_{16}H_{33}OH, carnaubyl, C_{24}H_{49}OH, ceryl,
C_{26}H_{53}OH, and melissyl, C_{30}H_{61}OH, are found in the esters which
constitute the major proportion of the common waxes.
=Cholesterol and phytosterol= are empirical names for certain closed-ring,
monohydric alcohols which are found in relatively small amounts in all
fats, the former term designating those found in animal fats and the latter
those of plant origin. Their composition has not yet been definitely
established. They are known to contain two, or three, closed rings,
probably of the phenanthrene type; to form dichlor- and dibrom- addition
products, showing that they contain one side-chain double linkage; and to
yield ketones when oxidized, indicating that they are secondary alcohols.
They form acetyl esters, or acetates, which can be separated from each
other and identified by their crystal forms and melting points. Because of
this fact and of the further fact that they are present in detectable
quantities in practically all fats and oils, they afford a qualitative
means of distinguishing between fats of animal and of plant origin. This
possibility is the most interesting fact known concerning these complex
alcohols; although their presence as esters in all plant and animal fats
indicates that they must have some biological function.
_Phytosterol_ is not a single alcohol, but a mixture of at least two, which
have been separated and studied as _sitosterol_, C_{27}H_{43}OH, and
_stigmasterol_, C_{30}H_{49}OH. As has been said, these are found in small
proportions in all vegetable fats, being present in largest amounts in oily
seeds, especially those of the legumes.
The saponification of esters of cholesterol and phytosterol is a difficult
and unsatisfactory process; but since this affords the only known means to
distinguish between fats of plants and of animal origin, its technique has
been fairly well worked out, and the process used in the study of the
changes which take place in plant fats when they are used by animals as
food.
HYDROLYSIS AND SYNTHESIS OF FATS
The reaction for the hydrolysis of fats has been discussed in connection
with the process for the manufacture of glycerine. This reaction takes
place very slowly with cold water alone, can be easily brought about by the
action of superheated steam, and much more easily and rapidly in the
presence of some catalyst (sulfuric acid is an especially effective
catalyst for this purpose).
Fats can be artificially synthetized by heating mixtures of glycerol and
fatty acids, under considerable pressure, for some time at temperatures of
200° to 240° C.; or by heating a mixture of the disulfuric ester of glycerol
with a fatty acid dissolved in sulfuric acid. Recently, fatty acids have
been prepared from carbohydrates, by first breaking the hexoses down into
three-carbon compounds, then carefully oxidizing these to pyruvic acid,
CH_{3}·CO·COOH, which can then be condensed into acids having longer
chains. The violent reagents and long-continued processes which must be
employed for the artificial hydrolysis or synthesis of the fats are in
sharp contrast with the easy and rapid transition of carbohydrates to
fats, and _vice versa_, which take place in both plant and animal
nutrition.
THE EXTRACTION OF OILS FROM PLANT TISSUES
There are three types of methods which are employed for the extraction of
oil from oil-bearing seeds, etc., either as a commercial industry or for
the purposes of scientific study. These are (1) by pressure; (2) extraction
with volatile solvents; and (3) boiling the crushed seeds or fruits with
water.
By the first method, the seeds are first cleaned, then "decorticated"
(hulls removed), crushed or ground, then subjected to intense pressure in
an hydraulic press. In the commercial process, the ground seeds are first
pressed at ordinary temperature, which yields "cold-drawn" oil, then the
press cake is heated and pressed again, whereby "hot-drawn" oil is
obtained. The crude oil is refined by heating it to coagulate any albumin
which it may contain, and is sometimes bleached by different processes
before it is marketed. The press cake from many seeds, such as flaxseed
(linseed), cottonseed, etc., is ground up and sold for use as stock feed.
In the second method, the finely crushed seeds are treated with solvents
such as gasoline or carbon bisulfide, in an apparatus which is so arranged
that the fresh material is treated first with solvent which has already
passed through various successive lots of material and has become highly
charged with the oil, followed by other portions which contain less oil,
and finally by fresh solvent, whereby the last traces of oil are removed
from the material. The saturated solvent is transferred to suitable boilers
and the solvent distilled off and condensed for repeated use, leaving the
oil in the boiler in very pure form.
Extraction by boiling with water is sometimes used in the preparation of
castor oil and olive oil. In such cases, the crushed seeds are boiled with
water and the oil skimmed off as fast as it rises to the surface.
IDENTIFICATION OF FATS AND OILS
Fats and oils are identified by determinations of their physical
properties, such as specific gravity, melting point, refractive index,
etc., and by certain special color reactions for particular oils; or by
measurements of certain chemical constants, such as the percentage of free
fatty acids which they contain, the saponification value (i.e., the number
of milligrams of KOH required to completely saponify one gram of the fat),
the iodine number (percentage by weight of iodine which is absorbed by the
unsaturated fatty acids present in the fat), percentage of water-insoluble
fatty acids obtained after saponification and acidifying the resultant
soap, etc., etc. Most of these tests must be carried out under carefully
controlled conditions in order to insure reliable identifications, and need
not be discussed in detail here. Full directions for making such tests,
together with tables of standard values for all common fats and oils, may
be found in any reference book on oil analysis.
PHYSIOLOGICAL USE OF FATS AND OILS
In animal organisms, fats are the one important form of energy storage.
They also form one of the most important supplies of energy reserve
material in plants. Carbohydrates commonly serve this purpose in those
plants whose storage reservoirs are in the stems, tubers, etc.; but in most
small seeds the reserve supply of energy is largely in the form of oil, and
even in those seeds which have large endosperm storage of starch, the
embryo is always supplied with oil which seems to furnish the energy
necessary for the first germinative processes.
Fats are the most concentrated form of potential energy of all the
different types of organic compounds which are elaborated by plants. This
is because they contain more carbon and hydrogen and less oxygen in the
molecule than any other group of substances of vegetable (or animal)
origin. It has been pointed out that a quantity of fat capable of yielding
100 large calories of heat will occupy only about 12 cc. of space, whereas
from 125 to 225 cc. of space in the same tissue would be required for the
amount of starch of glycogen necessary to yield the same amount of heat, or
energy, when oxidized.
The fats undoubtedly catabolize first by hydrolysis into glycerol and fatty
acids, and then by oxidation possibly first into carbohydrates and then
finally into the end-products of oxidation, namely, carbon dioxide and
water. The following hypothetical equation to represent the oxidation of
oleic acid into starch, suggested by Detmer, is interesting as a suggestion
of how much oxygen is required and how much heat would be liberated by such
a transformation:
C_{18}H_{34}O_{2} + 27O = 2(C_{6}H_{10}O_{5}) + 6CO_{2} + 7H_{2}O
Complete oxidation of oleic acid to the final end-products, carbon dioxide
and water, would require much more oxygen, thus:
C_{18}H_{34}O_{2} + 51O = 18CO_{2} + 17H_{2}O.
Hence, Detmer's reaction would yield only approximately one-half the total
energy available in the acid; but it does indicate the possibility of
redevelopment of fatty acids or fats from the unoxidized carbohydrate
material which remains in the equation. Moreover, there is abundant
evidence to show that, in both animal and plant tissues, energy changes are
brought about chiefly by the transformation of fats into carbohydrates and
_vice versa_.
Many different hypotheses have been put forward concerning the mode of
transformation of fats into carbohydrates, and the changes which take place
in oily seeds during their germination have been carefully studied by many
investigators. The following seem to be fairly well established facts.
First, that fats as such may be translocated from cell to cell, since
cell-walls and cell protoplasm seem to be permeable to oil if it is a
sufficiently fine emulsion; or they may be hydrolyzed into glycerol and
fatty acids and translocated from cell to cell in these forms and
recombined into fats in the new location. Second, that fats are formed from
glucose in some plants, from sucrose and from starch in others, and from
mannite and similar compounds in still other species. Third, that in
germination the fatty acids are used up in the order of their degree of
unsaturation, those which contain the largest number of double-bond
linkages being used first, and the saturated acids last of all. Fourth,
that the sugar produced by the oxidation of fats is derived either from the
glycerol or from the fatty acids of the fat, depending upon the nature of
the latter. If the fat is saturated, the glycerine is converted into sugar
while the fatty acids are oxidized; but if the fat contains large
proportions of unsaturated acids, these contribute to the formation of
sugar.
Recent studies seem to show that in the animal body fats serve an important
function in connection with the production of antibodies to disease germs.
But there is as yet no evidence to show that fats and oils have any similar
function in plant tissues. The fact that they are found almost wholly in
the storage organs of plants seems to indicate that their use as food
reserve material is their principal, if not their sole, function in the
plant economy.
B. THE WAXES
Waxes are most commonly found in or on the skin of leaves or fruits. They
are similar to fats in chemical composition, except that, instead of being
glycerides, they are esters of monohydric alcohols of high atomic weight.
The term wax, when used in the chemical sense, has reference to this
particular type of esters rather than to any special physical properties
which the compound possesses, and both solid and liquid waxes are known.
Carnauba wax, found on the leaves of the wax-palm (_Copernicia cerifera_)
contains ceryl alcohol (C_{23}H_{53}OH) and myricyl alcohol
(C_{30}H_{61}OH) esters of cerotic acid (C_{25}H_{51}·COOH) and carnaubic
acid (C_{23}H_{47}·COOH). It is the best known vegetable wax. Poppy wax is
composed chiefly of the ceryl ester of palmitic acid (C_{17}H_{35}·COOH).
Since waxes contain no glycerol, they give no odor of acrolein when heated
with dehydrating agents, do not become rancid, and are less easily
hydrolyzed than the fats. They are soluble in the same solvents as the
fats, but generally to a less degree.
The facts that waxes are impervious to water and usually occur on the
surfaces of plant tissues have led to the conclusion that their chief
function is to provide against the too-rapid loss of water by evaporation
from these tissues. This seems to be borne out by the common experience
that many fresh fruits and vegetables will keep longer without shriveling
if their waxy coating is undisturbed. No other function than that of
regulation of water losses has been suggested for the plant waxes.
C. THE LIPOIDS
The lipoids, or "lipins," as some authors prefer to call them, are
substances of a fat-like nature which are found in small quantities in
nearly all plant and animal tissues and in considerable proportions in
nerve and brain substance, in egg yolk, etc., and in the seeds of plants.
When hydrolyzed, they yield fatty acids or derivatives of fatty acids and
some other group containing either nitrogen only or both nitrogen and
phosphorus. The facts that they are extracted from tissues by the same
solvents which extract fats and that they yield fatty acids when hydrolyzed
account for the name "lipoid," which comes from the Greek word meaning fat.
Some writers, who object to the word "lipoid" as a group name, prefer to
call these substances the "fat-like bodies."
The first group of lipoids to be studied were those which occur in the
brain; and the name _cerebroside_ was given to those lipoids which, when
hydrolyzed, yield fatty acids, a carbohydrate and a nitrogen-containing
compound but no phosphoric acid; while those lipoids which contain both
nitrogen and phosphorus were called _phosphatides_. Substances which
correspond in composition to both these types are found in plant tissues
and the same class names are applied in a general way to lipoids of either
plant or animal origin.
Plant lipoids have not been studied to nearly the same extent as have those
which occur in the animal body; and certain observers believe that there
are significant differences between the lipoids of plants and those of
animal origin. However, most investigators use the same methods of study
and the same systems of nomenclature for these fat-like substances,
regardless of their origin.
LECITHIN
This phosphatide is by far the best-known lipoid. It occurs in the brain,
the heart, the liver, and in the yolk of the eggs of many animals; and
either lecithin or a substance so nearly like it in character as to be
regarded by most investigators as identical with it, is present in small,
but constant, quantities in nearly all seeds, especially those of
leguminous plants. In many legume seeds, it constitutes from 50 to 60 per
cent of the "ether extract," or "crude fat," which can be extracted from
the crushed seeds, using ether as the solvent.
Lecithin is a glyceride. Only two of the (OH) groups of the glycerol are
replaced by fatty acids, however; the third being replaced by phosphoric
acid, H_{3}PO_{4}, or PO(OH)_{3}, which, in turn, has one of its hydrogen
atoms replaced by the base _choline_. Choline is a nitrogenous base, or
amine, which may be regarded as ammonium hydroxide with three of its
hydrogen atoms replaced by methyl groups and the fourth by the ethoxyl
group, the latter being the ethyl group with an OH in place of one of its
hydrogens. Thus,
Ammonium hydride Choline
H H CH_{3} C_{2}H_{4}OH
\ / \ /
H-N CH_{3}-N
/ \ / \
H OH CH_{3} OH
Without the choline, lecithin would be a di-fatty acid derivative of
glycero-phosphoric acid. These relations may be seen in the following
formulas:
Glycerol Glycero-phosphoric acid Fatty Acid
CH_{2}OH CH_{2}OH OH HOOC·R
| | /
| | /
CHOH CH--O--P=O
| | \
| | \
CH_{2}OH CH_{2}OH OH
Choline Lecithin
HOC_{2}H_{4} CH_{2}OOC·R OH
\ | /
\ | /
(CH_{3})_{3}[trb]N CH---O---P=O C_{2}H_{4}
/ | \ / \
/ | \ / N[trb](CH_{3})_{3}
HO CH_{2}OOC·R O /
HO
fatty acid
/
Or, glycerol-fatty acid + Choline = Lecithin + H_{2}O
\
phosphoric acid
There are many different possible linkages of the constituent groups which
make up the lecithin molecule. In the first place, if the (OH) groups of
the glycerol molecule be numbered (1) and (2), thus,
CH_{2}OH (1)
|
CHOH (2)
|
CH_{2}OH (1)
the fatty acid radicals may be attached either in one (1) position and one
(2) position, or in the two (1) positions; hence, two forms of
glycero-phosphoric acid are possible, thus
fatty acid fatty acid
/ /
(A) glycerol--fatty acid (B) glycerol--phosphoric acid
\ \
phosphoric acid fatty acid
Again, the choline may be attached to the phosphoric acid either through
its alcoholic (OH) group or through its basic (N) group, thus
OH OH
/ /
-P=O C_{2}H_{4} -P=O
\ / \ \
O N[trb](CH_{3})_{3} or, O-N[trb](CH_{3})_{3}
/ \
HO C_{2}H_{4}OH
The facts that in the arrangement (B) the central carbon atom of the
glycerol would be asymmetric, and that both lecithin and the
glycero-phosphoric acid derived from it by hydrolysis are optically active,
prove that formula (B) correctly represents the arrangement of that part of
the lecithin molecule; and there is ample theoretical and experimental
evidence to prove that the choline linkage is through the alcoholic (OH)
group. Hence the formula for lecithin indicating the linkage as shown above
is the correct one.
The fatty acids in the lecithin molecule may be different in lecithins from
different sources, just as they are different in fats from different
sources. Both oleic acid and a solid fatty acid have been found in the
hydrolysis products of lecithin from leguminous seeds. In certain lupine
seeds, the fatty acids present in the lecithin appear to be palmitic and
stearic.
OTHER PLANT PHOSPHATIDES
Phosphatides other than lecithin are common in plants. In these, various
sugars replace part or all of the glycerol as the alcoholic part of the
ester. Percentages of sugar varying from mere traces up to 17 per cent of
the weight of material taken, have been found in the products of hydrolysis
of phosphatides prepared from vetch seeds, potato tubers, plant pollens,
and whole wheat meal.
Furthermore, betaine
CH_{2}
/ \
(tri-methyl glycocoll, OC N[trb](CH_{3})_{3})
\ /
O
and perhaps other vegetable amines (see Chapter XII) sometimes replace
choline as the basic group in the phosphatides.
PLANT CEREBROSIDES
Bodies similar to the animal cerebrosides seem to occur in many plant
tissues, since plant lipoids which yield no phosphorus when hydrolyzed have
often been isolated. The sugar which constitutes the alcoholic portion of
their structure appears to be galactose in every case which has been
reported. Beyond this, little is known of the structure of these plant
cerebrosides, as they are very difficult to prepare in pure form and not
easily hydrolyzed.
PHYSIOLOGICAL USES OF LIPOIDS
Lipoids are so universally present in plant and animal tissues and so
commonly found in those parts of the organism in which vital phenomena are
most pronounced (brain, heart, embryo of egg, embryo of seeds, etc.), that
it is evident that they must play some important rôle in the activity of
living protoplasm. There is, as yet, however, no definite and certain
knowledge of what this rôle is. Various theories concerning the matter have
been put forward in recent years. For example, Overton, in 1901, presented
the idea that every living cell is surrounded by a semi-permeable membrane
consisting of lipoid material, which regulates the passage into and out of
the cell of substances necessary to its metabolism and growth. Recent
investigations by Osterhout and others indicate, however, that Overton's
hypothetical lipoid membrane is not essential to a proper explanation of
the migration into and out of the cell protoplasm of nutritive materials,
etc. Other investigators have cited results which appear to indicate that
lipoids play an important, but as yet unknown, part in the process of fat
metabolism. Others go even further than this, and argue that since the
extraordinary rapidity of the chemical changes which take place in plant
protoplasm indicates the necessity of the presence there of exceedingly
labile substances, and since both fats and proteins are relatively stable
compounds, it is possible that the lipoids, which contain both nitrogenous
and fatty acid groups, play an exceedingly important part in the metabolism
processes. Bang, in particular, has pointed out (in 1911) that the lipoids
are probably the most labile of all the components which constitute the
colloidal system known as plant protoplasm. The importance of such
considerations will be more apparent after the relation of colloidal
phenomena to the activities of plant cell contents has been more fully
discussed (see Chapter XVI).
Experimental studies of the physiological uses of lipoids have thus far
been devoted almost exclusively to those of animal tissues. They have been
seriously hampered by the difficulty of securing properly purified extracts
of lecithin and similar lipoids. The same labile character which apparently
makes them so important in the chemical changes in the cell makes them
equally unstable compounds to work with in attempting to secure pure
preparations for the purposes of experimental study. On this account, there
is, as yet, no certain knowledge concerning their actual physiological
uses. It is evident, however, that they have some really important rôle to
play, which opens up a promising field for further study.
REFERENCES
ABDERHALDEN, E.--"Biochemisches Handlexikon, Band 3, Fette, Wachse,
Phosphatide, Cerebroside, ..." 340 pages, Berlin, 1911.
HOPKINS, E.--"The Oil-Chemist's Handbook," 72 pages, New York, 1902.
LEATHES, J. B.--"The Fats," 138 pages, _Monographs_ on Biochemistry,
London, 1913.
LEWKOWITSCH, J.--"Chemical Technology and Analysis of Oils, Fats, and
Waxes," Vol. I, 542 pages, 54 figs.; Vol. II, 816 pages, 20 figs.; and Vol.
III, 406 pages, 28 figs., London, 1909.
MACLEAN, H.--"Lecithin and Allied Substances," 206 pages, _Monographs_ on
Biochemistry, London, 1913.
SOUTHCOMBE, J. E.--"Chemistry of the Oil Industries," 204 pages, 13 figs.,
London, 1918.
CHAPTER XI
ESSENTIAL OILS AND RESINS
Included in this group are all those substances to which the characteristic
odors of plants are due, along with others similar in structure and
possessing characteristic resinous properties. They have no such uniformity
in composition as is exhibited by the oils which are included among the
fats and waxes; but belong to several widely different chemical groups.
Furthermore, there is no sharp dividing line between the essential oils and
certain esters of organic acids on the one hand and the fats on the other.
For example, if an aromatic fluid essence is a light fluid, non-viscid, and
easily volatile, it is usually classed with the organic esters; denser
liquid substances, of oily or waxy consistency, and with comparatively
slight odor and taste are usually fats, while oils of similar physical
properties but possessing strong characteristic odors are classed as
essential oils, regardless of their chemical composition.
Included in this general class are compounds having a great variety of
chemical structures; e.g., hydrocarbons, alcohols, phenols, organic
sulfides and sulfocyanides, etc. Many of these compounds are crystalline
solids at ordinary temperatures, but melt to oily fluids at higher
temperatures. The characteristic property which assigns any given plant
extract to this group is that it has a strikingly characteristic odor or
taste, often accompanied by some definite physiological effect, or
medicinal property.
These compounds may be either secretions or excretions of plants, sometimes
normally present in the healthy tissue, and sometimes produced as the
result of injury or disease.
The essential oils and the resins often occur associated together in the
plant; or, the resins may develop from the oily juice of the plant after
exposure to the air.
THE ESSENTIAL OILS
These may be divided, according to their chemical composition, into two
major groups; (1) the hydrocarbon oils, or terpenes, and (2) the oxygenated
and sulfuretted oils.
The =terpenes= are of three different types, namely: (_a_) the
hemiterpenes, C_{5}H_{8}, unsaturated compounds of the valerylene series,
of which _isoprene_ (found in crude rubber) is the best-known example;
(_b_) the terpenes proper, C_{10}H_{16}, which constitute the major
proportion of the whole group; and (_c_) the polyterpenes
(C_{5}H_{8})_{_n_}, of which _colophene_ and _caoutchouc_ are the most
common examples.
Eleven different terpenes having the formula C_{10}H_{16} have been
isolated from various plant juices, and their molecular arrangement
carefully worked out. The following three examples will serve as typical of
the general structural arrangement of these hydrocarbons:
Limonene Camphene Pinene
H
|
C
/|\
/ | \
/ | \
/ | \
/ | \
CH_{3} H / | \
| | / | \
C C /CH_{3} | CH_{3}\
/ \\ CH_{3} / | \ H_{3}C \|/ CH_{2}
/ \\ \ / | \ | C |
H_{2}C CH C | CH_{2} | / |
| | /| | | | / |
| | CH_{3} | HCH | | / |
| | | | | | / |
H_{2}C CH_{2} CH_{2}=C | CH_{2} | / |
\ / \ | / | / |
\ / \ | / | / |
CH C |/ |
| | HC CH
C H \ //
// \ \ //
H_{2}C CH_{3} \ //
\ //
\ //
\ //
\ //
\ //
C
|
CH_{3}
A discussion of the evidence which supports these formulas as properly
represented the molecular arrangements of the various isomeric forms would
be out of place here, as its only particular interest is in connection with
the medicinal effects of the different compounds. It is clear, however,
that they are six-membered hydrocarbon rings, with additional hydrocarbon
groups attached to one or more of the carbon atoms in the ring.
Different modifications, or varieties, of the terpenes constitute the main
proportions of the oils of turpentine, bergamot, lemon, fir needles,
eucalyptus, fennel, pennyroyal, etc.
The =oxygenated essential oils= may be either alcohols, aldehydes, ketones,
acids, esters, or phenols, derived from either five-membered or
six-membered closed-ring hydrocarbons. They are usually present in the
plant oil in mixtures with each other or with a terpene. Since most of them
have pronounced physiological or medicinal properties, their structure has
been well worked out, in most cases; but it seems to be hardly worth while
to present these matters in detail here, as they are of interest chiefly on
account of their medicinal properties rather than their botanical
functions.
_Borneol_, C_{10}H_{17}OH, and _menthol_, C_{10}H_{19}OH, are typical
_alcohols_. The latter is a crystalline substance, which melts at 42°,
which is present in peppermint oil, both as the free alcohol and as an
ester of acetic acid.
Amyl acetate, CH_{3}·COOC_{5}H_{11}, and linalyl acetate,
CH_{3}·COOC_{10}H_{17}, the latter occurring in the oils of lavender and
bergamot, are typical esters classed as essential oils.
As examples of the _aldehyde_ oils, benzoic aldehyde, C_{6}H_{5}CHO, "oil
of bitter almonds," and cinnamic aldehyde, C_{6}H_{5}CH=CHCHO, found in the
oils of cinnamon and cassia, may be cited.
Camphor, C_{10}H_{16}O, is a _ketone_, having the following structural
formula:
H
|
C
/ | \
/ | \
/ | \
/ | \
/ | \
/ | \
CH_{2} | CH_{2}
| | |
|CH_{3}-C-CH_{3}|
| | |
CH_{2} | C=O
\ | /
\ | /
\ | /
\ | /
\ | /
\ | /
C
|
CH_{3}
There are a considerable number of essential oils which are _phenols_.
_Thymol_, C_{6}H_{3}·(CH_{3})·(C_{3}H_{7})·OH, in oil of thyme, and
carvacrol, its isomer, in oil of hops, are familiar examples.
O-C=O
/ |
Coumarin, the anhydride of cinnamic acid C_{6}H_{4} | ,
\ |
HC=CH
is an example of an acid substance which is classed as an essential oil,
even though it is a solid at ordinary temperatures. It has an odor and
flavor similar to that of _vanillin_, the essential flavoring material of
the vanilla bean, and is often used as a substitute for the latter in the
preparation of artificial flavoring extracts.
Of the =essential oils containing sulfur=, there are two common examples;
oil of mustard, allyl isosulfocyanide, C_{3}H_{5}NCS, and oil of garlic,
allyl sulfide (C_{3}H_{5})_{2}S. The latter is present in onions, garlic,
water cress, radishes, etc., the difference in flavor of these vegetables
being due to the fact that the allyl sulfide is united with other different
groups in the glucoside arrangement, in the different plants. Similarly,
mustard oil is not present in mustard seeds as such, but as a glucoside
which, when hydrolyzed by the enzyme _myrosin_ which is always present in
other cells of the same seeds, yields C_{3}H_{5}NCS, KHSO_{4}, and
C_{6}H_{12}O_{6}.
THE RESINS
The resins were formerly supposed to be the mother substances from which
the terpenes are derived. It is now known, however, that they are the
oxidation products of the terpenes. Their exact structure is still a matter
of some uncertainty, as their peculiar "resinous" character makes them very
difficult to study by the usual methods of chemical investigations.
Resins are divided into two classes: (_a_) the balsams, and (_b_) the solid
or hard resins. Canada balsam and crude turpentine are familiar examples of
the first class. They consist of resinous substances, dissolved in or mixed
with fluid terpenes. Ordinary resin, or _colophony_, consists chiefly of a
monobasic acid having the empirical formula C_{20}H_{30}O_{2}, known as
sylvinic acid, whose exact structure is not known. Its sodium salt is used
as the basis for cheap soaps.
The hard resins are amorphous substances of vitreous character, which
consist of very complex aromatic acids, alcohols, or esters, combined with
other complicated structures, known as _resenes_, whose definite chemical
nature is not yet known. Among the hard resins are many substances which
are extensively used in the manufacture of varnishes, such as copal, amber,
dammar, sandarach, etc.
There are also resinous substances, such as asaf[oe]tida, myrrh, gamboge,
etc., which are mixtures of gums (see Chapter VI) and true resins. Some of
these have considerable commercial value for medicinal or technical uses.
PHYSIOLOGICAL USES AND BIOLOGICAL SIGNIFICANCE OF ESSENTIAL OILS
No theory has yet been advanced concerning the possibility of the use of
essential oils and resins by plants in their normal metabolic processes.
The very great diversity in their chemical nature makes it impossible that
they should all be considered as having the same physiological function,
if indeed any of them actually have any such function.
It is evident that those aromatic compounds which occur as normal
secretions of plants and which give to the plants their characteristic
odors may act either as an attraction to animals which might utilize the
plants as food and so serve to distribute the seed forms, or as a repellent
to prevent the too rapid destruction of the leaves, stems, or seeds of
certain species of plants whose slow-growing habits require the
long-continued growth of these portions of the plant for the perpetuation
of the species. The presence of these compounds in larger proportions in
those species of conifers, etc., which grow in tropical regions, in
competition with other rapid-growing vegetation, suggests the latter
possibility. It must be admitted, however, that their presence in such
cases may be the result of climatic conditions, as indicated by the fact
that most spice plants are tropical in habit, rather than the result of
their protective influence in the struggle for survival during past ages.
Many of the oils and resins which are secreted as the result of injury by
disease or wounds have marked antiseptic properties and undoubtedly serve
to prevent the entrance into the injured tissue of destructive organisms.
But apart from these possible protective influences which may have had an
important effect upon the preservation and perpetuation of the species of
plants which secrete them, there is no known biological necessity for the
presence of these aromatic substances in plants.
REFERENCES
ABDERHALDEN, E.--"Biochemisches Handlexikon, Band 7, Gerbstoffe,
Flechtenstoffe, Saponine, Bitterstoffe, Terpene, Aetherische Oele, Harze,
Kautschuk," 822 pages, Berlin, 1912.
ALLEN'S Commercial Organic Analysis, Vol. IV, "Resins, Rubber, Guttapercha,
and Essential Oils," 461 pages, 7 figs., Philadelphia, 1911 (4th ed.).
HEUSLER, F., trans. by Pond, F. J.--"The Chemistry of the Terpenes," 457
pages, Philadelphia, 1902.
PARRY, E. J.--"The Chemistry of Essential Oils and Perfumes," 401 pages, 20
figs., London, 1899.
CHAPTER XII
THE VEGETABLE BASES
We come, now, to the consideration of the characteristically nitrogenous
compounds of plants. None of the groups of compounds which have been
considered thus far have, as a group, contained the element nitrogen. This
element is present in the chlorophylls and in certain other pigments, but
not as the characteristic constituent of the molecular structure of the
group of compounds, nor do these compounds serve as the source of supply of
nitrogen for the plant's needs.
The characteristic nitrogen-containing compounds may all be regarded as
derived from ammonia, or ammonium hydroxide, by the replacement of one or
more hydrogen atoms with organic radicals of varying type and complexity.
If the group, or groups, which be considered as having replaced a hydrogen
atom in ammonia, in such compounds, is an alkyl group, the compound is
strongly basic in character and is known as an _amine_; whereas if the
replacing group is an acid radical, the resulting compound may be neutral
(known as _acid amides_), or weakly acid (known as _amino-acids_) in type.
Compounds of the first type constitute the _vegetable bases_; while those
of the second type are the _proteins_.
The vegetable bases may be divided into three groups. These are (_a_) the
_plant amines_, which are simple open-chain amines; (_b_) the _alkaloids_,
which are comparatively simple closed-ring amines, containing only one
nitrogen atom in any single ring; and (_c_) the _purine bases_, which are
complex compounds containing a nucleus with four carbon atoms and four
nitrogen atoms arranged alternately to form a double-ring group.
THE PLANT AMINES
The simple amines bear the relation to ammonia, or ammonium hydroxide,
represented by the following formulas, in which the R indicates any simple
alkyl radical:
H R R R R R H H
/ / / / \ / \ /
N-H N-H N-H N-R N-R N-H
\ \ \ \ / \ / \
H H R R R OH H OH
Ammonia Primary Secondary Tertiary Quaternary Ammonium
amine amine amine amine hydroxide
The simple amines which occur in animal tissues are known as "ptomaines"
and "leucomaines." The ptomaines are all decomposition products resulting
from the putrefactive decay of proteins caused by moulds or bacteria. Some
of these are highly toxic, producing the so-called "ptomaine-poisoning";
while others are wholly innocuous. They are all simple amines. Putrescine,
di-amino butane, NH_{2}·CH_{2}·CH_{2}·CH_{2}·CH_{2}·NH_{2}, and cadaverine,
di-amino pentane, HN_{2}·(CH_{2})_{5}·NH_{2}, are common non-toxic
ptomaines, resulting from the decay of meat. Neurine, trimethyl-ethylene
ammonium hydroxide, (CH_{3})_{3}(C_{2}H_{3})·NOH, is a violently poisonous
ptomaine produced in the decay of fish. Amines of similar structure to
these are occasionally found in living animal tissues. Such compounds are
known as _leucomaines_, to distinguish them from the _ptomaines_, which are
found only in dead material.
Corresponding in structure and properties to these amines of animal origin,
there is a series of basic substances, found in many plants, known as the
_plant amines_. The following are common examples:
=Trimethyl amine=, (CH_{3})_{3}N, is a very volatile compound, found in the
flowers of several species of the Rose family, the leaves of certain weeds,
etc. When crushed, these tissues give off a very fetid odor, which is due
to this amine.
=Choline=, =muscarine=, and =betaine= are plant amines which are closely
related to each other and to neurine (the toxic ptomaine) in composition
and structure, as shown in the following formulas:
CH_{2}CH_{2}OH CH_{2}CHO
/ /
(CH_{3})_{3}N (CH_{3})_{3}N
\ \
OH OH
Choline Muscarine
CH_{2}CO CH=CH_{2}
/ / /
(CH_{3})_{3}N / (CH_{3})_{3}N
\ / \
O OH
Betaine Neurine
Choline and betaine are non-toxic; while muscarine and neurine are violent
poisons.
Choline and muscarine occur in certain toadstools. Betaine and choline
often occur together in the germs of many plants. Betaine is found in the
beet root and the tubers of Jerusalem artichoke. Choline occurs alone in
the seeds and fruits of many plants, sometimes as the free amine, but more
often as a constituent of lecithin (see page 141).
Phenyl derivatives of simple amines are sometimes found in
---
/ \
plants. _Hydroxyphenylethyl amine_, HO-C CH_{2}·CH_{2}·NH_{2},
\ /
---
---
/ \
found in _ergot_, and _hordeine_, HO-C CH_{2}·CH_{2}·N·(CH_{3})_{2},
\ /
---
found in barley, are examples. The former has marked medicinal properties.
There is no known physiological use for these simple amines in plants. By
some investigators, they are regarded as intermediate products in the
synthesis or decomposition of proteins; but it would seem that if this were
a normal procedure, these amines would occur in varying proportions in all
plants, under different conditions of metabolism, instead of in practically
constant proportions in only a few species, as they do.
ALKALOIDS
These are a group of strong vegetable bases whose nitrogen atom is a part
of a closed-ring arrangement.
As a rule, alkaloids are colorless, crystalline solids, although a few are
liquids at ordinary temperatures. They are generally insoluble in water,
but easily soluble in organic solvents. Being strong bases, they readily
form salts with acids, and these salts are usually readily soluble in
water.
Alkaloids are usually odorless; although nicotine, coniine, and a few
others, have strong, characteristic odors. Most of them have a bitter
taste, and many of them have marked physiological effects upon animal
organisms, so that they are extensively used as narcotics, stimulants, or
for other medicinal purposes.
Most of the alkaloids contain asymmetric carbon atoms and are, therefore,
optically active, usually levorotatory, although a few are dextrorotatory.
The alkaloids are precipitated out of their solutions by various solutions
of chemical compounds, known as the "alkaloidal reagents": iodine dissolved
in potassium iodide solution gives a chocolate-brown precipitate; tannic
acid, phosphotungstic acid, phosphomolybdic acid, and mercuric iodide
solutions give colorless, amorphous precipitates; while gold chloride and
platinic chloride solutions give crystalline precipitates, many of which
have sharp melting points and can be used for the identification of
individual alkaloids. There are a great many specific color reactions for
individual alkaloids, which are important to toxicologists and pharmacists,
but which it would not be desirable to consider in detail here.
The alkaloids are conveniently divided into groups, according to the
characteristic closed-ring arrangements which they contain. The several
closed-ring arrangements which are found in common alkaloids, and upon
which their grouping is based, may be illustrated by the following
formulas:
H H H
H{2}_C---CH_{2} | \ /
| | C C
| | // \ / \
H{2}_C CH_{2} HC CH H_{2}C CH_{2}
\ / | | | |
N HC CH H_{2}C CH_{2}
| \\ / \ /
H N N
|
Pyrrolidine, C_{4}H_{9}N Pyrridine, C_{5}H_{5}N H
Piperidine, C_{5}H_{11}N
H H
\ / H
C |
/ \ H_{2}C-----C-----CH_{2}
H_{2}C CH_{2} | | |
| | | NH CH_{2}
HC CH | | |
|\ /| or H_{2}C-----C-----CH_{2}
| N | |
| /|\ | H
|/ H \|
H_{2}C-----CH_{2}
Tropane, C_{7}H_{13}N
H H H H
| | | |
C C C C
// \ / \\ // \ / \\
HC C CH HC C CH
| ║ | | ║ |
HC C CH HC C N
\\ / \ // \\ / \ //
C N C C
| | |
H H H
Quinoline, C_{9}H_{8}N Isoquinoline, C_{9}H_{8}N
The common alkaloids are distributed in the several groups as follows:
Pyrridine--piperidine group; piperine, coniine, nicotine.
Pyrrolidine group; hygrine and stachydrine.
Tropane group; atropine, hyoscine, cocaine, lupinine.
Quinoline group; quinine, cinchonine, strychnine, brucine.
Isoquinoline group; papaverine, hydrastine, morphine, codeine, berberine.
The composition and properties of the individual alkaloids have been
extensively studied, because of their medicinal uses. As they have no known
metabolic use to the plants which elaborate them, it will not be worth
while to consider all of these investigations in detail here. The following
facts with reference to certain typical members of each group will serve to
illustrate the general constitution and properties of the alkaloids.
=Piperine=, C_{17}H_{19}O_{3}, is found in black peppers. Its constitution
is represented by the following formula, the group which is united to the
piperidine ring, in this case, being piperic acid:
H_{2}
║
C
/ \
H_{2}C CH_{2}
| | /\ --O
H_{2}C CH_{2} | | \
\ / | | CH_{2}
N----OC·CH=CH·CH=CH--| | /
\/ --O
=Coniine=, C_{8}H_{17}N, is found in the umbelliferous plant, _Conium
maculatum_. Structurally, it is a propyl-piperidine, represented by the
following formula:
H_{2}
║
C
/ \
H_{2}C CH_{2}
| |
H_{2}C CH-C_{3}H_{7}
\ /
N
|
H
=Nicotine=, C_{10}H_{14}N_{2}, is the alkaloid of tobacco leaves. It is an
extremely poisonous, oily liquid, with a strong odor and a burning taste.
Its structural formula shows it to contain both a pyrridine ring and a
pyrrolidine ring, linked together thus
H
|
C H_{2}C---CH_{2}
/ \\ | |
HC C------HC CH_{2}
║ | \ /
HC CH N
\ // |
N CH_{3}
=Hygrine=, C_{7}H_{13}NO, from coca leaves, is an acetic acid salt of
pyrrolidine, represented by the following formula:
H_{2}C---CH--OC·CH_{3}
| |
H_{2}C CH_{2}
\ /
N
|
CH_{3}
=Atropine= and =hyoscyamine=, C_{17}H_{23}NO_{3}, are optical isomers.
Atropine is an extremely poisonous, white crystalline compound, which is
obtained from deadly nightshade and henbane, and used in medicine, in
minute doses, as an agent for reducing temperature in acute cases of
fevers. Structurally, it is a tropic acid ester of tropane, represented
by the following formula:
H_{2}C---CH--------CH_{2} C_{6}H_{5}
| | | |
| N-CH_{3} CHOOC---CH
| | | |
H_{2}C---CH--------CH_{2} CH_{2}OH
=Cocaine=, C_{17}H_{21}NO_{4}, is found in coca leaves. It is a white
crystalline solid, which is largely used as a local anæsthetic for minor
surgical operations. Its structural formula is
H_{2}C---CH--------HC-OOC·CH_{3}
| | |
| N-CH_{3} HC-OOC·C_{6}H_{5}
| | |
H_{2}C---CH---------CH_{2}
It is, therefore, a di-ester of acetic and benzoic acids with tropane.
=Cinchonine=, C_{19}H_{22}N_{2}O, and =quinine=, C_{20}H_{24}N_{2}O_{2},
are alkaloids found in cinchona bark. They are white crystalline solids,
which are extensively used in medicine. They have been shown to contain a
quinoline group combined with modified piperidine groups, as represented in
the following formulas:
H
|
N C
/ \ / \ /|\
| | | / | \
| | | H_{2}C HCH CH-CH=CH_{2}
\ / \ / | | |
CH-CHOH-HC HCH CH_{2}
\ | /
N
Cinchonine
H
|
N C
/ \ / \ / \
| | | / \
| | | H_{2}C CH-CH=CH_{2}
\ / \ / | |
CH-CHOH-HC CH_{2}
\ /
N
Quinine
=Strychnine=, C_{21}H_{22}N_{2}O_{2}, =brucine=,
C_{21}H_{20}(OCH_{3})N_{2}O_{2}, and =curarine= are three alkaloids which
are present in the seeds of several species of _Strychnos_. They are all
highly poisonous. Beyond the fact that when they are hydrolyzed they yield
quinoline and indole, their composition is unknown.
=Morphine=, C_{17}H_{19}NO_{3}, is the chief alkaloid of opium, which is
the dried juice of young pods of the poppy. Both the alcoholic solution of
opium (known as "laudanum") and morphine itself are extensively used in
medicine as narcotics to deaden pain. Morphine has an exceedingly complex
structure, being a combination of an isoquinoline and a phenanthrene
nucleus, which is probably correctly represented by the following formula:
H H_{2}
| ║
C C
// \ / \
HOC C CH_{2}
| ║ |
C C N-CH_{3}
/ \\/ \ /
/ C CH
O | |
\ HC CH_{2}
\ / \ /
HC C
| ║
H_{2}C CH
\ /
C
║
H_{2}
=Codeine=, C_{17}H_{18}(OCH_{3})NO_{2}, which is also found in opium, is a
methyl derivative of morphine. =Papaverine=, =laudanosine=, =narcotine=,
and =narceine= are four other alkaloids found in opium. They each contain
an isoquinoline nucleus, combined by one bond to a benzene ring, with one
or more methyl groups and three or more methoxy (OCH_{3}) groups attached
at various points around the three characteristic rings. The following
formula for laudanosine will illustrate their structure:
/ \ / \
CH_{3}O | |
| | | OCH_{3}
CH_{3}O | N-CH_{3} / \
\ / \ / | |OCH_{3}
| | |
| \ /
H_{2}C----------------CH
The above discussions of the composition of typical alkaloids clearly
indicate the extreme complexity of their molecular structure. It is
generally supposed that they are formed by the decomposition of proteins.
But they are developed in only a few particular species of plants and are
always present in these plants in fairly constant quantities. Hence, it
appears that, in these species, the production of alkaloids is in some way
definitely connected with protein metabolism; but it is certain that this
is not a common relationship, as it is manifested by such a limited number
of species of plants, and there is absolutely no knowledge as to its
character and functions. Some authorities prefer to regard the alkaloids as
waste-products of protein metabolism; but here, again, it is difficult to
understand why such products should result in certain species of plants and
not in others.
THE PURINE BASES
This is a group of compounds, widely distributed in both plant and
animal tissues, all of which are derivatives of the compound known as
_purine_, C_{5}H_{4}N_{4}. All of the naturally occurring compounds of
this group may be regarded as derived from purine, either by the
addition of oxygen atoms, or by the replacing of one or more of its
hydrogen atoms with a methyl (CH_{3}) group or an amino (NH_{2}) group.
The following structural formula represents the arrangement of the
purine nucleus, the numbers being used to designate the nitrogen or
carbon atoms to which the additional atoms, or groups, are attached in
the more complex compounds of the group. In purine itself, the four
hydrogen atoms are attached in the 2, 6, 7, and 8 positions.
6
1N==C--
| | 7
--2C 5C---N--
║ ║ \
║ ║ 8C--
║ ║ //
3N--4C--9N
The double bonds, in each case except those between the 4 and 5 carbon
atoms, are easily broken apart and readjusted, so that other atoms or
groups can be attached to any atom in the nucleus except the 4 and 5 carbon
atoms. In all of the statements with reference to the structure of the
purine bases, the term "oxy" is used to mean an oxygen atom attached by
both its bonds to one of the carbons in the nucleus, instead of its
customary use to mean the monovalent OH group replacing a hydrogen, as in
the case of all other nomenclature of organic compounds. With this
understanding, reference to the numbered nucleus formula above will make
plain the structure of all of the purine bases which are included in the
following list:
Hypoxanthine, C_{5}H_{4}N_{4}O, = 6-monoxypurine.
Xanthine, C_{5}H_{4}N_{4}O_{2}, = 2,6-dioxypurine.
Uric acid, C_{5}H_{4}N_{4}O_{3}, = 2,6,8-trioxypurine.
Adenine, C_{5}H_{3}N_{4}NH_{2}, = 6-aminopurine.
Guanine, C_{5}H_{3}N_{4}ONH_{2}, = 2-amino-6-oxypurine.
Theobromine, C_{5}H_{2}N_{4}O_{2}(CH_{3})_{2} =
3,7-dimethyl-2,6-dioxypurine, or dimethyl xanthine.
Theophylline, C_{5}H_{2}N_{4}O_{2}(CH_{3})_{2} =
1,3-dimethyl-2,6-dioxypurine.
Caffeine, C_{5}HN_{4}O_{2}(CH_{3})_{3} = 1,3,7-trimethyl-2,6-dioxypurine,
or trimethyl xanthine.
In order to make these structural relationships quite clear, the following
formulas for uric acid and for caffeine are presented as typical examples:
H N--C=O CH_{3}--N--C=O
| | | |
O=C C--N-H O=C C--N-CH_{3}
| ║ \ | ║ \
| ║ C=O | ║ CH
| ║ / | ║ //
HN--C--N-H CH_{3}-N--C--N
Uric acid Caffeine
=Uric acid= is found in the excrement of all animals; in the urine of
mammals, and in the solid excrement of birds and reptiles. It is not known
to occur in plants.
=Xanthine= and =hypoxanthine= occur in animal urine, and also in the
tissues of both plants and animals.
=Adenine= and =guanine= are constituents of all nucleic acids (see below)
and, hence, are found in all plant and animal tissues. Guanine is the chief
constituent of the excrement of spiders, and is found also in Peruvian
guano. It is also a constituent of the scales of fishes.
=Caffeine=, =theophylline=, and =theobromine= are not found in animal
tissues, but are fairly widely distributed in plants. Caffeine and
theobromine are the active constituents of tea leaves and coffee seeds and
are found also in cacao beans and kola nuts. The use of these three
compounds in the metabolism of the plants which elaborate them is wholly
unknown. They are not so directly related to protein metabolism as are the
other purine bases.
The purine bases, other than the three mentioned in the preceding
paragraph, are undoubtedly intermediate products in protein metabolism. In
animals, they constitute a large proportion of the waste-products from the
use of proteins in the body. It is not clear that there are similar
waste-products in plant metabolism, however. In both plants and animals,
the purine bases which are a part of the nucleic acids undoubtedly play an
important and essential part in growth, since they form the major
proportion of the nucleus, from which all cell-division proceeds.
THE PYRIMIDINE BASES
These compounds do not occur free in plants; but since they are constituent
groups in the plant nucleic acids (see below), a brief explanation of their
composition is desirable. They are nitrogenous bases, similar to, but
somewhat simpler than, the purine bases. Their general composition and
structural relationships are illustrated by the following typical formulas:
N==C-H H-N--C=O
| | | |
H-C C-H O=C C-H
║ ║ | ║
N--C-H H-N--C-H
Pyrimidine Uracil
C_{4}H_{4}N_{2} C_{4}H_{4}N_{2}O_{2}
2,6-dioxypyrimidine
N==C-NH_{2} H-N--C=O
| | | |
O=C C-H O=C C-CH_{3}
| ║ | ║
H-N--C-H H-N--C-H
Cytosine Thymine
C_{4}H_{3}N_{2}ONH_{2} C_ {4}H_{3}N_{2}O_{2}CH_{3}
2,oxy-6-amino-pyrimidine 2,6-dioxy-5-methyl-pyrimidine
THE NUCLEIC ACIDS
The nuclei of cells are composed almost wholly of complex organic salts, in
which _proteins_ constitute the basic part and _nucleic acids_ the acid
part. These salts, or esters, are known under the general name
"nucleoproteins." The composition of the proteins is discussed in detail in
the following chapter, and it seems desirable to present a brief discussion
of the constitution of the nucleic acids here; although they are
essentially acids rather than vegetable bases.
The nucleic acids are complex compounds consisting of a carbohydrate,
phosphoric acid, two purine bases, and two pyrimidine bases. So far as is
known, all animal nucleic acids are identical and all plant nucleic acids
are identical; but those of plant origin differ from those found in animal
cells in the character of the carbohydrate and that of one of the
pyrimidine bases which are present in the molecule, as shown in the
following tabulation of their composition:
Animal nucleic acid Plant nucleic acid
Phosphoric acid Phosphoric acid
Hexose (levulose) Pentose (_d_-ribose)
Guanine Guanine
Adenine Adenine
Cytosine Cytosine
Thymine Uracil
The structure of the plant nucleic acid may be represented by the following
formula:
OH
|
O=P--O--carbohydrate-guanine group
|
O
|
O=P--O--carbohydrate-adenine group
|
O
|
O=P--O--carbohydrate-uracil group
|
O
|
O=P--O--carbohydrate-cytosine group
|
OH
That this is probably a correct representation of the general arrangement
in this compound, is indicated by the fact that by different methods of
hydrolysis it is possible to split off either the purine and pyrimidine
bases, leaving a carbohydrate ester of phosphoric acid; or the phosphoric
acid, leaving carbohydrate combinations with the nitrogenous bases.
Nucleic acid, prepared from animal glands which contain large proportions
of it, is a white powder, which is insoluble in water, but when moistened
forms a slimy mass. It is almost insoluble in alcohol, but dissolves
readily in alkaline solutions, forming a colloidal solution which readily
gelatinizes (see chapter on Colloids). Solutions of nucleic acids are
optically active, probably because of the carbohydrate constituents.
From their structure and properties, it is apparent that nucleic acids are
on the border line between carbohydrates, plant amines, and proteins. They
undoubtedly play an important part, both in cell-growth and in the
synthesis of proteins from carbohydrates and ammonium compounds.
References
BARGER, GEO.--"The Simpler Natural Bases," 215 pages, _Monographs_ on
Biochemistry, London, 1914.
FISCHER, E.--"Untersuchungen in der Puringruppe, 1882-1906," 608 pages,
Berlin, 1907.
HENRY, T. A.--"The Plant Alkaloids," 466 pages, Philadelphia, 1913.
JONES, W.--"The Nucleic Acids," 118 pages, _Monographs_ on Biochemistry,
London, 1914.
PICTET, A.--"La Constitution Chimique des Alcaloides Vegetaux," 421 pages,
Paris, 1897 (2d ed.).
VAUGHAN, V. C. and NOVY, F. G.--"Ptomaines, Leucomaines, Toxins and
Antitoxins," 604 pages, Philadelphia, 1896, (3d ed.).
WINTERSTEIN, E. and TRIER, G.--"Die Alkaloide," 340 pages, Berlin, 1910.
CHAPTER XIII
PROTEINS
The proteins are the most important group of organic components of plants.
They constitute the active material of protoplasm, in which all of the
chemical changes which go to make up the vital phenomena take place.
Combined with the nucleic acids, they comprise the nucleus of the cell,
which is the seat of the power of cell-division and, hence, of the growth
of the organism. Germ-cells are composed almost exclusively of protein
material. Hence, it is not an over-statement to say that proteins furnish
the material in which the vital powers of growth and repair and of
reproduction are located. A recognition of their importance is reflected in
the use of the name "protein," which comes from a Greek word meaning
"pre-eminence," or "of first importance."
In addition to the proteins which constitute the active protoplasm, plants
also contain large amounts of reserve, or stored, proteins, especially in
the seeds. In the early stages of growth, the proteins are present in
largest proportions in the vegetative portions of the plant; but as
maturity approaches, a considerable proportion of the protein material is
transferred to the seeds.
GENERAL COMPOSITION OF PROTEINS
The plant proteins are fairly uniform in their percentage composition. The
analyses of some sixteen different plant proteins show the following
maximum limits of percentages of the different chemical elements which they
contain: Carbon, 50.72-54.29; hydrogen, 6.80-7.03; nitrogen, 15.84-19.03;
oxygen, 20.86-24.29; sulfur, 0.17-1.09. Animal proteins vary more widely,
both in percentage composition and in properties, than do those of plant
origin.
Protein molecules are very large and, in the case of the so-called
"conjugated proteins" in particular, their structure is very complex.
The molecular weight of some of the proteins has been determined
directly, in the case of those particular ones which can be prepared in
proper form for the usual determination of molecular weight by the
osmotic pressure method; and has been computed for various others, from
the percentage of sulfur found on analysis, or (in the case of the
hæmoglobin of the blood) from the proportion by weight of oxygen
absorbed. From these determinations and computations, the following
formulas for certain typical proteins have been calculated: for zein
(from Indian corn), C_{736}H_{1161}N_{184}O_{208}S_{3}; for gliadin
(from wheat), C_{685}H_{1068}N_{196}O_{211}S_{5}; for casein (from
milk), C_{708}H_{1130}N_{180}O_{224}S_{4}P_{4}; for egg-albumin,
C_{696}H_{1125}N_{175}O_{220}S_{8}. These few examples will serve to
illustrate the enormous size and complexity of the protein molecule. The
conjugated proteins are still more complex than the simple proteins
whose formulas are here presented.
Fortunately for the purposes of the study of the chemistry of the proteins,
however, it has been found that most of the common plant proteins, known as
the "simple proteins," can easily be hydrolyzed into their constituent unit
groups, which are the comparatively simple amino-acids, whose composition
and properties are well understood. A study of the results of the
hydrolysis of some twenty common plant proteins has shown that it is rarely
possible to recover the amino-acids in sufficient quantities to account for
a full 100 per cent of the material used, the actual percentage of
amino-acids recovered usually totaling from 60 to 80 per cent. The
remaining material is supposed to be also composed of amino-acids which are
linked together in some arrangement which is not broken apart by any method
of hydrolysis which has yet been devised. This view is borne out by the
fact that substances which exhibit all the characteristic properties of
proteins have been artificially synthetized, by using only amino-acid
compounds. Animal proteins often show a much larger proportion of
unhydrolyzable material than do plant proteins.
AMINO-ACIDS AND PEPTID UNITS
The products of hydrolysis of the common simple proteins are all
amino-acids. These are ordinary organic acids with one (or more) of the
hydrogen atoms of the alkyl group replaced by a --NH_{2} (or sometimes by a
--NH--) group. They may be regarded as ammonia, NH_{3}, with one of its
hydrogen atoms replaced by an acid radical; or as the acid with one of its
hydrogens replaced by the NH_{2} group. For example, an amino-acid derived
from acetic acid, CH_{3}·COOH, is glycine, or amino-acetic acid,
CH_{2}NH_{2}·COOH; from propionic acid, CH_{3}·CH_{2}·COOH, there may be
obtained either [alpha]-amino-propionic acid, CH_{3}·CHNH_{2}·COOH, or
[beta]-amino-propionic acid, CH_{2}NH_{2}·CH_{2}·COOH, etc.
All of the amino-acids which result from the hydrolysis of proteins are
[alpha]-amino-acids, that is to say, the NH_{2} group is attached to the
[alpha]-carbon atom, i.e., the one nearest to the COOH group. Hence, the
general formula for all the amino-acids which are found in plants is
R·CHNH_{2}·COOH.
These amino-acids contain both the basic NH_{2} group and the acid COOH
group. For this reason, they very easily unite together, in the same way
that all acids and bases unite, to form larger molecules, the linkage
taking place between the basic NH_{2} group of one molecule and the acid
COOH group of the other, as indicated by the following equation:
R R R R
| ---------- | | |
HOOC·C·N-|H + HO|OC·C·NH_{2} = HOOC·C·N-OC·C·NH_{2} + H_{2}O
| | ---------- | | | |
H H H H H H
It is obvious that the compound thus formed still contains a free NH_{2}
group and a free COOH group, and is, therefore, capable of linking to
another amino-acid molecule in exactly the same way; and so on
indefinitely. In actual laboratory experiments, as many as eighteen of
these amino-acid units have been caused to unite together in this way, and
the resulting compounds thus artificially prepared have been found to
possess the characteristic properties of natural proteins.
These artificially prepared, protein-like, substances have been called
"polypeptides," and the individual amino-acids which unite together to form
them are called "peptides." Thus, a compound which contains three such
units linked together is called a "tripeptid"; one which contains four, a
"tetrapeptid." The use of the term "peptid" was suggested by the fact that
these amino-acids are produced from the hydrolysis of proteins by the
digestive enzyme _pepsin_.
The peptid units of any such complex as those which have been referred to
in the preceding paragraphs may be linked together in a great variety of
ways. Thus, in a tetrapeptid containing units which may be designated by
the letters _a_, _b_, _c_, and _d_, the arrangement may be in the orders
_abcd_, _bacd_, _acbd_, _dbca_, etc., etc. Similarly, the same peptid unit
may appear in the molecule in two or more different places. Hence, the
number of possible combinations of amino-acids into protein molecules is
very great. Further, it is possible that the peptid units in natural
proteins may be united together through other linkages than the one
illustrated above, as they often contain alcoholic OH groups in addition to
the basic NH_{2} groups, and these OH groups may form ester-linkages with
the acid (COOH) groups of other units. Still other acid and basic groups
are present in some of the amino-acids which have been found in natural
proteins, so that the possibility of variation in the polypeptid linkages
is almost limitless.
INDIVIDUAL AMINO-ACIDS FROM PROTEINS
About twenty different amino-acids have been isolated from the products of
hydrolysis of natural proteins, and this number is being added to from time
to time, as the methods of isolation and identification of these compounds
are improved. Many of these same amino-acids have been found in free form
in plant tissues, particularly in rapidly growing buds, or shoots, or in
germinating seeds, where they undoubtedly exist as intermediate products in
the transformation of proteins into other types of compounds.
These amino-acids, grouped according to the characteristic groups which
they contain, are as follows:
A. Monoamino-monocarboxylic acids:
Glycine, C_{2}H_{5}NO_{2}, CH_{2}NH_{2}·COOH, amino-acetic acid.
Alanine, C_{3}H_{7}NO_{2}, CH_{3}·CHNH_{2}·COOH, amino-propionic acid.
Serine, C_{3}H_{7}NO_{3}, CH_{2}OH·CHNH_{2}·COOH, oxy-amino-propionic
acid.
CH_{3}
\
Valine, C_{5}H_{11}NO_{2}, CH·CHNH_{2}·COOH,
/
CH_{3}
amino-isovalerianic acid.
CH_{3}
\
Leucine, C_{6}H_{13}NO_{2}, CH·CH_{2}·CHNH_{2}·COOH,
/
CH_{3}
amino-isocaproic acid.
CH_{3}
\
Isoleucine, C_{6}H_{13}NO_{2}, CH·CHNH_{2}·COOH,
/
C_{2}H_{5}
amino-methylethyl-propionic acid.
/ \
Phenylalanine, C_{9}H_{11}NO_{2}, | |CH_{2}·CHNH_{2}·COOH,
| | phenyl-amino-propionic acid.
\ /
/ \
Tyrosine, C_{9}H_{11}NO_{3}, | |CH_{2}·CHNH_{2}·COOH,
OH| | paraoxy-phenylalanine.
\ /
Cystine, C_{6}H_{12}N_{2}O_{4}S_{2}, HOOC·CHNH_{2}·CH_{2}S-SH_{2}C·
CHNH_{2}·COOH, di(thio-amino-propionic acid).
B. Monoamino-dicarboxylic acids:
Aspartic acid, C_{4}H_{7}NO_{4}, HOOC·CH_{2}·CHNH_{2}·COOH,
amino-succinic acid.
Glutamic acid, C_{5}H_{9}NO_{4}, HOOC·CH_{2}·CH_{2}·CHNH_{2}·COOH,
amino-glutaric acid.
C. Diamino-monocarboxylic acids:
Ornithine, C_{5}H_{12}N_{2}O_{2},
H_{2}N·CH_{2}·CH_{2}·CH_{2}·CHNH_{2}·COOH, di-amino-valerianic acid.
Lysine, C_{6}H_{14}N_{2}O_{2},
H_{2}N·CH_{2}·CH_{2}·CH_{2}·CH_{2}·CHNH_{2}·COOH,
di-amino-caproic acid.
Arginine, C_{6}H_{14}N_{4}O_{2},
NH_{2}
/
HN=C
\
NH·CH_{2}·CH_{2}·CH_{2}·CHNH_{2}·COOH,
guanidine-amino-valerianic acid.
Di-amino-oxysebacic acid, C_{11}H_{12}N_{2}O_{3}.
Di-amino-trioxydodecanic acid, C_{12}H_{26}N_{2}O_{3}.
D. Monoimido-monocarboxylic acids:
Proline, C_{5}H_{9}NO_{2}, H_{2}C---CH_{2}
| |
H_{2}C CH·COOH, pyrrolidine-carboxylic
\ / acid
N
|
H
Oxyproline, C_{5}H_{9}NO_{3}, proline with one (OH) group.
E. Monoimido-monoamino-monocarboxylic acids.
Histidine, C_{6}H_{9}N_{3}O_{2}, HC===C--CH_{2}·CHNH_{2}·COOH,
| | imidazole-amino-propionic
N NH acid.
\\/
C
|
H
Tryptophane, C_{11}H_{12}N_{2}O_{2},
/ \
| |---C--CH_{2}·CHNH_{2}·COOH, indole-amino-propionic acid.
| | ║
| | CH
\ /\ /
N
|
H
As has been said, other amino-acids are being found, from time to time, as
additional proteins are examined, or as better methods of examination of
the cleavage products of the natural proteins are devised.
COMPOSITION OF PLANT PROTEINS
The distribution of the different amino-acids in some of the different
plant proteins which have been examined in this way is shown in the
following table:
Letter code for column headings:
A = Gliadin (wheat).
B = Hordein (barley seed).
C = Zein (corn).
D = Legumin (vetch).
E = Edestin (hemp).
F = Globulin (squash seed).
G = Amandin (almonds).
--------------+-------+-------+-------+-------+-------+-------+-------
| A | B | C | D | E | F | G
--------------+-------+-------+-------+-------+-------+-------+-------
Glycine | 0.02 | 0.00 | 0.00 | 0.39 | 3.80 | 0.57 | 0.51
Alanine | 2.00 | 0.43 | 9.79 | 1.15 | 3.60 | 1.92 | 1.40
Valine | 0.21 | 0.13 | 1.88 | 1.36 | 6.20 | 0.26 | 0.16
Leucine | 5.61 | 5.67 | 19.55 | 8.80 | 14.50 | 7.32 | 4.45
Proline | 7.06 | 13.73 | 9.04 | 4.04 | 4.10 | 2.82 | 2.44
Phenylalanine | 2.35 | 5.03 | 6.55 | 2.87 | 3.09 | 3.32 | 2.53
Aspartic acid | 0.58 | ..... | 1.71 | 3.21 | 4.50 | 3.30 | 5.42
Glutamic acid | 42.98 | 43.19 | 26.17 | 18.30 | 18.84 | 12.35 | 23.14
Serine | 0.13 | ? | 1.02 | ? | 0.33 | ? | ?
Cystine | 0.45 | ? | ? | ? | 1.00 | 0.23 | ?
Tyrosine | 1.20 | 1.67 | 3.55 | 2.42 | 2.13 | 3.07 | 1.12
Arginine | 3.16 | 2.16 | 1.55 | 11.06 | 14.17 | 14.44 | 11.85
Histidine | 0.61 | 1.28 | 0.43 | 2.94 | 2.19 | 2.63 | 1.58
Lysine | ..... | ..... | ..... | 3.99 | 1.65 | 1.99 | 0.70
Tryptophane |present|present| absent|present|present|present|present
Ammonia | 5.11 | 4.87 | 3.64 | 2.12 | 2.28 | 1.55 | 3.70
+-------+-------+-------+-------+-------+-------+-------
| 71.46 | 78.16 | 85.27 | 62.65 | 82.38 | 55.77 | 59.00
--------------+-------+-------+-------+-------+-------+-------+-------
At the time when these analyses were made, a method for the quantitative
estimation of tryptophane had not been devised, although one is now
available. The addition of the percentages of tryptophane and of other
amino-acids for which methods of determination are not yet known, would
bring the total, in each case, more nearly up to the full 100 per cent.
These data will serve to show how widely the different plant proteins vary
in the proportions of the different amino-acids which they contain. Animal
proteins have been found to be still more variable in composition.
In the use of the proteins as food for animals, it appears that the
different amino-acids are in some way connected with the different
physiological functions which the proteins have to perform in the animal
body: thus, _tryptophane_ is absolutely essential to the maintenance of
life, but does not promote growth; _lysine_, on the other hand, definitely
promotes growth, so that animals which have been maintained without any
increase in weight for many months immediately begin to grow when furnished
with a diet in which lysine is a constituent; while _arginine_ seems to be
definitely associated with the reproductive function; and _cystine_, with
the growth of hair, feathers, etc. It is not known whether there is any
similar relation of amino-acids to the functions of different proteins in
plant metabolism.
The separation of the individual amino-acids from the mixture which results
from the hydrolysis of any given protein is a long and tedious process and,
at best, yields only moderately satisfactory results. For that reason, it
has recently been almost entirely abandoned in favor of the separation
devised by Van Slyke, which divides the total nitrogenous matter in the
mixture resulting from the hydrolysis of a protein into the following
groups; ammonia N, humin (or melanin) N, cystine N, arginine N, histidine
N, lysine N, amino N of the filtrate, and non-amino N of the filtrate.
These groups can be conveniently and fairly accurately separated out of the
hydrolysis mixture, by means of various precipitating agents, and the
quantity of N in the several precipitates determined by the usual Kjeldahl
method. The actual process for these separations need not be discussed
here, as it is given in detail in all standard text-books dealing with the
methods of biochemical analysis. The distribution of the nitrogen in any
given protein into these various groups is characteristic for that
particular protein, and the process serves both as a means of
identification of individual proteins and a method for tracing their
changes through various vital, or biochemical, transformations.
GENERAL PROPERTIES OF THE PROTEINS
Individual proteins differ slightly in their characteristics, but in
general they are all alike in the following physical and chemical
properties.[5]
=Physical Properties.=--(1) The proteins are all
_colloidal_ in character, that is, they form solutions in water, out of
which they cannot be dialyzed through parchment, or other similar
membranes. (2) All natural proteins, when in colloidal solution, may be
_coagulated_, forming a semi-solid _gel_, which cannot again be rendered
soluble except by decomposition. The most familiar example of this type of
coagulation is that of egg-albumin, when eggs are cooked. This coagulation
may be produced by heat, by the action of certain enzymes, or by the
addition of alcohol to the solution. (3) All solutions of plant proteins
are optically active, rotating the plane of polarized light to the left, in
every case. (4) Proteins are precipitated out of their solutions, without
change in the composition of the protein, by saturating the solution with
various neutral salts of the alkali, or alkaline earth, metals, such as
sodium chloride, ammonium sulfate, magnesium sulfate, etc. This is only
another way of saying that the proteins are insoluble in strong salt
solutions. Separation from solution by the addition of salts is different
from coagulation by heat, etc., as in this case simple dilution of the salt
solution will cause the protein to redissolve, whereas a coagulated protein
cannot be redissolved without some change in its composition.
=Chemical Properties.= (1) Precipitation reactions.--The proteins have both
acid and basic properties (due to the presence in their molecules of both
free NH_{2} groups and free COOH groups). Bodies of this kind are known as
"amphoteric electrolytes," since they yield both positive and negative
ions, if dissociated. The proteins readily form salts, which are generally
insoluble in water, with strong acids. For this reason, they are generally
precipitated out of solution by the addition of the common mineral acids.
They are also precipitated by many of the "alkaloidal reagents," to which
reference has been made in the preceding chapter, namely, phosphotungstic,
phosphomolybdic, tannic, picric, ferrocyanic, and trichloracetic acids, the
double iodide of potassium, mercuric iodide, etc. The precipitates produced
by strong mineral acids are often soluble in excess of the acid, with the
formation of certain so-called "derived proteins," which are probably
products of the partial hydrolysis of the protein.
The proteins are also precipitated out of solution by the addition of small
amounts of salts of various heavy metals, such as the chlorides, sulfates,
and acetates of iron, copper, mercury, lead, etc. This precipitation is
different than that caused by the saturation of the solution with the salts
of the alkali metals, as in this case the metal unites with the protein to
form definite, insoluble salts, which cannot be redissolved except by
treatment with some reagent which removes the metal from its combination
with the protein (hydrogen sulfide is commonly used for this purpose).
(2) Color reactions.--Certain specific groups which are present in most
proteins give definite color reactions with various reagents. It is
apparent that any individual protein will respond to a particular color
reaction, or will not do so, depending upon whether the particular group
which is responsible for the color in question is present in that
particular protein. Color reactions to which most of the common plant
proteins respond are the following ones:
(_a_) _Biuret Reaction._--Solutions of copper sulfate, added to an alkaline
solution of a protein, give a bluish-violet color if the substance contains
two, or more, --CONH-- groups united together through carbon, nitrogen, or
sulfur atoms. Inasmuch as most natural proteins contain several such
groups, the biuret reaction is a very general test for proteins.
(_b_) _Millon's Reaction._--A solution of mercuric nitrate containing some
free nitrous acid (Millon's reagent) produces a precipitate which turns
pink or red, whenever it is added to a solution which contains tyrosine, or
a tyrosine-containing protein.
(_c_) _Xanthoproteic Acid Reaction._--This is the familiar yellow
coloration which is produced whenever nitric acid comes in contact with
animal flesh. It is caused by the action of nitric acid on tyrosine. The
color is intensified by heating, and is changed to orange-red by the
addition of ammonia.
(_d_) _Adamkiewicz's Reaction._--If concentrated sulfuric acid be added to
a solution of a protein to which some acetic acid (or better, glyoxylic
acid) has previously been added, a violet color is produced. This color
will appear as a ring at the juncture of the two liquids, if the sulfuric
acid is poured carefully down the sides of the tube, or throughout the
mixture if it is shaken up. It depends upon the interaction of the
glyoxylic acid (which is generally present as an impurity in acetic acid)
upon the tryptophane group, and is therefore given by all proteins which
contain tryptophane.
(_e_) _Molisch's reaction_ for furfural will be shown by those proteins
which contain a carbohydrate group. In applying this test, the solution to
be tested is first treated with a few drops of an alcoholic solution of
[alpha]-naphthol, and then concentrated sulfuric acid is poured carefully
down the sides of the test-tube. If carbohydrates are present, either free
or as a part of a protein molecule, a red-violet ring forms at the juncture
of the two liquids.
(_f_) _Sulfur Test._--If a drop of a solution of lead acetate be added to a
solution containing a protein, followed by sufficient sodium hydroxide
solution to dissolve the precipitate which forms, and the mixture is heated
to boiling, a black or brown coloration will be produced if the protein
contains cystine, the sulfur-containing amino-acid.
FOOTNOTES:
[5] Since the proteins are essentially _colloidal_ in nature, many of the
terms used in the discussions of their properties, and these properties
themselves, will be better understood after the chapter dealing with the
colloidal condition of matter has been studied. A more logical arrangement
so far as the systematic study of these properties is concerned would be to
take up chapter XV before undertaking the study of the proteins (this order
is actually followed in some texts on Physiological Chemistry). But from
the standpoint of the consideration of the various groups of organic
components of plants, it seems a better arrangement to consider these
groups in sequence, and then to discuss the various physical-chemical
phenomena which govern their activity. However, it is recommended that the
student refer at once to Chapter XV for an explanation of any terms used
here, which may not be familiar to him; and that after the study of Chapter
XV, he return to this chapter dealing with the proteins for an illustrative
study of the applications of the principles presented there.
THE CLASSIFICATION OF THE PROTEINS
Formerly, the classification of proteins was based almost wholly upon their
solubility and coagulation reactions. More recently, since their products
of hydrolysis have been extensively studied, their classification has been
modified, in attempts to make it correspond as closely as possible to their
chemical constitution and physical properties. As knowledge of these
matters progresses, the schemes of classification change. On that account,
no one definite scheme is universally used. For example, the English system
varies considerably from the one commonly used by American biochemists,
which is the one presented below.
The proteins are divided into three main classes, as follows:
(1) Simple proteins, which yield only amino-acids when hydrolyzed.
(2) Conjugated proteins, compounds of proteins with some other non-protein
group.
(3) Derived proteins, decomposition products of simple proteins.
The first two of these classes comprise all the natural proteins; while the
third includes the artificial polypeptides and proteins which have been
modified by reagents.
These major classes are further subdivided into the following sub-classes,
which depend in part upon the solubilities of the individual proteins, and
in part upon the nature of their products of hydrolysis:
1. _The Simple Proteins_
A. Albumins--soluble in water and dilute salt solutions, coagulated
by heat.
B. Globulins--insoluble in water, soluble in dilute salt solutions,
coagulated by heat.
C. Glutelins--insoluble in water or dilute salt solutions, soluble
in dilute acids or alkalies, coagulated by heat.
D. Prolamins--insoluble in water, etc., soluble in 80 per cent
alcohol.
E. Histones--soluble in water, insoluble in ammonia, not coagulated
by heat.
F. Protamines--soluble in water and ammonia, not coagulated by
heat, yielding large proportions of diamino-acids on hydrolysis.
G. Albuminoids--insoluble in water, salt solutions, acids, or
alkalies.
2. _Conjugated Proteins_
A. Chromoproteins--compounds of proteins with pigments.
B. Glucoproteins--compounds of proteins with carbohydrates.
C. Phosphoproteins--proteins of the cytoplasm, containing
phosphoric acid.
D. Nucleoproteins--proteins of the nucleus, containing nucleic
acids.
E. Lecithoproteins--compounds of proteins with phospholipins.
F. Lipoproteins--compounds of proteins with fats, existence in
nature doubtful, artificial forms easily prepared.
3. _Derived Proteins_
A. Primary protein derivatives.
_a._ Proteans--first products of hydrolysis, insoluble in
water.
_b._ Metaproteins--result from further action of acids or
alkalies, soluble in weak acids and alkalies, but
insoluble in dilute salt solutions.
_c._ Coagulated proteins--insoluble forms produced by the
action of heat or alcohol.
B. Secondary protein derivatives.
_a._ Proteoses--products of hydrolysis, soluble in water,
not coagulated by heat, precipitated by saturation of
solution with ammonium sulfate.
_b._ Peptones--products of further hydrolysis soluble in water,
not coagulated by heat, not precipitated by ammonium
sulfate, give biuret reaction.
_c._ Peptides--individual amino-acids, or poly-peptides, may or
may not give biuret reaction.
The plant proteins which have been investigated, thus far, fall into these
groups as follows:
1A. _Albumins_
Leucosin, found in the seeds of wheat, rye, and barley.
Legumelin, " " pea, horse-bean, vetch, soy-bean,
lentil, cowpea, adzuki-bean.
Phaselin, " " kidney-bean.
Ricin, " " castor-bean.
1B. _Globulins_
Legumin, found in the seeds of pea, horse-bean, lentil and vetch.
Vignin, " " cowpea.
Glycinin, " " soy-bean.
Phaseolin, " " beans (_Phaseolus spp._)
Conglutin, " " lupines.
Vicilin, " " pea, horse-bean, lentil.
Corylin, " " hazel nut.
Amandin, " nuts of almond and peach.
Juglansin, " seeds of walnut and butternut.
Excelsin, " " Brazil nut.
Edestin, " hemp seed.
Avenalin, " oats.
Maysin, " corn.
Castanin, " the seeds of European chestnut.
Tuberin, " potato tubers.
And, crystalline globulins found in the seeds of flax, squash, castor-bean,
sesame, cotton, sunflower, radish, rape, mustard, and in cocoanuts,
candlenuts, and peanuts.
1C. _Glutelins_
Glutenin, found in the seeds of wheat.
Oryzenin, " " rice.
1D. _Prolamins_
Gliadin, found in the seeds of rye, wheat, with glutenin forms
"gluten."
Hordein, " " barley.
Zein, " " corn.
1E-1G. _Histones, Protamines and Albuminoids._--So far as is now known, no
representatives of these classes are found in plants.
2. Conjugated Proteins.--There is no conclusive evidence of the existence
in plants of any of the conjugated proteins, other than the nucleoproteins
and the chromoproteins, the composition and properties of which have been
discussed in previous chapters. The nucleoproteins undoubtedly occur in the
embryos of many, if not all, seeds.
3. Derived Proteins.--Representatives of the various types of derived
proteins are undoubtedly found as temporary intermediate products in
plants, both as products of hydrolysis produced during the germination of
seeds and as intermediate forms in the synthesis of proteins. So far as is
known, however, they do not occur as permanent forms in any plant tissues.
They have been prepared in large numbers and quantities, by the hydrolysis
of the natural proteins and the artificial synthesis of polypeptides.
In the present state of our knowledge concerning the functioning of the
proteins, no significance in the physiology of plant life, or metabolism,
is to be attached to the particular type of protein material which it
contains, at least so far as the simple proteins of the cytoplasm are
concerned.
DIFFERENCES BETWEEN PLANT AND ANIMAL PROTEINS
A much larger variety of protein materials is found in animal tissues than
in plants. This is undoubtedly because different animal organs perform so
much more varied physiological functions than do those of plants. Three
groups of simple proteins, the histones, the protamines, and the
albuminoids, which are quite common in animal tissues, are entirely unknown
in plants. Further, conjugated proteins of greater complexity and more
varied structure are found in animal tissues, especially in the brain,
nerve-cells, etc., than in plants.
Plant proteins, in general, usually contain larger proportions of proline
and of glutamic acid than are found in animal proteins; also more arginine
than is found in any of the animal proteins except the protamines, which
contain as high as 85 per cent of this amino-acid.
Of the twenty-five plant proteins which have thus far been hydrolyzed and
studied from this standpoint, all contained leucine, proline,
phenylalanine, aspartic acid, glutamic acid, tyrosine, histidine, and
arginine; two gave no glycine; two others, no alanine; four contained no
lysine; and one, no tryptophane. Zein, the principal protein of corn
contains no glycine, lysine, or tryptophane. It is not sufficient to
support animal life and promote growth, if used as an exclusive source for
protein for food.
THE EXTRACTION OF PROTEINS FROM PLANT TISSUES
Since proteins are indiffusible, it is essential that the cell-walls of the
tissue shall be thoroughly ruptured as the first step in any process for
the extraction of these compounds from plant tissues. This is usually
accomplished by grinding the material as finely as possible, preferably
with the addition of sharp quartz sand, or broken glass, to aid in the
tearing of the cell-wall material.
The solvent to be used in extracting the proteins from this finely ground
material depends upon the nature and solubility of the proteins which are
present, and also upon whether it is desired to separate the proteins which
may be present in the plant, during the process of the extraction. A glance
at the scheme of classification of the proteins will show the following
solubilities which serve as a guide to the procedure to be followed: (_a_)
proteoses, albumins, and some globulins may be extracted with water; (_b_)
globulins and most of the water-soluble proteins may be extracted by using
a 10 per cent solution of common salt; (_c_) prolamines are extracted by
70-90 per cent alcohol; glutelins and prolamins dissolve in dilute acids or
dilute alkali.
A common procedure is to extract groups (_a_) and (_b_), using a 10 per
cent salt solution as the solvent, and then to separate the albumins,
globulins, etc., from this solution by suitable precipitants; then to treat
the material with 80 per cent alcohol, to extract the prolamines; and
finally with dilute alkali, to extract the glutelins. The dissolved
proteins in each extract can be subsequently purified by dialysis,
precipitation, etc. The insoluble proteins can be studied only after
removing the other materials associated with them in the tissue, by
suitable mechanical or chemical means.
THE SYNTHESIS OF PROTEINS IN PLANTS
The synthesis of proteins in plants is not a process of photosynthesis, as
it can take place in the dark and in the absence of chlorophyll, or any
other energy-absorbing pigment. However, protein-formation normally takes
place in conjunction with carbohydrate-formation. The carbon, hydrogen,
and oxygen necessary for protein synthesis are undoubtedly obtained from
carbohydrates. The nitrogen and sulfur come from the salts absorbed from
the soil through the roots and brought to the active cells in the sap.
Atmospheric nitrogen cannot be used by plants for this purpose, except in
the case of certain bacteria and other low plants, notably the bacteria
which live in symbiosis with the legumes in the nodules on the roots of the
host plants. In general, the sulfur must come in the form of sulfates and
the nitrogen in the form of nitrates; although many plants can make use of
ammonia for protein-formation. Presumably, the nitrate nitrogen must be
reduced in the plant to nitrites, and then to ammonia form, in order to
enter the amino-arrangement required for the greater proportion of the
protein nitrogen.
The mechanism by which ammonia nitrogen becomes amino-acids in the plant is
not understood. Artificial syntheses of amino-acids, by the action of
ammonia upon glyoxylic acid and sorbic acid, both of which occur in plants
and may be obtained by the oxidation of simple sugars, have been
accomplished, and it seems probable that similar reactions in the plant
protoplasm may give rise to the various amino-acids which unite together to
form proteins. Nothing is known, however, of the process by which the more
complicated closed-ring amino-acid compounds, such as proline, histidine,
or tryptophane, are synthetized.
The condensation of amino-acids into proteins, or the reverse
decomposition, is very readily accomplished in all living protoplasm, under
the influence of special protein-attacking enzymes, which are almost
universally present in the cytoplasm. These reactions in connection with
the proteins are similar to the easy transformation of sugars to starches,
and _vice versa_, under the action of the corresponding
carbohydrate-attacking enzymes.
PHYSIOLOGICAL USES OF PROTEINS
There can be no doubt that the all-important rôle of proteins, in either
plant or animal tissue, is to furnish the colloidal protoplasmic material
in which the vital phenomena take place. Their occurrence in seeds, and
other storage organs, is, of course, in order to provide the
protoplasm-forming material for the young seedling plant.
They are, moreover, the source for the material which goes into some of the
secretion groups of organic compounds; as they are easily broken down by
various agents of decomposition into nitrogen-free alcohols, aldehydes, and
acids, which produce the essential oils, pigments, etc.
Much, if not all, of their physiological activity is due to their colloidal
nature, the importance and effects of which will be more apparent after the
chapters dealing with the colloidal condition of matter and with the
physical chemistry of protoplasm have been studied.
REFERENCES
ABDERHALDEN, E.--"Neuere Ergebnisse auf dem Gebiete der Speziellen
Eiweisschemie," 128 pages, Jena, 1909.
FISCHER, E.--"Untersuchungen über Aminosäuren, Polypeptide, und Proteine,
1899-1906," 770 pages, Berlin, 1906.
MANN, G.--"Chemistry of the Proteids," 606 pages, London, 1906.
OSBORNE, T. B.--"The Vegetable Proteins," 138 pages, _Monographs_ on
Biochemistry, London, 1909.
PLIMMER, R. H. A.--"The Chemical Constitution of the Proteins, Part I,
Analysis," 188 pages; and "Part II, Synthesis, etc." 107 pages,
_Monographs_ on Biochemistry, London, 1917. (3d ed.).
ROBERTSON, T. B.--"The Physical Chemistry of the Proteins," 477 pages, New
York, 1918.
SCHRYBER, S. B.--"The General Characters of the Proteins," 86 pages,
_Monographs_ on Biochemistry, London, 1909.
UNDERHILL, F. P.--"The Physiology of the Amino-acids," 169 pages, 13 figs.
1 plate. Yale University Press, 1915.
CHAPTER XIV
ENZYMES AND THEIR ACTION
The characteristic difference between the reactions of inorganic compounds
and those of organic substances lies in the rapidity, or velocity, of the
chemical changes involved. Speaking generally chemical reactions take place
between substances which are in solution, so that they may come into
sufficiently intimate contact that chemical action between them can take
place. There are, of course, occasional examples of reactions between dry
solids, such as the explosion of gunpowder, etc., but the general rule is
that reacting materials must be in either colloidal or true solutions.
Inorganic materials, when dissolved in water, usually ionize very readily.
That is, they are not only disintegrated into individual _molecules_, but a
considerable proportion of these molecules separate into their constituent
_ions_. When solutions containing ionized compounds are brought together,
conditions for chemical interaction are ideal, and the reaction proceeds
with such tremendous rapidity as to be completed almost instantaneously, in
most cases.
Organic compounds, on the other hand, ionize only very slowly, if at all.
Hence, reactions between organic compounds, even when they are in solution,
proceed very slowly unless carried on at high temperatures, under increased
pressure, or under the influence of some catalytic agent. Even under the
stimulation of these reaction-accelerating agencies, most chemical changes
in organic compounds when carried on in the laboratory, require several
hours or even days and sometimes weeks, for their completion. But when
similar reactions take place in living organisms, they proceed with
velocities which resemble those of inorganic compounds in the laboratory.
This difference between the velocity of organic reactions when carried on
under artificial conditions in the laboratory (often spoken of as "_in
vitro_") as compared with that of the same reactions when they take place
in a living organism ("_in vivo_"), is due to the universal presence in
the living protoplasm of certain organic catalysts, known as _enzymes_.
ENZYMES AS CATALYSTS
The phenomenon known as "catalysis" is of common occurrence in both
inorganic and organic chemistry. The effect of a small amount of manganese
dioxide in aiding in the liberation of oxygen from potassium chlorate is an
example which is familiar to all students of elementary chemistry.
Similarly, spongy platinum accelerates the oxidation of sulfur dioxide to
sulfur trioxide, in the commercial manufacture of sulfuric acid. Again, the
hydrolysis of sucrose into fructose and glucose proceeds very slowly in the
presence of water alone, but if a little hydrochloric acid or sulfuric acid
be added to the solution, the velocity of the hydrolysis is enormously
accelerated. Many other examples of the accelerating effect of various
chemicals upon reactions into which they do not themselves enter, might be
cited.
The essential features of all such catalytic actions are: (1) the velocity
of the reaction is greatly altered, usually accelerated; (2) the catalytic
agent does not appear as one of the initial substances, or end-products, of
the reaction, and is not itself altered by the chemical change which is
taking place; (3) the accelerating effect is directly proportional to the
amount of the catalyst which is present; (4) relatively small amounts of
the catalyst produce very large results in the reacting mixture; and (5)
the catalysts cannot themselves initiate reactions, but only influence the
velocity of reactions which would otherwise take place at a different rate
(usually much more slowly) in the absence of any catalytic agent.
Enzymes conform to all of these properties of catalysts, and are commonly
defined as the "catalysts of living matter." They are almost universally
present in living organs of every kind, and perform exceedingly important
functions, both in the building-up of synthetic materials and in the
rendering soluble of the food of both plants and animals, so that it can be
translocated from place to place through the tissues of the organism.
Enzymes differ from inorganic catalysts in being destroyed by heat, in not
always carrying the reaction to the same stage as does the inorganic
catalyst which may accelerate the same reaction, and in producing
different changes in the same substance by different enzymes.
The name "enzyme" comes from Greek words meaning "in yeast," as the nature
and effect of the enzyme involved in the alcoholic fermentation of sugars
by yeast were those which were first recognized and understood. It was at
first thought, by Pasteur and his students, that fermentation is the direct
result of the life activities of the yeast plant. Later, it was found that
water extracts from sprouted barley, from almond seeds, and from the
stomach, pancreas, etc., were able to bring about the decomposition of
starch, of amygdalin, and of proteins, respectively, in a way which seemed
to be quite comparable to the fermentative action of yeasts. Hence, it was
thought that there were two varieties of active agents of this kind, one
composed of living cells and the other non-living chemical compounds, and
these were called the "organized ferments" and the "unorganized ferments,"
respectively. However, in 1897, Büchner found that by grinding yeast cells
with sharp sand until they were completely disintegrated and then
submitting the mass to hydraulic pressure, he could obtain a clear liquid,
entirely free from living cells, which was just as active in producing
fermentation as was the yeast itself. This discovery paved the way for a
long series of investigations, which have conclusively demonstrated that
there is no distinction between "organized" and "unorganized" ferments,
that all living organisms perform their characteristic functions by means
of the enzymes which they contain, and that these enzymes can bring about
their characteristic catalytic effects outside the cell, or tissue which
elaborates them, just as well as within it, provided only that the
conditions of temperature, acidity or alkalinity of the medium, etc., are
suitable for the particular enzyme action which is under consideration.
GENERAL PROPERTIES OF ENZYMES
Since enzymes are catalysts, it is plain that an accurate description of
their activity should, in each case, refer to the influence which they
exert upon some definite reaction velocity. But since the phrases necessary
to describe such an effect are cumbersome and inconvenient, and since most
of the reactions which are accelerated by the catalytic action of enzymes
are either simple hydrolyses, changes in oxygen content, or other simple
decompositions or condensations, which will otherwise proceed so slowly as
to be practically negligible, it is customary to speak of the enzyme as
"acting upon" the material in question. It should be understood, however,
that this is a misstatement, as the enzyme cannot actually initiate a
reaction, or "act upon" any substance; it only acts as a catalyzer to
accelerate the action of water, oxygen, etc., upon the material in
question.
Generally speaking, most enzymes are colloidal in form and, hence, do not
diffuse through membranes such as the cell-walls. Some of them perform
their characteristic functions only within the cell, or organ, which
elaborates them, and can be obtained outside these tissues for purposes of
study only by first rupturing the cell-wall or other membrane with which
they are surrounded. Such enzymes are known as "intracellular." Others are
regularly secreted by glands which discharge them onto other organs, as the
stomach or intestines of animals, where they perform their useful
functions; or, as in the case of germinating seeds, they move to other
parts of the organ, and can be extracted from the tissue by simple
treatment with water. These are known as the "extracellular" enzymes.
Enzymes are specific in their action. Any given enzyme affects only a
single reaction; or at most acts only upon a single group of compounds
which have similar molecular configuration. Usually it is only a single
compound whose decomposition is accelerated by the action of a particular
enzyme; but there are a few enzymes, such as _maltase_ (which acts on all
[alpha]-glucosides) and emulsin (which acts on all [beta]-glucosides) which
act catalytically upon groups of considerable numbers of similar compounds.
Enzymes, like all other catalysts, act more energetically at increased
temperatures; but for each particular enzyme there is an "optimum
temperature," (usually between 40° and 65°) above which the destructive
effect of the temperature upon the enzyme itself more than offsets the
accelerating influence of the increased temperature. At still higher
temperatures (usually 80° to 100°) the enzymes are "killed," i.e., rendered
permanently inactive. All enzymes are "killed" by boiling the solutions in
which they are contained. Dry preparations of enzyme material can withstand
somewhat higher temperatures, for somewhat longer periods of time, than can
the same enzyme in moist condition or in solution. When an enzyme has once
been inactivated by heating, or "killed," it can never be restored to
activity again.
Enzymes are extremely sensitive to acids, bases, or salts, their activity
being often enormously enhanced or, in other cases, entirely inhibited, by
the presence in the reacting medium of very small amounts of free acids, or
bases, or even of certain neutral salts. For example, pepsin, the enzyme of
the stomach will act only in the presence of a slightly acid medium and is
wholly inactive in a mixture which contains even the slightest amount of
free alkaline material; while trypsin, the similar enzyme of the intestine,
acts only under alkaline conditions. Practically all enzymes are rendered
inactive, but not destroyed, by the presence of either acid or alkali in
excess of N/10 strength. Many will act only in the presence of small
quantities of certain specific neutral salts; while, on the other hand,
other salts are powerful inhibitors of enzyme action. Enzymes often differ
from the protoplasm which secretes them in their response to antiseptics,
such as toluene, xylene, etc., which inhibit the activity or growth of the
cell, but have no effect upon the activity of the enzymes which it
contains.
THE CHEMICAL NATURE OF ENZYMES
Nothing is known with certainty concerning the chemical nature of enzymes.
Being colloidal in nature, they adsorb carbohydrates, proteins, fats, etc.,
so that active enzyme preparations often respond to the characteristic
tests for these groups of substances; and many investigators have reported
what has, at first, seemed to be conclusive evidence that some particular
enzyme which they have studied is either a carbohydrate, a protein, or some
other type of organic compound. Later investigations have always shown,
however, that if the preparation in question be submitted to the digestive
action of the enzymes which hydrolyze the particular type of substances to
which it is supposed to belong, the material will lose its characteristic
protein, or carbohydrate, etc., properties, without losing its specific
activity, thus clearly indicating that the substance which responds to the
characteristic tests for some well-known type of organic compounds is
present as an impurity and is not the enzyme itself.
The present state of knowledge concerning the nature of enzymes seems to
indicate that, like the inorganic catalysts, they may vary widely in
chemical composition; and that their tremendous catalytic effects are due,
in part at least, to their colloidal nature. This will be better understood
and appreciated after the phenomena associated with the colloidal condition
have been considered (see the following chapter).
NOMENCLATURE AND CLASSIFICATION
Since nothing is known of the chemical composition of enzymes, they can
only be studied by considering the effects which they produce. This is
reflected in the systems which have been adopted for their nomenclature and
classification.
As they were first supposed to be proteins, the earlier representatives of
the group were given characteristic names ending with the suffix _in_,
similar to that of the proteins. Since this idea has been found to be
incorrect, however, a system of nomenclature has been adopted which assigns
to each enzyme the name of the material upon which it acts, followed by the
suffix _ase_. Thus, cellulase is the enzyme which accelerates the
hydrolysis of cellulose; glucase, that acting upon glucose; amylase, that
acting upon starch (_amylum_), etc.
The substance upon which the enzyme acts (or, strictly speaking, the
substance whose hydrolysis, oxidation, or other chemical change, is
catalytically affected by the enzyme) is called the _substrate_.
Most enzymes are catalysts for hydrolysis reactions and are, hence, classed
as _hydrolytic_ in their action, and may be spoken of as "hydrolases."
Those which accelerate oxidation are called "oxidases"; while those that
stimulate reduction reactions are "reductases"; those that aid in the
splitting off of ammonia, or amino-acid groups, are "deaminases"; and those
that aid in the splitting off of CO_{2} from COOH groups are
"carboxylases," etc.
The hydrolytic enzymes are further subdivided into the sucroclastic
(sugar-splitting), or sucrases; the lipoclastic (fat-splitting), or
lipases; the esterases (ester-splitting); proteoclastic
(protein-splitting), or proteases; etc.
OCCURRENCE AND PREPARATION FOR STUDY
Enzymes are present in all living matter. In animal tissues, they occur in
the largest amounts in those glands or organs where active vital processes
take place, as in the brain, the digestive tract, blood, etc. In plants,
they may be found in all living cells, and are especially abundant in the
seeds, where they serve to render soluble and available to the young plant
the stored food materials. The enzymes of moulds, and other parasitic
plants, are usually extracellular in type, being secreted for the purpose
of making the material of the host plant available to the parasite.
Extracellular enzymes are also developed in seeds during germination, in
order that the stored food material of the endosperm may be rendered
soluble and translocated into the tissues of the growing seedling. But most
other plant enzymes are intracellular in type. Hence, in all preparations
of plant enzymes for study, or for commercial use, the first step in the
process is, necessarily, a thorough rupturing of the cell-walls of the
plant material.
The rupturing of the cells may be accomplished in a variety of ways, as
follows: (1) mechanical disintegration, as by grinding in a mortar with
sharp sand; (2) freezing the material, by treatment with liquid air, then
grinding; (3) killing the cells by drying, by treatment with alcohol or
acetone, then grinding the mass in a paint mill with toluene; (4) killing
the cells by chemicals (sulfuric acid, 0.5 to 1.0 per cent, or other
suitable agents) followed by extraction with water; (5) autolysis, or
self-digestion, in which the cells are mixed with toluene or some other
antiseptic which kills the cells without injuring the enzymes, then the
material is minced or ground up and suspended in water containing the
antiseptic, until the enzymes dissolve the cell-walls and so escape into
the liquid--this process being especially adapted to the preparation of
active extracts from yeasts, which contain the necessary cell-wall
dissolving enzymes to facilitate autolysis.
Enzymes may be separated out of the aqueous extract obtained from cells
ruptured by any of the above methods, by precipitation with alcohol,
acetone, or ether, in which they are insoluble; but if this is done, the
precipitate must be at once filtered off and rapidly washed and dried, as
prolonged contact with these precipitating agents greatly diminishes the
activity of most enzymes. Or, they may be adsorbed out of solution on
gelatinous, or colloidal, materials, like aluminium hydroxide, or various
hydrated clays. If the dry preparations obtained in any of these ways are
contaminated by carbohydrates, proteins, etc., these may be removed by
treatment with suitable digesting enzymes obtained from the saliva,
gastric, and pancreatic juices, and the digested impurities washed out with
60 to 80 per cent alcohol, leaving the enzyme preparation in a purified but
still active form.
In any study of the "strength," or possible catalytic effects, of an enzyme
preparation, it is necessary, first, to determine what particular reaction
it affects, by qualitative tests with various substrate materials, such as
starch, sugars, glucosides, proteins, etc., and then to determine
quantitatively its accelerating effect upon the reaction in question. The
latter may be done by measuring either the _time_ required to carry a unit
quantity of the substrate material through any determined stage of chemical
change, or the _quantity_ of the substrate which is changed in a unit
period of time. It would not be profitable to go into a detailed discussion
here of the methods of making these quantitative measurements of enzyme
activity. Such discussions must necessarily be left to special treatises on
methods of study of enzyme action. It may be said, however, that generally
both the qualitative tests for, and the quantitative measurements of, the
accelerating influence of enzymes depend upon the observation of some
change in the physical properties of the substrate material, such as the
optical activity, electrical conductivity, or viscosity, of its solution.
In some cases, it is convenient to make an actual quantitative
determination of the amount of end-products produced in a given time, as in
the inversion of cane sugar, the hydrolysis of maltose, etc., but such
determinations necessarily involve the removal of some of the reaction
mixture for the purposes of the determinations, and are not, therefore,
suitable for the study of the progressive development of the reaction which
is being studied.
Enzymes are found in all parts of the animal organism and those which are
active in the digestion of food, the metabolism of digested material, the
coagulation of blood, etc., have been extensively studied. A discussion of
these animal enzymes would be out of place in such a text as this, however,
and the following list includes only enzymes which are known to occur in
plant tissues. These well-known enzymes will serve as examples of the
several general types which have thus far been isolated and studied.
------------------- -+-----------+-------------+-------------+-----------
Class and Type. | Enzyme. | Substrate. |End-products.| Found in.
---------------------+-----------+-------------+-------------+-----------
I. Hydrolases | | | |
(_a_) Esterases |Lipase |Fats |Glycerol and |Oily seeds
| | |fatty acids |
| | | |
(_b_) Carbohydrases |Sucrase or |Sucrose |Glucose and |Yeasts
|invertase | |fructose |
| | | |
|Maltase |Maltose and |Glucose, etc.|Barley malt
| |all [alpha]- | |
| |glucosides | |
| | | |
|Dextrinase |Dextrin |Maltose |Malt
| | | |
|Inulase |Inulin |Fructose |Artichokes,
| | | |etc.
| | | |
|Amylase or |Starch |Maltose |Malt, etc.
|diastase | | |
| | | |
|Cellulase |Cellulose |Maltose |Bacteria
| | | |
|Pectinase |Pectose |Arabinose |Fruits
| | | |
|Cytase |Hemi- |Mono- |Nuts,
| |celluloses |saccharides |seeds, etc.
| | | |
(_c_) Glucosidases |Emulsin |Amygdalin |Glucose, |Almond
| |and all |etc. |kernels,
| |[beta]- | |etc.
| |glucosides | |
| | | |
|Maltase |All [alpha]- |Glucose, |Barley malt
| |glucosides |etc. |
| | | |
|Myrosin |Sulfur- |Glucose, |Mustard
| |containing |etc. |seeds
| |glucosides | |
| | | |
|Rhamnase |Xanthorhamnin|Rhamnose, |_Rhamnus
| | |etc. |spp._
| | | |
|Phytase |Phytin |Inosite and |Bran coats
| | |H_{3}PO_{4} |of seeds
| | | |
(_d_) Proteases |Erepsin |Proteins |Amino-acids |Many plants
| | | |
|Papain |Proteins |Amino-acids |Papaws
| | | |
|Bromelin |Proteins |Amino-acids |Many plants
| | | |
|Nuclease |Nucleo- |Proteins and |Many plants
| |proteins |nucleic acid |
| | | |
II. Oxidases | | | |
(_a_) Catalases | ........ |Hydrogen |Water and |Nearly
| |peroxide |oxygen |all plants
| | | |
(_b_) Peroxidases | ........ |Organic |"Active" |Nearly
| |peroxides |oxygen |all plants
| | | |
(_c_) Oxidases | ........ |Chromogens |Pigments |Many plants
| | | |
| |Alcohols and |Acids |Many plants
| |phenols | |
(_d_) Reductases | ........ | ......... | ......... |Many plants
| | | |
III. Deaminases |Urease |Urea |Ammonia and |
| | |CO_{2} |
| | | |
|Guanase |Guanine |Xanthine |
| | | |
|Adenase |Adenine |Hypoxanthine |
| | | |
IV. Carboxylases | | | |
(_a_) | ........ |Keto-acids |Aldehydes |
| | |and CO_{2} |
| | | |
(_b_) | ........ |Amino-acids |Amines and |
| | |CO_{2} |
| | | |
V. Coagulation |Pectase |Coagulates | .......... |Fruits
enzymes | |pectic | |
| |bodies | |
| | | |
VI. Fermentation |Zymase |Glucose, etc.|Alcohol and |Yeasts
enzymes | | |CO_{2} |
| | | |
|Lactic acid|Fatty acids |Lactic acid |Bacteria
|ferment | | |
| | | |
|Butyric |Fatty acids |Butyric acid |Bacteria
|acid | | |
|ferment | | |
| | | |
---------------------+-----------+-------------+-------------+----------
The above list includes only the more common and best-known plant enzymes.
It seems reasonable to suppose that for every individual type of organic
compound which may occur in general plant groups, or even in single
species, there is a corresponding enzyme available to affect its
physiological alterations. Indeed, new preparations of active enzymes from
special types of plants and new evidences of the existence of enzymes in
various plant organisms are continuously being reported.
A few of the most common specific representatives of individual groups of
enzymes may be briefly described, as follows:
=Amylase= (or =diastase=, as it was first named and is still commonly
called) is probably the most widely distributed enzyme of plants. It is
found in practically all bacteria and fungi; in practically all seeds (it
has been found in active form in seeds which were known to be over fifty
years old); in all roots and tubers; and in practically all leaves, where
it is located in the stroma of the chloroplasts.
It appears to exist in two modifications, known, respectively, as (_a_)
translocation diastase and (_b_) diastase of secretion. The first form is
found in the cells of ungerminated seeds, in leaves, shoots, etc. It
remains in the cells where reserve starch is stored and aids in the
transformation of starch into soluble materials for translocation from cell
to cell. It is active at a lower temperature than the second form, its
optimum temperature being 45° to 50°. The second form is secreted by the
scutellum, and perhaps by the aleurone cells, of germinating seeds, being
produced by special glandular tissue. It aids in the hydrolysis of the
starch for the use of the growing embryo. Its optimum temperature is 50° to
55°.
The activity of amylase is accelerated by the presence of small quantities
of neutral salts, especially by sodium chloride and disodium phosphate. It
acts best in neutral solutions, its activity being inhibited, although the
enzyme itself is not destroyed, by the presence of more than minute traces
of free mineral acid or alkali.
=Sucrase= (or invertase) is present in almost all species of yeasts, where
it serves to convert unfermentable sucrose into glucose and fructose, which
are readily fermentable. Invertase is also present in moulds and other
microorganisms; and in the buds, leaves, flowers, and rootlets of those
higher order plants which store their carbohydrate reserves in the form of
sucrose. It appears that sucrose, while easily soluble, is not readily
translocated, or utilized, by plants until after it has been hydrolyzed
into its constituent hexoses.
The optimum temperature for invertase is 50° to 54°; it is killed if
heated, in the moist condition, to 70°. Its activity is increased by the
presence of small amounts of free acids; but is inhibited by free alkalies.
=Zymase= is the active alcoholic fermentation enzyme of yeasts. It
accelerates the well-known reaction for the conversion of hexose sugars
into alcohol and carbon dioxide, namely,
C_{6}H_{12}O_{6} = 2C_{2}H_{5}OH + 2CO_{2}.
Because of its scientific interest and industrial importance in the
fermentation industries, its action has been extensively studied. It acts
only in the presence of soluble phosphates and of a coenzyme (see below)
which is dialyzable and not destroyed, which is probably an organic ester
of phosphoric acid. The significance of the molecular configuration of the
hexose sugars in their susceptibility to action by zymase has already been
discussed in detail (see page 56).
The optimum temperature for zymase action is 28° to 30°. The enzyme is
killed by heating to 45° to 50° in solution, or to 85° if in dry
preparation.
=Proteases= of the erepsin type, i.e., those which break proteins down to
amino-acids instead of only to the proteose or peptone stage, as is
characteristic of the enzymes of the trypsin type, are widely distributed
in plants. Except in the case of the two which occur in large amounts in
certain special fruits (papain in papaws, and bromelin in pineapples), they
are very difficult to prepare in pure form for study. In general, all
proteolytic actions, even when accelerated by active enzymes, proceed much
more slowly than do the hydrolyses of carbohydrates or fats. It seems that
metabolic changes of the complex protein molecules are much more difficult
to bring about and take place much more slowly than do those of the
energy-producing types of compounds.
The presence of proteolytic enzymes in most vegetative cells, and in seeds,
may be demonstrated, however, by studying the action of extracts of these
tissues upon soluble proteins. The best-known example of this type of
enzymes is the protease of yeast; but similar ones may be found in
germinating seeds. These vegetable proteases are usually most active in
neutral or only faintly alkaline solutions, and their activity is nearly
always inhibited by even traces of free acids.
Most laboratory studies of proteolytic enzymes are carried on with
preparations of the powerful members of this class of enzymes which are
found in the digestive tract of animals, namely, the pepsin of the gastric
juice, which acts in the acid medium, in the stomach, and the trypsin of
the pancreatic juice, which acts in the alkaline medium of the intestinal
tract. But even these powerful proteases require several hours for the
transformation of an amount of soluble albumin into its amino-acid
constituents which is equivalent to the amount of starch which is
hydrolyzed to maltose by diastase in a very few minutes.
Enzymes which govern oxidative changes, known respectively, as _catalases_
and _oxidases_, are almost universally present in plants. Catalase
decomposes peroxides, with the liberation of free oxygen. It is, therefore,
necessary to the final step in the process of photosynthesis, as elucidated
by Usher and Priestley (see page 26), and serves to prevent the destructive
action of hydrogen peroxide upon chlorophyll. The almost universal presence
of oxidases in plant tissues has been repeatedly demonstrated. They are
present in especially large amounts in tissues which are being acted upon
by parasitic fungi or are combating unfavorable conditions of growth. The
oxidases, in such cases, seem to be the agents by which the plant is able
to stimulate its metabolic activities to overcome the unfavorable
environment for its normal development.
In vegetables and fruits, the common browning, or blackening, of the
tissues when cut surfaces are exposed to the air has been demonstrated to
be due to the catalytic oxidation of the tannins or of certain amino-acids,
especially tyrosine, under the influence of the oxidases which are present
in the tissues. In fact, most pigmentation phenomena are due to changes in
the oxygen content of the chromogens of the cells of the plant, under the
influence of the oxidases which are present in the protoplasm of the cells
in question. Hence, the oxidases may be said to be the controlling agencies
for both the energy-absorbing activities and for respiration in plants.
THE NATURE OF ENZYME ACTION
The mechanism by which an enzyme accomplishes its catalytic effects has
been the object of extensive studies during recent years, especially since
the discovery by Büchner that enzymes could be isolated in solutions
entirely free from the disturbing influence of growing cells. Several
theories concerning the mode of this catalytic action have been advanced.
The earliest and simplest of these was that the enzyme simply creates an
environment favorable for the particular chemical reaction to take place,
as by exposing large surfaces of the substance in question to the action of
the hydrolytic, or other effective, agent, by means of surface adsorption
of the substrate material on the colloidal enzyme.
However, more recent investigations clearly indicate that there is an
actual combination between the substrate material and the enzyme, which
combination then breaks down with a resultant change in the substrate
material and a freeing of the enzyme for repeated recombination with
additional substrate, with the net result that the chemical change in the
substrate material is enormously accelerated. That such a combination
between substrate and enzyme actually exists has been demonstrated in two
different ways: (_a_) experimentally, by mixing together solutions of an
enzyme and of its substrate, each of which is filterable through paper or
through a porous clay filter, with the result that the active material in
the combined solutions will not pass through these same filters; and (_b_)
mathematically, by a study of the curves representing the reaction
velocities of typical reactions which are proceeding under the influence of
an enzyme, which show that so long as there is a large excess of substrate
material present, the accelerating influence of the catalyst is uniform
over given successive periods of time, but that when the quantity of
substrate material becomes smaller than that which permits the maximum
combining power of the enzyme to be exercised, the reaction velocity
immediately slows up.
Again, the fact that the specificity of the action of an enzyme, i.e., the
limitation of the action of that enzyme to a specific single compound or
group of similar compounds, is definitely related to the molecular
configuration of the molecule of the substrate, as has been found to be
true in all those cases where the molecular configuration of the substrate
material has been established (see pages 56 to 58), is an added indication
that there is some kind of a union between the enzyme and the substrate as
a first step in the catalytic process.
As to the nature of this supposed combination of substrate and enzyme, two
theories are held. The first is that this union is in the form of an actual
molecular combination, or chemical compound, and the other is that it is a
purely physical, or colloidal complex. The latter view has by far the
greater weight of theoretical and experimental evidence in its support. The
relation of electrolytes to the catalytic effect of enzymes, the appearance
of the reacting masses under the ultramicroscope, and the effect of heat
upon the reacting mixtures, all point to the conclusion that the phenomenon
is colloidal rather than molecular in character. This view also makes the
remarkable catalytic effects which take place in living protoplasm, which
undoubtedly exists in the colloidal condition, much more easily understood.
This phase of the matter will be much more apparent after the chapter
dealing with the physical chemistry of the protoplasm has been studied.
A further indication that the mechanism of enzyme activity is colloidal in
character lies in the fact that, so far as is known, all reactions which
are catalyzed by specific enzymes are reversible and the same enzyme will
accelerate the velocity of the reaction in either direction, the direction
in which the reaction goes being determined by the conditions surrounding
the reacting material at the time. It was formerly supposed that enzymes
catalyze only decomposition reactions and that the synthetic reactions of
living tissues are produced by means of some other force or agency. This
view supported the idea of a chemical union of the enzyme with the
substrate which, when it breaks down, breaks the molecule of the substrate
material into some simpler form, or forms. But it is now known that the
reaction which is influenced by the enzyme will be catalyzed in either
direction by the specific enzyme which "fits" the particular substrate
material at every point of its molecular configuration, as the glove fits
the hand. The contrast between this fitting of the enzyme to the entire
configuration of the molecule, and the union at a single point or group
which is characteristic of chemical linkages, is apparent. As examples of
the synthetic action of the same enzyme which, under other conditions,
accelerates the decomposition of the same material, there may be cited the
demonstrated synthesis of isomaltose from glucose by maltase; the
production of ethyl butyrate from alcohol and butyric acid; and the
synthetic production of artificial fats, by the aid of the pancreatic
lipase; and the apparent synthesis of a protein from the same amino-acids
which may be obtained from it by hydrolysis under the influence of the same
protease, but under different environmental conditions.
ACTIVATORS AND INHIBITORS
The activity of enzymes is strongly influenced by the presence in the
solution of other bodies, usually, although not always, electrolytes. This
is probably due, in most cases at least, to the action of the electrolyte
upon the colloidal condition of the enzyme. All enzymes do not respond
alike to the action of the same electrolyte, however. The activity of
certain enzymes is enormously increased by the presence of a small amount
of acid; while the action of another may be absolutely inhibited by the
same acid in the same concentration. Thus, the activity of the amylase
found in the endosperm of many seeds is instantly stopped by adding to the
solution enough sulfuric acid to make it two-hundredth normal in strength;
while the same concentration of acid actually accelerates the activity of
some of the proteases.
Formaldehyde, hydrocyanic acid, and soluble fluorides usually inhibit both
the activity of a cell and of the enzymes which it contains; while other
antiseptics, such as toluene, xylene, etc., prevent the growth of the cell,
or organism, without interfering with the activity of the enzymes which may
be present. By the use of this latter type of antiseptics, it is possible
to distinguish between chemical changes which are involved in the actual
development of a cell and those which can be brought about in other media
by means of the enzymes which are contained in the cell.
Any substance which increases the catalytic activity of an enzyme is known
as an "accelerator," or "activator"; while one which prevents this activity
is called an "inhibitor," or "paralyzer."
A type of accelerating influence quite different from that of electrolytes
is found in the effect of certain amino-acids upon enzyme action. The
influence of small amounts of asparagine in enormously increasing the
hydrolytic effect of amylase is an example. There is no known explanation
for this type of activation of the enzyme.
The influence of activators, or inhibitors, in providing favorable or
unfavorable conditions for the action of an enzyme, should not be confused
with the relation to the enzyme itself of what are known as "coenzymes" and
"antienzymes," discussed in the following paragraph.
COENZYMES AND ANTIENZYMES
In the cases of many enzymes of animal tissues, it has been found that they
are absolutely inactive unless accompanied by some other substance which is
normally present in the gland, or protoplasm, which secretes them. Thus,
the bile salts are absolutely necessary to the activity of trypsin, in its
characteristic protein-splitting action. Such substances are known as
"coenzymes." They can usually be separated from their corresponding enzymes
by dialysis, the coenzyme passing through the parchment membrane. Such
coenzymes are not killed by boiling the dialyzate, and the activity of the
enzyme is restored by adding the boiled dialyzate to the liquid which
remains within the dialyzer.
The best known example of a coenzyme in plant tissues is in connection with
the activity of the zymase of yeast cells. If yeast juice be filtered
through a gelatin filter, the colloidal enzymes which are left behind are
entirely inactive in producing fermentation, but may be restored to
activity again by mixing with the filtrate. An examination of this
filtrate, which contains the coenzyme for zymase, shows that it contains
soluble phosphates and some other substance whose exact nature has not yet
been determined, both of which are necessary to the activity of the zymase.
The phosphates seem to enter into some definite chemical combination with
the substrate sugars, while the other coenzyme seems to be necessary in
order to make possible the final breaking down of the sugar-phosphate
complex by the zymase. This phenomenon of coenzyme relationship is not very
frequently observed in plant enzyme studies, probably because the coenzyme
(if there be such, in the case which is under observation) usually
accompanies the enzyme itself through the various processes of extraction
and purification of the material for study. However, care must be taken in
all cases when dialysis is employed, to see that a possible coenzyme is not
separated from an otherwise active preparation.
An entirely different type of phenomenon is that exhibited by
"antienzymes." These are found in the various intestinal worms which live
in the digestive tracts of animals; and prevent the digestive action of the
enzymes of the stomach and intestines upon these worms. Probably similar
"antienzymes" are located in the mucous linings of the intestinal tract
itself, and serve to prevent the auto-digestion of these organs by the
active enzymes with which they are almost continually in contact.
The difference between an antienzyme, which protects material which would
otherwise be subject to the attack of an enzyme, and an inhibitor, which
renders the enzyme itself inactive, is apparent.
So far as is known, however, no such substances as antienzymes are present
in plant tissues; although the question as to why the proteoclastic enzymes
which are elaborated by a given mass of protoplasm do not attack the
protoplasm itself, might well be raised.
ZYMOGENS
It is apparent that, since enzymes are produced by protoplasm for the
special needs of any given moment or stage of development, there must be a
preliminary stage, or condition, in which they do not exert their
characteristic catalytic effect. When in this stage, the compound is known
as "proenzyme," or "zymogen." In this stage, it is inactive, but can be
made to exhibit its catalytic effect, usually by bringing it into contact
with a suitable activator. When once so activated, however, it cannot be
returned again to the inactive state.
This phenomenon has been studied in connection with the zymogens of the
digestive proteases, pepsin and trypsin. Trypsinogen may be rendered active
by contact with either calcium salts or with another substance (apparently
itself an enzyme) known as enterokinase, which is secreted in the
intestinal tract.
Similarly, proenzymes have been reported as occurring in numerous plant
tissues. These proenzymes are believed to be present in the plant cells in
the form of definite characteristic granules, which may be observed under
the microscope, and which disappear when the enzyme becomes active. Thus,
"proinulase" has been reported as occurring in artichoke tubers:
"prolipase," in castor beans; "proinvertase," in several species of fungi;
and, probably, "prooxidase," in tobacco leaves. In the case of the
last-named zymogen, it has been observed that after the zymogen has been
once activated, as in response to the need for increased activity due to
the entrance of the germs of certain leaf-diseases, it can once again
produce a second supply of the enzyme, but the process cannot again be
repeated.
Calcium salts, or very dilute acids, are usually energetic activators of
proenzymes.
PHYSIOLOGICAL USES OF ENZYMES
There can be no doubt that enzymes exert a tremendously important influence
in vital phenomena, by determining the rate at which the chemical changes
which are involved in these phenomena shall proceed. Since they do not
initiate reactions, and since they may catalyze reversible reactions in
either direction, it cannot be said that they determine the type of
reactions which will take place in any given mass of protoplasm; but,
undoubtedly, they do exert a determining influence upon the rate at which
the reaction will proceed, after the protoplasmic activity has determined
the direction in which it shall go.
Without the intervention of these catalyzing agents, it would be impossible
for reactions between these non-ionized organic components of the cell
contents to come to completion with anything like the marvelous rapidity
with which these changes must take place in order to permit the organism to
grow, to perform its necessary vital functions, or to adjust itself to the
changes in its environmental conditions.
Since the number of different reactions which take place within a living
cell is very great, and since these chemical changes are extremely variable
in type, it follows that the number of different enzymes which must exist
in either a plant or an animal organism is likewise very large. For
example, fourteen different enzymes have been isolated from the digestive
system, and at least sixteen from the liver, of animals. They are
universally present in living protoplasm of every kind, from the most
minute bacterium to the largest forest trees, in the plant kingdom; and
from the am[oe]ba to the whale, in animals.
While there is a great variety of enzymes which may be produced by a single
individual organism, the same enzyme may be found in the greatest variety
of organisms; as, for example, the protease trypsin, which has been found
in several species of bacteria, in the carnivorous plant known as "Venus'
Fly Trap," and in the human pancreas, as well as that of all other animals.
FURTHER STUDIES NEEDED
From the discussions which have been presented in this chapter, it is
apparent that the enzymes play a tremendously important part in vital
phenomena, by controlling the rate at which the biochemical reactions take
place in the cells of the living organism.
The means by which the protoplasm elaborates these all-important chemical
compounds are as yet absolutely unknown. Even the nature of the enzymes
themselves is still a matter of speculation and study. Much intensive study
is needed and should be given to these matters, for the purpose of
elucidating the methods by which the enzymes accomplish their remarkable
catalytic effects, and, if possible, the actual chemical nature of the
enzymes themselves. It is conceivable, of course, that if the latter object
of these studies should ever be reached, it might be possible to synthetize
enzymes artificially, and so to develop a means for the artificial
duplication of the synthesis of organic compounds with the same velocity
that this is done in the plant cells. Such a result would have a scientific
interest fully as great as did Wöhler's artificial synthesis of urea, which
proved that there is no essential difference in character between the
compounds which are the products of living organisms and those which are
produced in the laboratory; and, at the same time, might have an immensely
more important practical bearing, since it would lead the way to the
artificial production of the carbohydrates, proteins, fats, etc., for which
we are now dependent upon plant growth as the source of these materials for
use as human food.
References
BAYLISS, W. M.--"The Nature of Enzyme Action," 186 pages, _Monographs_ on
Biochemistry, London, 1919 (4th ed.).
EULER, H., trans. by POPE, T. H.--"General Chemistry of the Enzymes," 319
pages, 7 figs., New York, 1912.
EFFRONT, J., trans. by PRESCOTT, S. C.--"Enzymes and their
Application,--Enzymes of the Carbohydrates," 335 pages, New York, 1902.
EFFRONT, J., trans. by PRESCOTT, S. C.--"Biochemical Catalysts in Life and
Industry--Proteolytic Enzymes," 763 pages, New York, 1917.
GREEN, J. R.--"The Soluble Ferments and Fermentation," 512 pages,
Cambridge, 1901, (2d ed.).
GRUS, J.--"Biologie und Kapillaranalyse der Enzyme," 227 pages, 58 figs., 3
plates, Berlin, 1912.
HARDEN, A.--"Alcoholic Fermentation," 156 pages, 8 figs., Monographs on
Biochemistry, London, 1914.
PLIMMER, R. H. A.--"The Chemical Changes and Products Resulting from
Fermentations," 184 pages, London, 1903.
OPPENHEIMER, C., trans. by MITCHELL, C. A.--"Ferments and their Actions,"
343 pages, London, 1901.
CHAPTER XV
THE COLLOIDAL CONDITION
Reference has frequently been made, in preceding chapters, to the fact that
proteins, enzymes, lipoids, etc., exist in the protoplasm of plants and
animals in the colloidal condition. The properties and uses of these
compounds by plants depend so much upon this fact that, before proceeding
to the consideration of the actual physical chemistry of protoplasm itself,
it will be appropriate and profitable to give some attention to the nature
and significance of the colloidal condition of matter and of some of the
phenomena which grow out of it.
Every discussion of the colloidal condition in general properly begins with
reference to the work of the English physicist, Thomas Graham, who carried
on his investigations of the so-called "colloids" through a period of forty
years, beginning with 1851. His most important results were published,
however, from 1861 to 1864. Graham studied the diffusibility of substances
in solution through the parchment membrane of a simple dialyzer. As a
result of his earlier investigations, he divided all the chemical compounds
which were known to him into two groups, which he called "crystalloids" and
"colloids," respectively, the first including those substances which
readily diffused through the parchment membrane and the second those which
diffused only very slowly or not at all. He at first thought that
crystalloids are always inorganic compounds, while colloids are of organic
origin. He soon learned, however, that this distinction in behavior is not
always related to the organic or inorganic nature of the compound. He
further discovered that the same individual chemical element or compound
may exist sometimes in crystalloidal, and sometimes in colloidal, form.
This latter discovery led to the conclusion that diffusibility depends upon
the _condition_, rather than upon the _nature_, of the material under
observation.
As a result of the long series of investigations which were stimulated by
Graham's work, the modern conception is that diffusibility is a
_condition_ of matter when in minute subdivision, or in solution, in some
liquid, as contrasted with its _state_, or condition, when existing alone.
That is, the _state_ of a substance may be either gaseous, liquid, or
solid; and its _condition_ when in solution may be either crystalloidal or
colloidal. Substances which are in crystalloidal form, in true solution,
exist there in molecular or ionized condition; but, as will be pointed out
below, when in the colloidal condition they exist in aggregates which are
somewhat larger than molecules, but not large enough to be visible as
individual particles under the ordinary microscope, even under the highest
magnification which has yet been obtained. Colloidal particles are,
however, generally visible under the Zigmondy "ultramicroscope." (See
below.)
The use of the word "colloid" as a noun, or as the name for a substance
which is in the colloidal condition, is of the same nature as the use of
the words "gas," "liquid," and "solid," in such statements as "ice is a
solid," "water is a liquid," or "steam is a gas," etc.; i.e., the noun
represents a state or condition rather than an actual object or thing.
Hence, the expression "enzymes are colloids," means only that enzymes exist
in the colloidal condition, and not that enzymes represent a definite type
of substances having the group name "colloids."
THE COLLOIDAL CONDITION A DISPERSION PHENOMENON
When one substance is distributed through the mass of another substance,
the mixture is said to be a "two-phase system," composed of the _dispersed
phase_, or substance, and the _dispersion medium_, or _continuous_ phase,
through which the other substance is distributed. The following examples
illustrate the possibilities of such two-phase systems:
(1) Dispersion medium a gas.
(_a_) Disperse phase a liquid--mist in the air.
(_b_) Disperse phase a solid--smoke or dust in air.
(2) Dispersion medium a liquid.
(_a_) Disperse phase a gas--foams.
(_b_) Disperse phase a liquid--emulsions.
(_c_) Disperse phase a solid--suspensions.
(3) Dispersion medium a solid.
(_a_) Disperse phase a gas--solid foams, pumice stone, etc.
(_b_) Disperse phase a liquid--liquid inclusions in minerals.
(_c_) Disperse phase a solid--alloys, colored glass, etc.
Although the same general principles of physical chemistry apply to all
two-phase systems, the term "colloidal condition" is commonly used only in
connection with a particular type of dispersions, in which the dispersion
medium is a liquid and the dispersed material is either a solid or a
liquid.
Thorough and careful studies have shown that when a solid or a liquid is
introduced into another liquid, and becomes dispersed or distributed
through it, the mixture may be either a true solution, a colloidal
solution, or a mechanical suspension. The characteristic differences
between these three conditions may be tabulated as follows: although the
significance of some of the phrases used will not be apparent until the
phenomena in question have been considered in some detail.
-----------------------+------------------------+------------------------
True Solutions. | Colloidal Solutions. | Suspensions.
-----------------------+------------------------+------------------------
| |
(_a_) Particles of the | |
disperse phase are: | |
| |
In molecular | In colloidal | In mechanical
subdivision | subdivision | subdivision
| |
Invisible | Visible under | Visible under
| "ultrascope" | microscope or to naked
| | eye
| |
Less than 1µµ | 1µµ to 1µ in diameter | Greater than 1µ in
in diameter[6] | | diameter
| |
Pass through filters | Pass through filters | Do not pass through and
parchment membranes | but not through | filters or parchment
| parchment |
| |
In molecular motion | In Brownian movement | In gravitational
| | movement
| |
(_b_) The system | |
exhibits: | |
| |
High osmotic pressure | Low osmotic pressure | No osmotic pressure
| |
Transparency | "Tyndall phenomenon" | Is generally opaque
| |
No gel-formation | Forms gels | No gel-formation
| |
-----------------------+------------------------+------------------------
FOOTNOTES:
[6] 1µ is one-thousandth of a millimeter; 1µµ is one-thousandth of a µ, or
one millionth of a millimeter.
It is recognized by all students of these matters that it is not possible
to draw a sharp dividing line between these three types of conditions, and
that they shade into each other, in many cases; but in general it may be
said that a colloidal solution is one in which the dispersed particles are
usually between 5µµ and 200µµ in diameter, are difficultly or not at all
diffusible through the membrane of a simple dialyzer, cannot be filtered
out of solution, do not settle out under the action of gravitation, and are
visible only under the "ultramicroscope"; and one which has certain
peculiar optical, osmotic, and other physical and chemical properties.
Since colloidal particles are very minute in size, they possess very large
relative surface areas as compared with their total mass or volume, very
high surface tension, and a relatively high surface energy as compared with
their total, or molecular, energy. These properties bring into play, in a
substance which is in the colloidal condition, in a remarkable degree, all
the phenomena which are associated with surface boundaries between solids
and liquids, liquids and gases, etc.
The properties arising out of the colloidal condition are of such
tremendous importance in connection with the vital phenomena exhibited by
cell protoplasm that it is necessary to give some detailed consideration to
them here. Many large volumes dealing with this condition of matter have
been written, and it is very difficult to condense even the most important
facts concerning it into a few pages, but an attempt has been made to
present in this brief summary the most essential facts and principles
involved in the colloidal phenomena.
NOMENCLATURE AND CLASSIFICATION
Colloidal mixtures may exist in two different forms: one, in which the
mixture is fluid and mobile, like a true solution, is known as a "sol"; and
the other, which is a semi-solid, or jelly-like, form, is known as a "gel."
Sols may be easily converted (or "set") into gels, by changes of
temperature or of the electrolyte content, or by changes in the
concentration of the mixture, etc., and in most cases gels can be converted
again into sols. In some cases, however, gel-formation is irreversible, the
gels are permanent and cannot be changed back again into sols by any known
change in environmental conditions.
Depending upon whether the liquid dispersion medium is water, alcohol,
ether, etc., sols are known as "hydrosols," "alcosols," "ethersols," etc.;
and gels as "hydrogels," "alcogels," etc.
Sols in which the disperse phase is a solid are known as "suspensoids";
while those in which it is a liquid are "emulsoids." Thus, sols of most
inorganic compounds, of dextrin, gelatin, and (probably) of casein, etc.,
are suspensoids; while sols of egg-albumin, of oils, etc., are emulsoids.
The classification of these substances into suspensoids and emulsoids is,
however, more a matter of convenience than of real difference in
composition, since it is practically impossible to say whether many of the
organic substances which normally exist in colloidal form are themselves
liquids or solids, when in the non-dispersed form.
CONDITIONS NECESSARY TO THE FORMATION OF SOLS
Suspensoids differ from mechanical suspension of solids in a liquid in that
in the latter the solid particles settle toward the bottom of the mixture,
because of the effect of the attraction of gravity upon them. The rate at
which such particles settle depends upon the size and density of the
particle and the viscosity of the liquid, and can be roughly calculated
from the formula for Stokes' law for the rate of falling of a spherical
body in a liquid. This formula is
_V_ = 2_r_^2(_s_ - _s_´)_g_ / 9_n_;
_V_ = velocity of the falling body, in millimeters per second;
_r_ = radius of the particle, in millimeters;
_s_ = specific gravity of the solid;
_s_´ = specific gravity of the liquid;
_g_ = the attraction of gravity, in dynes;
_n_ = the viscosity of the liquid.
For example, if this formula be applied to determine the rate at which the
particles of gold of the size of those in a red gold sol would settle, if
they were in mechanical suspension in water (_r_ = 10µµ, or
one-ten-thousandth of a millimeter; _s_ = 19.3; _s_´ = 1; _g_ = 980, and
_n_ = 0.01), it will be found that such particles will settle at the rate
of approximately 0.0146 millimeter per hour, or a little over 10 mm. (0.4
inch) per month. Hence, the settling of such particles, if in mechanical
suspension, would be measurable, although very slow. Shaking up the
_suspension_ would cause the particles to rise through the liquid again.
But in a gold sol, or _suspensoid_, which contains particles of gold of the
size used in this calculation, the gold particles do not settle, even at
the slow rate as calculated above. They remain uniformly distributed
throughout the liquid for an indefinite period or time. The reason for
this phenomenon undoubtedly lies in the fact that these minute particles
carry an electric charge, which, is of the same sign for all of the
particles and results in a repellent action which keeps the particles in
constant motion. This constant motion may easily be conceived to keep the
particles uniformly distributed throughout the liquid, just as constant
shaking would keep those of a mechanical suspension uniformly distributed
through the mixture.
The sign of the electric charge on the particles of a sol may be either
negative or positive, depending upon the chemical nature and dielectric
constants of the two phases of the system. The proportion of the total
electric charge of the system which is of the opposite sign to that borne
by the dispersed particles is, of course, borne by the liquid which
constitutes the other phase. The origin of this electric charge on the
colloidal particles is, as yet, not known with certainty; but it seems
probable that it is due to a partial ionization of these small particles,
similar to, but not so complete as, that which takes place when compounds
which are soluble go into true solution in water, or other solvents which
bring about the dissociation of dissolved substances.
The conditions necessary to bring a solid substance into a colloidal
mixture with some liquid, or, in other words, to produce a suspensoid sol,
require that the proportion of liquid to solid shall be large and some
means of disintegrating the material which is to be dispersed into very
fine particles. Many common chemical reactions, if carried out in very
dilute solutions, result in the production of sols, especially if a small
amount of some emulsoid is present in the reacting mixture; sols produced
in this way are very stable, and the emulsoid which is used in stabilizing
the sol is known as a "protective colloid." Direct methods of
disintegration; such as reduction by chemical agents, discharge of a strong
electrical current through the substance which is to be dispersed while it
is submerged in the liquid, alternate treatment of finely ground material
with alkali and acid so as to frequently change the electric charge, etc.,
are utilized for bringing inorganic compounds into the colloidal state.
Suspensoids usually contain less than 1 per cent of the solid dispersed
through the liquid. In fact, extreme dilution is one of the necessary
conditions for suspensoid-formation.
Emulsoids are much more easily produced than are suspensoids. The property
of forming an emulsoid seems to be much more definitely a characteristic of
the substance in question than does the formation of sols from solids
which, under other conditions, may form true solutions. This difference may
be due to the fact that the liquids which easily form emulsoids (usually
those of organic origin) have very large molecules, so that the transfer
from molecular to colloidal condition involves much less change in such
cases than it does in the case of solid (inorganic) substances of
relatively low molecular weight. This view of the matter is further borne
out by the fact that solids which have very large molecules (generally of
organic origin) take on the colloidal form much more readily than do those
of small molecular size.
At the same time, a given liquid may form a true emulsoid when introduced
into one other liquid and a true solution when introduced into another.
Thus, soaps form emulsoids with water (true hydrosols); but dissolve in
alcohol to true solutions, in which they affect the osmotic pressure, the
boiling point of the liquid, etc., in exactly the same way that the
dissolving of other crystalloids in water affects the properties of true
aqueous solutions. Again, ordinary "tannin," when dissolved in water,
produces a sol, which froths easily, is non-diffusible, etc.; but when
dissolved in glacial acetic acid, it produces a true solution.
The concentration of the disperse phase may be much greater in the case of
emulsoids than it can be in suspensoids. This is probably because the
dispersed particles do not carry so large an electric charge and are not in
such violent motion.
GEL-FORMATION
The one property which most sharply distinguishes sols from true solutions
is their ability to "set" into a jelly-like, or gelatinous semi-solid,
mass, known as a "gel," without any change in chemical composition, or
proportions, of the two components of the system. In the gel, the two
components are still present in the same proportions as in the original
sol; but the mixture becomes semi-solid instead of fluid in character.
Thus, an agar-agar sol containing 98 per cent of water sets into a stiff
gel; while many other gels which contain 90 to 95 per cent of water can be
cut into chunks with a knife and no water will ooze from them. The water is
not in chemical union with the solid matter in the form of definite
chemical hydration, however, as the same gel is formed with all possible
variations in the water content.
Gels may be either rigid, as in the case of those of silicic acid, etc., or
elastic, as are those of gelatin, egg-albumin, agar-agar, etc. The latter
are the common type of gels among organic colloids. They can be easily
changed in shape, or form, without any change in total volume.
In gel-formation, the two phases of the system take a different
relationship to each other. The disperse, or solid, phase becomes
associated into a membrane-like, or film, structure, surrounding the liquid
phase in a cell-like arrangement. That is, the whole mass takes on a
structure similar to a honeycomb except that the cells are roughly
dodecahedral in shape, instead of the hexagonal cylinders in which the bees
arrange their comb cells, in which the original disperse phase constitutes
the cell-walls and the original liquid, or continuous phase, represents the
cell-contents. The cells of an elastic gel resemble closely the cells of a
plant tissue in many of their physical properties. They are roughly
twelve-sided in shape, as this is the form into which elastic spherical
bodies are shaped when they are compressed into the least possible space.
=Imbibition and Swelling of Gels.=--When substances which are natural gels,
such as gelatin, agar-agar, various gums, etc., are submerged in water,
they imbibe considerable quantities of the liquid and the cells become
distended so that the mass of the material swells up very considerably.
This swelling will take place even against enormous pressures. For example,
it has been found that the dry gel from sea-weeds will swell to 330 per
cent of its dry volume, if immersed in water under ordinary atmospheric
pressure; but that it will increase by 16 per cent of its own volume when
moistened, if under a pressure of 42 atmospheres.
During the swelling of gels by imbibition of water, the total volume of the
system (i.e., that of the original dry gel plus that of the water absorbed)
becomes less. For example, a mixture of gelatin and water will, after the
gelatin has swelled to its utmost limit, occupy 2 per cent less space than
the total volume of the original gelatin and water. It has been computed
that a pressure equivalent to that of 400 atmospheres would be necessary to
compress the water to an extent representing this shrinkage in volume.
On the other hand, gels when exposed to the air lose water by evaporation,
shrink in volume, and finally become hard inelastic solids, as in the case
of the familiar forms of glue, gelatin, agar-agar, gum arabic, etc.
The difference in the relation of gels and that of non-colloidal solids to
water may be illustrated by the different action of peas, beans, etc., and
of a common brick, when immersed in water. Each of these substances, under
these conditions, absorbs, or "imbibes," water; but the peas and beans
swell to more than twice their original size and become soft and elastic,
while the brick undergoes no change in size, elasticity, or ductility. In
all cases of colloidal swelling, the swollen body possesses much less
cohesion, and greater ductility, than it had before swelling. The essential
difference in the two types of imbibition is that in the case of the
non-swelling substances the cohesion, or internal attraction of the
molecules of the material, is too great to permit them to be forced apart
by the water; while in colloidal swelling, the particles are forced apart
to such an extent as to make the tissue soft and elastic. It is possible,
of course, to make this separation go still further, until there is an
actual segregation of the molecules, when a true solution is produced; for
example, gum arabic when first treated with water swells into a stiff gel,
then into a soft gel, and finally completely dissolves into a true
solution.
=Reversibility of Gel-formation.=--In some cases, the change of a sol to a
gel is an easily reversible one. Glue, gelatin, various fruit jellies,
etc., "melt" to a fluid sol at slightly increased temperatures and "set"
again to a gel on cooling, and the change can be repeated an indefinite
number of times. On the other hand, many gels cannot be reconverted into
sols; that is, the "gelation" process is irreversible. For example,
egg-albumin which has been coagulated by heat cannot be reconverted into a
sol; casein of milk when once "clotted" by acid cannot again be converted
into its former condition, etc. Irreversible gelation is usually spoken of
as "coagulation." Some coagulated gels, by proper treatment with various
electrolytes, etc., can be converted into sols, the process being known as
"peptization"; but in such "peptized" hydrosols, the material usually
exists in a different form than originally, having undergone some chemical
change during the peptization, and the coagulation and peptization cannot
be repeated, that is, the process is not a definitely reversible one.
=Importance of Gel-formation.=--From the physiological point of view,
gel-formation is undoubtedly the most important aspect of colloidal
phenomena. In the first place, the ability to absorb and hold as much as 80
to 90 per cent of water in a semi-solid structure is of immense
physiological importance. In no other condition can so large a proportion
of water, with its consequent effect upon chemical reactivity, be held in a
structural, or semi-solid, mass. But a vastly more significant feature of
the conditions supplied by the gel lies in the fact that the non-water
phase, or phases, of the system are spread out in a thin film, or membrane,
thus giving it enormous surface as compared with its total volume. This
effect is easily apparent if one thinks of the enormous surface which is
exposed when a tiny portion of colloidal soap is blown out into a
"soap-bubble" several inches in diameter. This condition brings into play
all the phenomena resulting from surface boundaries between solids and
liquids, liquids and liquids, liquids and gases, etc., from surface
tension, surface energy, etc. Among these effects may be cited those of
adsorption, increased chemical reactivity due to enlarged areas of contact,
permeability and diffusion, etc., the importance of which in the vital
phenomena of cell-protoplasm will be discussed in detail in the following
chapter.
GENERAL PROPERTIES OF COLLOIDAL SOLUTIONS
=Non-diffusibility.=--The most characteristic property of all sols is the
failure of the suspended particles to pass through a parchment, or any
similar dialyzing membrane.
=Visibility under the "Ultramicroscope."=--The particles of a sol, in
contrast with the molecules of a true solution, are visible as bright
scintillating points under the ultramicroscope. This is a modification of
the type of dark-field illumination of the ordinary microscope, as applied
to microscopic studies, in which the solution to be studied is contained in
a small tube or box of clear glass which is mounted on the stage of an
ordinary microscope and instead of being illuminated from below by
transmitted light is illuminated by focusing upon it the image of the sun,
or of some other brilliant source of light such as an electric arc, by
passing the rays from the source of light through a series of condensing
lenses which are adjusted at the proper distance and angles to bring the
image of the illuminating body within the tube containing the substance
which is to be examined and in the line of vision of the microscope.
Obviously, this results in intense illumination of any particles in the
solution which come within this brilliant image of the sun, or arc, and
therefore renders visible particles which are of less diameter than the
wave-length of ordinary light (450µµ to 760µµ for the visible spectrum)
and, hence, are not visible by the ordinary means of illumination in the
direct line of vision. It will be apparent that what is seen in the field
of the ultramicroscope is not the particles themselves, but rather the
image of the sun (or other illuminating body) falling upon the particles
which come within the image, just as one does not see the paper but only
the image of the sun when the rays from the sun are brought to a focus upon
a sheet of paper through any ordinary convex lens, or "burning glass."
Hence, the ultramicroscope gives no idea of the shape, color, or size of
the particles upon which the image falls; but it does permit the counting
of the number of particles within a given area, and a study of their
movements, from which it is possible, by mathematical computations, to
calculate the relative size of the particles themselves. Repeated studies
have shown that particles of the sizes between 5µµ and 250µµ in diameter,
which are visible under the ultramicroscope, are sufficiently small to
bring about the surface phenomena which are known as properties of
colloidal solutions. Further, the ultramicroscope permits the observation
of the growth, or disintegration, under various chemical reagents, of the
individual colloidal particles, which appear as scintillating points in the
field of the microscope; and the study of changes in relationships during
gel-formation, peptization, etc.
=The "Tyndall Phenomenon."=--Colloidal solutions exhibit this phenomenon;
that is, if a bright beam of light be passed through a sol which is
contained in a clear glass vessel having parallel vertical sides, and the
solution be viewed from the side, it appears turbid and often has a more or
less bluish sheen. This effect is due to the small particles in the sol, of
polarizing the light which is reflected from them, the blue rays being bent
more than are those in the other part of the spectrum. The Tyndall
phenomenon is similar in its effect in making the tiny particles of the sol
visible to the illumination of the dust particles in the air of a darkened
room when a ray or narrow beam of light passes through it. In a true
molecular solution, the particles are too small to be visible by this mode
of illumination.
=Other Optical Properties.=--Sols are generally translucent and opalescent;
many of them are highly colored, some of the sols of gold, platinum and
other heavy metals possessing particularly brilliant colors. In general,
metallic suspensoids are red, violet, or some other brilliant color; while
inorganic suspensoids are bluish white, and emulsoids generally blue to
bluish white.
=Formation of Froth, or Foam.=--Colloidal solutions, especially those of
the natural proteins, fats, glucosides, gums, and the artificial soaps,
have a strong tendency to produce froth, or foam, when shaken; this being
due to the enormous surface tension resulting from the finely divided
condition of the dispersed material.
=Low Osmotic Pressure.=--All colloidal solutions exhibit a very low osmotic
pressure; the freezing point of the dispersion medium is lowered only very
slightly and its boiling point is only very slightly raised by the presence
of the dispersed particles in it.
=Precipitation by Electrolytes.=--Sols of all kinds are precipitated, or
caused to form gels, by the addition of electrolytes, since these cause a
disturbance of the electric charge on the dispersed particles, to which the
colloidal condition is due. In the case of most emulsoids and of a few of
the suspensoids, this change converts the mass into a stiff gel; but in
that of many of the metallic suspensoids, the dispersed particles are
gathered together into larger aggregates, which settle out of the liquid in
the form of a gelatinous precipitate. In the latter case, the effect is
usually spoken of as "precipitation" by electrolytes; while in the former,
it is called "coagulation," or "gelation."
The effectiveness of the various electrolytes in bringing about this change
is proportional to their valency; bivalent ions are from 70 to 80 times,
and trivalent ions about 600 times as effective as monovalent ions.
Further, all sols in which the dispersed particles carry a charge of the
opposite sign likewise precipitate both suspensoids and emulsoids.
A demonstration of the presence of an electric charge on the particles of a
sol and a determination of its sign can be made by placing the solution in
a U tube, with a layer of distilled water above the sol in each arm of the
tube, and then passing an electric current through the contents of the
tube, keeping the electrodes in the distilled water, so that the migration
of the particles toward one pole or the other can be observed by their
appearance in the clear water at that end of the tube; or by passing an
electric current through the observation chamber of an ultramicroscope, in
which the solution under examination has been placed, and observing the
migration of the particles across the field toward either one or the other
(positive or negative) electrode.
_Emulsoids and suspensoids_ differ in their properties in the following
respects. Suspensoids are always very dilute, containing less than 1 per
cent of the dispersed solid; while emulsoids may be prepared with widely
varying proportions of the two component liquids. Suspensoids have a
viscosity which is only slightly greater than that of the liquid phase when
it exists alone, and their viscosity varies with the proportion of
dispersed solid which is present in the sol; while emulsoids have a very
high viscosity in all cases. Emulsoids usually form stiff gels when treated
with electrolytes; while suspensoids more commonly yield gelatinous
precipitates under the same conditions.
Suspensoids and emulsoids which carry electric charges of opposite sign
mutually precipitate each other. But emulsoids often protect suspensoids
from precipitation by electrolytes, by forming a protective film around the
particles of the suspensoids, which prevents the aggregation of the
particles into the precipitate form.
ADSORPTION
If a sol be precipitated or coagulated by the action of an electrolyte,
substances which may be present in solution in the liquid of the sol are
carried out of solution and appear in the gel or precipitate. This
phenomenon is known as "adsorption," which means the accumulation of one
substance or body upon the surface of another body, as contrasted with
"absorption," which means the accumulation of one substance within the
interior of another. Since substances which are in the colloidal form have
very large relative surface areas, it follows that the opportunity for
surface adsorption on colloidal materials is very great.
Surface adsorption is a common phenomenon. It was extensively studied by
the physicist, Willard Gibbs, who showed that adsorption will take place
whenever the surface tension of the adsorbing body will be lowered by the
concentration in its surface layer of the material which is available in
the solution or other surrounding medium.
As applied to colloidal phenomena, adsorption may be exhibited in either
one of four different ways, as follows: (1) A crystalloidal substance which
is in solution may be adsorbed on the colloidal particles of a hydrosol, so
that if the mixture be dialyzed, or filtered through a so-called
"ultrafilter" (i.e., a filter with pores so small that it will retain
colloidal particles) the dissolved crystalloid will remain with the
separated colloidal particles, or the dissolved crystalloid will not react
chemically as it would in a free solution. For example, if to a solution of
methylene blue, which dyes wool readily, there be added a small quantity of
albumin (a colloidal substance), the dye is adsorbed by the albumin and
will no longer color wool with anything like the same readiness. (2) During
gel-formation, electrolytes and other soluble substances which may be
present in solution in the liquid may adsorbed out of the solution and
appear in the gel. For example, a precipitate of aluminium hydroxide, or of
silicic acid, is nearly always contaminated with the soluble salts which
are present in the solution, and can be prepared in pure form only by
repeated filtering, redissolving, and reprecipitating. (3) Colloidal
substances may be removed from sols by being adsorbed upon porous materials
like charcoal, fuller's earth, hydrated silicates, etc. For example, animal
charcoal (or bone black) is used commercially for the clarification of
sugar solutions, because it adsorbs out of these solutions the colloidal
proteins, coloring matters, etc., with which they are contaminated. (4)
Finally, colloids mutually adsorb each other, as in the case of the
"protective colloids" previously referred to.
Certain characteristics of adsorption phenomena are of interest and
importance from both the physiological and the industrial point of view.
The following may be mentioned: (_a_) _Amount of adsorption._ Relatively
more material is adsorbed out of dilute solutions than out of more
concentrated ones. An increase of ten times in the concentration of the
dissolved material results in only four times as much adsorption by the
colloidal substance which may be introduced into the two solutions. In
this, adsorption differs from chemical action, as the latter is
proportional to the concentration of the reacting material which is present
in the solution. (_b_) _Adsorption out of different liquids_, by the same
adsorbing body, is different in amount. It is usually greatest out of
water. Hence, many dyes may be adsorbed out of water by charcoal, porous
clay, etc., and if the latter be then introduced into alcohol, or ether,
the dye goes back into solution in these latter liquids. This process is
often used industrially and in the laboratory for the purification of such
substances when they are present in impure form in aqueous solutions. (_c_)
_Selective adsorption._ Different substances are not adsorbed out of the
same solvent to the same extent by the same adsorbing agent. Advantage is
taken of this fact when filter paper is used in the so-called "capillary
analysis" to separate different dyes, or other colloidal materials which
have been stained different colors, into alternate layer by reason of the
different rate at which the paper adsorbs the different materials out of
the solution in which they are present together. (_d_) _Similar relative
adsorption by different adsorbing agents._ Although different adsorbing
agents may possess varying active surfaces and hence, variable adsorbing
power, or rates of adsorption, they adsorb the same relative amounts of
different materials; i.e., if substance _A_ adsorbs more of _X_ than it
does of _Z_ out of any given solution, substance _B_ will likewise adsorb
more of _X_ than of _Z_ out of the same solution; although the actual
amounts adsorbed by _A_ may be quite different from those adsorbed by _B_.
CATALYSIS AFFECTED BY THE COLLOIDAL CONDITION
The velocity of a chemical reaction is the net result of opposing
influences. It is directly proportional to the chemical affinity of the
reacting bodies and inversely proportional to the so-called "chemical
resistance." The first factor, chemical affinity, is not easily measured,
as it depends upon both the mass of the reacting molecules, atoms, or ions,
and their attraction for each other. But if, as the result of chemical
affinity, a reaction takes place, it is evident that the time required for
its completion (which measures the velocity of the reaction) is made up of
two separate periods. The first is the time required for the reacting
molecules to come into contact; and the second is that required for the
molecular rearrangement which constitutes the reaction. Clearly, the time
required for the substances to come into molecular contact will be greatly
diminished if they are mutually adsorbed in large quantities on the
extended surface area of some colloidal catalyst which is present in the
mixture rather than scattered throughout its entire volume. The application
of this principle to the catalysis of hydrolytic reactions is not apparent,
if it is considered that the H_{2}O molecules which cause the hydrolysis
are those of the solvent itself; but is clear on the assumption (which is
discussed in the following chapter) that the water which enters into a
colloidal complex is in multimolecular form, represented by the formula
(H_{2}O)_{_n_}, in which the oxygen atoms are quadrivalent and, hence, much
more active chemically than as illustrated in the simple solvent action of
water.
Hence, the surface adsorption of reacting bodies by a colloidal catalyst
may have a very important influence in decreasing the time required to
bring the reacting molecules into intimate contact, and so increasing the
velocity of the reaction.
But the colloidal condition of the catalyst may also aid in decreasing the
"chemical resistance" which tends to slow up the reaction. Chemical
resistance may be understood to be the internal molecular friction of the
densely packed atoms within the reacting molecule, which tends to prevent
the molecular rearrangement and so to prolong the second period of the
reaction time. To overcome this friction and so decrease the reaction time,
some form of energy is necessary. If there be present in the solution in
which the reaction is taking place some colloidal catalyst, and if the
reacting bodies are concentrated at the surface boundaries between the two
phases of the colloidal system, they may be conceived to be within the
sphere of influence of the surface energy of the dispersed particles of the
catalyst, so that this may furnish the energy necessary to overcome the
chemical resistance of the reacting bodies, and so to speed up the second
portion of the reaction time.
From these considerations, it would appear that the colloidal condition of
such catalysts as enzymes, etc., has much to do with their ability to
increase reaction velocities, both by reducing the time necessary for the
reacting bodies to come into molecular contact and by furnishing the energy
to overcome the chemical resistance to the molecular rearrangement which
constitutes the reaction itself. Evidence in favor of the accuracy of this
view of the nature of the catalytic action of colloidal substances is
afforded by the facts that catalysts accelerate the velocity of reversible
reactions in either direction and that they do not change the point of
final equilibrium, in any case; that is, they do not affect the nature or
direction of the reaction, but only accelerate a chemical change which
would otherwise take place more slowly because of the stability (or
chemical resistance) of the molecules involved, or their inability to come
quickly into intimate molecular contact.
These facts and principles have been clearly established in many studies of
the nature of enzyme action (enzymes are typical colloidal catalysts) and
probably apply equally well to the action of other types of colloidal
catalysts. On the other hand, the catalytic action of certain inorganic and
non-colloidal substances, such as the action of acids in accelerating the
hydrolysis of carbohydrates, etc., may be conceived to be due to chemical
influences upon the internal molecular resistance, which are similar in
their effects, but entirely different in their mechanism, from the physical
effects of the surface boundary phenomena of the colloidal catalysts.
INDUSTRIAL APPLICATIONS OF COLLOIDAL PHENOMENA
Large numbers of industrial processes are based upon colloidal phenomena.
Many of these processes were known and practiced long before the nature of
the phenomenon itself was understood. But with the coming of the knowledge
of the nature, causes, and possibilities of the control, of the colloidal
condition of the materials involved, immense improvements in the economy of
the process, or the quality of the end-products, have been worked out, in
many cases. Many volumes of treatises concerning the industrial
applications of colloidal phenomena have been written. Any discussion of
these would be out of place here; but the following list of examples will
serve to illustrate the immense importance of these matters both in
industry and to the needs of everyday life: the tanning of leather; the
dyeing of fabrics; vulcanizing rubber; mercerizing cotton; sizing textile
fabrics; manufacture of mucilages and glues; manufacture of hardened casein
goods; manufacture of celluloid; production of colloidal graphite for
lubrication; the prevention of the smoke nuisance by electric deposition;
the purification of sewage; the manufacture of soaps; the manufacture of
butter, cheese, and ice cream; fruit jellies, salad dressings, etc. This
list could be extended to a great length, but is already long enough to
emphasize the very great importance and practical value of colloidal
phenomena in daily life.
NATURAL COLLOIDAL PHENOMENA
Many of the phenomena of nature are colloidal in character. These may be
observed in the mineral, the animal, and the vegetable kingdoms. Here,
again, a lengthy discussion of the nature of these phenomena would be out
of place in this connection, and a few typical examples will serve to
illustrate the general importance in nature of this property of matter.
In the soil, the following properties are easily recognizable as definite
colloidal phenomena: water-holding capacity of clays, silts, loams, etc.;
adsorption (or "fixation") of soluble plant foods so that they are not
readily leached out of the soil by drainage; flocculation and
deflocculation of clay, etc.
In the animal body; the contraction of muscles, the conveyance of nerve
stimuli, etc., are undoubtedly accomplished by colloidal changes; and the
existence of insoluble casein and fat in colloidal form in milk insures the
proper nourishment of the young of nearly all species of animals.
In both plants and animals, as will be pointed out in the following
chapter, practically all the vital activities of the cell protoplasm are
definite manifestations of colloidal phenomena. Enzymes perform their
catalytic functions by reason of their colloidal form. Proteins exist in
colloidal form and are the seat of all vital functions. The regulation of
the passage of materials into and out of the cell is governed by minute
changes in the electrolyte concentration, etc., which produce enormous
changes in the colloidal character of the protoplasm.
It is apparent, therefore, that the study of the colloidal condition of
matter and of the properties arising out of it is of immense importance to
the biochemist. No other single field is capable of yielding more fruitful
results to the plant physiologist, in his studies of the response of plants
to changes in their environment, or of the mechanism by which plants
perform their internal functions.
References
BECHHOLD, H., trans. by BULLOWA, J. G. M.--"Colloids in Biology and
Medicine," 463 pages, 54 figs., New York, 1919.
BURTON, E. F.--"The Physical Properties of Colloidal Solutions," 200 pages,
18 figs., London, 1916.
CASSUTO, L.--"Der Kolloide Zustand der Materie," 252 pages, 18 figs.,
Dresden and Leipzig, 1913.
LIESEGANG, R. E.--"Beiträge zu einer Kolloidchemie des Lebens," 144 pages,
Dresden, 1909.
OSTWALD, W., trans. by FISCHER, M. H.--"Theoretical and Applied Colloid
Chemistry," 218 pages, 43 figs., New York, 1911.
OSTWALD, W., trans. by FISCHER, M. H.--"A Handbook of Colloid-Chemistry,"
278 pages, 60 figs., Philadelphia, 1915.
TAYLOR, W. W.--"The Chemistry of Colloids," 328 pages, 22 figs., New York,
1915.
ZIGMONDY, R., trans. by ALEXANDER, J.--"Colloids and the Ultramicroscope,"
238 pages, 2 plates, New York, 1909.
ZIGMONDY, R., trans. by SPEAR, E. B.--"The Chemistry of Colloids," 274
pages, 39 figs., New York, 1917.
CHAPTER XVI
THE PHYSICAL CHEMISTRY OF PROTOPLASM
Thus far, we have considered the chemical nature of the various groups of
compounds which are found in the tissues of living organisms, laying
emphasis upon those which are of plant origin. These compounds constitute
the material, or machinery, of the cell, and their various transformations
furnish the energy for its operation. We come now to a study of the mode of
its operation, or the processes of vital phenomena.
Our knowledge of these matters is not yet far enough advanced to permit a
definite statement as to whether there is any difference between the
protoplasm of plant tissues and that of animal origin in their modes of
action, or in the physical-chemical changes which constitute the vital
phenomena in the two groups of living organisms. Thus far, no such
differences have been discovered. Hence, in the following discussions, no
attempt is made to differentiate between animal and plant protoplasm. Most
of the facts and principles which are here presented have been developed as
the result of the study of the physiological chemistry of animal life. No
similar careful study of plant chemistry has yet been carried out; but
preliminary studies seem to indicate that the same general principles apply
to all protoplasm, regardless of whether it is of plant or of animal
origin. It is possible, of course, that further studies of plant protoplasm
will render necessary some modifications of some of these views as applied
to the growth of plants; but they are believed to represent the best which
is now known of the physical chemistry of the plant-cell activities.
HETEROGENEOUS STRUCTURE OF THE CELL
Examination of cell protoplasm under the microscope reveals that it is not
a simple homogeneous mass. In the first place, it has a definite
structure, composed of (_a_) a nucleus; (_b_) numerous granular bodies of
different sizes and kinds; and (_c_) a clear mass of colloidal material,
which (if observed under the ultramicroscope, or photographed by
ultra-violet light) is apparently made up of very minute particles of many
different types of materials; the whole mass, in the case of plant
protoplasm, being generally surrounded by (_d_) a differentiated layer
known as the cell-wall. The actual internal structural arrangement of the
clear colloidal mass is uncertain; but its properties indicate that it may
be considered to be like a mass of foam (resembling a compact mass of
soap-bubbles) the compartments of the foam being, of course, very minute
and the films themselves almost infinitely thin, the contents of each
compartment being probably liquid, and the whole composing a typical
colloidal gel of complex composition.
This conception may not be accurate in every detail, but it seems to fit
very closely the conditions and reactions of cell protoplasm. Furthermore,
it is obvious that the definite structure, or form, of the cell is
essential to its life; since, if the structure be destroyed by any kind of
mechanical injury (freezing of the cell contents, resulting in the
puncturing of the membranes by ice crystals; rupturing of the films, or
cell-walls, by grinding with sharp sand, etc.) so as to bring about an
intermingling of the parts which are segregated from each other in the
organized structure, there results an immediate exhibition of abnormal
chemical actions, accompanied by the liberation of carbon dioxide, and the
death of the cell.
A proper mental picture of the organization of the cell structure and of
the interrelation of all its working parts is suggested by the figure of a
well-organized chemical factory, with the different chemical
transformations which are involved in the whole process being carried on in
different portions, or rooms, of the factory, with the various intermediate
and final products regularly and systematically transported from one room
to another as they are needed to keep each individual step in the whole
process going at the proper rate, and with the different parts of the whole
factory working in smooth coordination with each other. Any disturbance of
the mechanism in any particular room, or any abnormal condition which
breaks down the coordination or results in the mixing of the reagents or
processes of adjoining rooms in improper order or proportions, produces
instant destruction of the normal process, abnormal reactions take place,
and the factory output is interrupted.
No other conception than this one of a definite structure and coordination
of the different working parts of a cell can adequately account for the
great variety of chemical changes which are constantly going on in any
given cell. It is wholly inconceivable that a homogeneous mass of all the
varying chemical compounds which are contained in any given quantity of
protoplasm could either exist or produce any regular sequence of chemical
reactions. Structure, or organization of the cell-contents into separate
colloidal compartments, and the segregation of cell-contents into masses
having different functions, is essential to any reasonable conception of
how the cell performs its various activities.
The best understanding of the structural arrangement is afforded by the
conception that protoplasm consists of a colloidal gel, or sometimes a very
viscid sol, containing water, salts, carbohydrates, fats, proteins, and
enzymes. Evidence in favor of this conception is afforded by the appearance
of protoplasm under a high-power microscope, and by the close resemblance
of the processes which go on in it, and its responses to external stimuli,
to those of an artificial gel of similar chemical composition.
Two different conceptions of the form in which the chemical components
exist in this mass have been advanced. One is that they are in true
molecular unions, known as "biogens," and that the reactions which take
place in the mass may, therefore, be studied from the same basis as are
reactions between similar substances when they take place in a beaker or
test tube in the laboratory. It would seem, however, that the constantly
varying proportions of the materials themselves, and the lack of
homogeneity of cell contents, afford insurmountable difficulties to this
conception as a basis for the study of cell activities. The other, and
seemingly more reasonable, conception is that these bodies exist in the
form of colloidal complexes, whose composition may vary within wide limits
and whose reactions are responsive to the usual phenomena incident to the
colloidal condition of matter.
According to the latter conception, vital activities of cell protoplasm may
be due to changes in water content, to electrical disturbances, to the
phenomena resulting from the conditions brought about by surface boundaries
between the different phases of the gel, to varying osmotic pressure, to
changes in chemical reaction, etc., and may be controlled by various
stimuli of chemical, physical, or mechanical nature. This conception seems,
therefore, to fit most closely the actual conditions under which the
protoplasm exists and carries on its vital functions.
With this conception in mind, we may now proceed to a consideration of how
the various components of the complex organic colloidal system, and their
specific properties, can affect its chemical activities.
The components of the system are, of course, water, salts, and the various
organic compounds (fats, proteins, carbohydrates, and enzymes in all cells;
and other groups, such as essential oils, tannins, pigments, etc., in cells
which have certain special functions to perform) which constitute the solid
phase of the colloidal mixture. In addition to the definite chemical
properties of each of these component groups, which have been studied in
detail in preceding chapters, there are many physical, or
physical-chemical, properties of the system as a whole, and of its
component parts, which are of the utmost importance in the physiological
activities of the protoplasm. These we may now proceed to consider in some
detail.
WATER
Water constitutes the largest proportion of the weight of active
protoplasm. In living cell contents (except those of such bodies as resting
seeds, etc.), water comprises from 70 to 95 per cent of the total weight of
the substance; the average proportion being usually between 85 and 90 per
cent. The fact that protoplasmic material can exist in turgid form with
such high percentages of water as these is due, as has been pointed out, to
its existence as a colloidal gel. It is because of this condition that
increases in the proportion of water generally increase the turgidity, or
turgor, of the protoplasm; instead of, as in all other cases, rendering the
mixture less solid and more labile. Losses of water from the protoplasmic
gel decrease its "swollen" condition and so render the tissue soft and
flabby; while increases in water content swell the gel and make the tissue
stiff and turgid. No other condition than that of a colloidal gel could
respond in this way to changes in water content.
The formula which is commonly assigned to water is the simplest possible
one; namely, H_{2}O. But if the water were really as simple as this, the
compound would boil at a very low temperature, would have a very low
surface tension, etc.; whereas its actual boiling point, surface tension,
etc., are much higher than those of other compounds having a higher
molecular weight than is indicated by the formula H_{2}O. Actual
measurements of the physical properties of water indicate that at the
temperature at which water is a vapor its formula is at least (H_{2}O)_{2};
while at lower temperatures, at which it exists as a liquid, its formula
may be (H_{2}O)_{3}, or (H_{2}O)_{4}, or even more complex still. The cause
for this association of the compound into multiple molecules undoubtedly
lies in the extra valences of the oxygen. In many organic compounds oxygen
is undoubtedly tetravalent, and it may be easily conceived that in these
complex molecular groupings in the water it exhibits this same property;
the possible molecular arrangements being represented by the formulas
H H
H H \ /
\ / H-O-O-H
O=O and | | etc.
/ \ H-O-O-H
H H / \
H H
Such molecules may be conceived to break down very easily, leaving the
extra valences of the oxygen available to form linkages with other atoms or
molecules. This may constitute one of the ways in which water exerts its
remarkable effects both as a solvent and as an accelerator of all kinds of
chemical reactions. Other organic compounds which contain tetravalent
oxygen are exceedingly active chemically, and there seems to be much to
commend this view of the chemical structure of the water molecule.
Probably the most remarkable property of water is its power of solution. No
other liquid surpasses water as a solvent. This power, as has been pointed
out, is supposed to be due to, or in some way correlated with, the extra
valences of the oxygen atoms, which may perhaps unite with similar extra
valences of other substances with which the water is brought into contact,
and so cause the latter to enter into solution. All kinds of substances
dissolve in water, and when in solution, or even when only moistened, are
much more active chemically than when dry. This property of water
contributes greatly to the possibilities of the chemical reactions which
constitute life processes.
Water, likewise, has a higher dielectric constant than any other common
liquid. This means that it does not readily conduct electricity, or readily
permit electric equilibrium to be established in it; or, in other words,
that it is a good insulator. This property permits the existence in it
simultaneously of materials having opposite electric charges, or the
so-called ionization phenomena; hence, water is the best-known ionizing
medium, and ionization favors chemical reactivity.
Again, water has a very high specific heat, a fact which is of the utmost
biological importance. It takes more heat to raise the temperature of one
gram of water through one degree than is required to produce the same
result in any other known substance; or, stated the other way around, a
given amount of heat will cause less change in temperature of water than of
any other known substance. Further, the latent heat of liquefaction and of
vaporization (i.e., the amount of heat required to change the substance
from solid to liquid and from liquid to gaseous state, respectively) is
greater for water than for any other common substance. These facts are of
very great importance in cell-protoplasm. The high specific heat of water
provides that the heat liberated by the chemical reactions which take place
in the protoplasm can be absorbed by the water of the cell contents, and
given off again to other reactions, with very slight effect upon the
temperature of the protoplasm itself. Hence, violent changes in
temperature, which might be disastrous to the life of the cell, are
prevented by the high specific heat of the water which it contains.
Similarly, the high latent heat of liquefaction of water, resulting in the
giving up of large quantities of heat before it can become solid, or
"freeze," tends to prevent freezing and thawing of the cell contents with
sudden changes of external temperatures at or near the freezing temperature
of water.
As a result of its physical properties, as just briefly described, water
accelerates all kinds of chemical reactions in protoplasm, both by solution
and by ionization of such substances as undergo electric dissociation; and
serves to regulate the temperature of the protoplasmic mass. Furthermore,
in organic tissues, most of the important chemical reactions of the
protoplasm are reversible hydrolyses; i.e., water actually enters into the
reaction or is liberated by it, and the equilibrium point of the reaction
is changed by the proportions of water which are present in the reacting
mass. Hence, the presence of large proportions of water in the colloidal
complex known as protoplasm has a very important influence upon its
possibilities of biological reactions.
SALTS
Active protoplasm contains mineral salts in solution. These are of the same
general nature as those found in sea-water, which is the original habitat
of the earlier evolutionary forms of living matter. Or, it might be said
that both plants and sea-water derive their mineral salts from the same
source, namely the soluble salts of the soil. Recent investigations have
shown that the proportions of sodium ions to calcium ions in sea-water are
precisely those which maintain fats, proteins, etc., in a true colloidal
emulsion; and that comparatively small variations in the ratio of these two
cations produce very marked effects upon the colloidal conditions of these
substances in an artificial colloidal preparation, which resemble very
closely the changes which apparently take place in cell protoplasm under
the influence of narcotics, or nerve stimulants, in blood-coagulation, in
the parthogenetic development of germ cells, in cancerous growth of
tissues, etc. In other words, in so far as it has been studied in this
respect, cell plasma exhibits exactly the same responses to variations in
the proportions of salts (electrolytes) in solution, that artificial
emulsions of oils (fats) in water do; and the normal, or critical,
equilibrium proportion of these electrolytes for all colloidal complexes is
that in which they occur in sea-water. It must be admitted that there is as
yet no definite evidence that the observations which have been made upon
the protoplasm of animal tissues will apply equally well to plant cell
protoplasm. But many of the phenomena which have been studied in animal
tissues have what are apparently similar, if not identical, effects in
plant tissues, and it seems reasonable to suppose that these conclusions
apply generally to protoplasm of either animal or plant origin.
The effects which salts produce in protoplasm are undoubtedly due to the
fact that, when in solution, they readily ionize and conduct the electric
current. A discussion of the nature and importance of the theory of
dissociation of electrolytes in solution, or the so-called "ionization
theory," which has done so much to clear up otherwise unexplainable
properties of solutions, would be out of place here. But it may be noted
that the ionized condition of salts in solution accounts for the avidity,
or "strength," of acids and bases; for the increased osmotic pressure of
such solutions; for the conduction of the electric current through
solutions; and for the effects of these dissolved electrolytes upon the
colloidal condition of many substances, since this is due to the electric
charge on the dispersed particles.
Hence, the presence of salts in solution in the water of the protoplasm has
a tremendous influence upon the osmotic pressure (which governs the
movement of dissolved materials into and out of the cell protoplasm); upon
the colloidal condition of the cell contents (which controls all the
effects due to the surface boundary phenomena which are discussed below and
which are responsible for a large part of the remarkable chemical activity
of the protoplasm); upon the electrical phenomena (which constitute many of
the stimulations which the protoplasm receives); and upon the acidity or
alkalinity of the cell contents (which determine the nature of the
respiratory, or oxidation, reactions of the protoplasm and, indirectly, its
life or death).
The general nature of these physical-chemical properties of the protoplasm
and of the relation of electrolytes in solution to them may now be
considered in some detail.
OSMOTIC PRESSURE
Osmotic pressure is one of the chief factors in controlling the amount of
water in the protoplasm. As is well known, the phenomenon known as
"osmosis" is the passage of solvents, or of dissolved substances, into or
out of any tissue, or substance, through the membrane which surrounds it.
In the case of a cell, the membrane in question may be either the cell-wall
or the internal colloidal films which are distributed throughout the entire
mass of the cell contents.
From the standpoint of their relation to osmosis, membranes may be either
_impermeable_, in which case neither solvent nor dissolved materials can
pass through them; _semi-permeable_, which permit the passage of the
solvent, but not that of dissolved crystalloidal substances; or
_permeable_, which permit the free passage through them of both solvents
and solutes. The first and last of these types of membranes have no effect
upon osmotic pressure; but osmotic pressure is at once set up whenever a
semi-permeable membrane is interposed between solutions of different
concentrations. It is due to the molecular motion of both the liquid and
the dissolved solids, as a result of which a greater number of molecules
are "bombarding," or pressing upon the membrane from the side of the more
concentrated solution. This sets up an unequal pressure upon the two sides
of the membrane, and if the latter be semi-permeable there will result a
passage of the liquid through the membrane toward the denser solution so as
to equalize the pressure. The resultant tendency is for the solutions on
the two sides of the membranes to become equal in concentration by movement
of the liquid from the less dense to the more dense portion, instead of by
movement of the dissolved materials toward the less dense part of the
solution as in the case of diffusion when solutions of different
concentrations are brought in contact with no membrane to interfere with
free diffusion.
Osmotic pressure tends, therefore, to force the movement of solvents
through semi-permeable membranes from more dilute toward more concentrated
solutions. Protoplasm acts in general as an approximately semi-permeable
membrane or material. For example, if the concentration of sugar in any
given mass of protoplasm becomes greater, by reason of the photosynthetic
activity, osmotic pressure is set up and water enters the mass, thus
preventing loss of turgidity due to increased concentration. Similarly, any
other increase in concentration of synthetic products is compensated for by
entrance of water because of increased osmotic pressure, unless the
products are insoluble and, therefore, incapable of effecting the osmotic
pressure.
Hence, osmotic pressure provides for the movement of water into and out of
protoplasm and so tends to keep the proportion of water uniform throughout
the entire tissue. It will at once occur to the reader, however, that if
the statements in the preceding paragraph were unqualifiedly true, and if
the protoplasmic mass were absolutely semi-permeable in character, there
would be no possibility of the passage of dissolved solids into or out of
the cell; i.e., if the protoplasm acted as an ideally semi-permeable
membrane, only water could pass into or out of it. But we know that mineral
salts from the soil must pass into any cell before the synthesis of
proteins, etc., can proceed, and that the fats, carbohydrates, proteins,
etc., which are synthetized in vegetative cells pass from these to other
organs of the plant for use or storage. The obvious explanation for this
condition of things in the plant is that protoplasm (and, indeed, this is
equally true for practically all known membranes) is not absolutely
impermeable to dissolved crystalloids; or, in other words,
semi-permeability generally means only that the solvent passes through the
membrane more readily and more rapidly than do the dissolved materials in
it. Even colloidal materials will diffuse through most common membranes,
although at so slow a rate that the process is scarcely observable by
ordinary methods of study. Hence, the actual permeability of the protoplasm
permits the movement of both water and dissolved solids from one part of
the organism to another; but its approximation of semi-permeability
produces osmotic pressure and induces freer movement of water than of
dissolved substances, and so provides for turgidity of the cells and for
equalization of the water content of different portions of the protoplasmic
mass.
It is clear, therefore, that osmotic pressure plays an important part in
the physical mechanism of cell activities and in the regulation of the
proportion of water contained in the protoplasm, with its consequent
effects upon the chemical reactions which may go on in the cell.
Actual measurements of the osmotic pressure of plant cell have been made.
The results are more or less uncertain, because, as has been pointed out, a
plant cell is not a definite quantity of uniform protoplasm surrounded by
an ideal semi-permeable membrane, but is instead a mass of living matter
which is approximately semi-permeable throughout its entire volume and is
in a constantly changing condition because of the anabolic and catabolic
activities which are going on in it; but values have been obtained which
show a normal osmotic pressure as high as fourteen atmospheres in the cells
of very turgid plants, such as those of some of the green algæ. Animal
cells probably have an osmotic pressure similar to that of the blood which
circulates around them, which is approximate that of seven atmospheres.
SURFACE BOUNDARY PHENOMENA
In the preceding chapter, a brief consideration of the phenomena arising at
surface boundaries was presented. It was pointed out that when any
substance exists in the colloidal, or dispersed, condition, it has
relatively enormous surface area and that, consequently, enormous surface
boundaries between the dispersed phase and the dispersion medium exist in
all colloidal mixtures. Since protoplasm is conceived to exist in the form
of a colloidal gel, having a foamlike structure, it is apparent that it
has these enormous surface boundaries between the different phases of the
system, and that the phenomena arising from this condition are of great
importance in its biological activities. The following necessarily brief
discussion will serve to give some indication of the physiological
importance of the surface boundaries in such a system.
It is easy to see that the molecules which are in the surface layers at the
interface, where two phases of a colloidal system are in contact, are under
the influence of forces quite different from those which are acting upon
the molecules in the interior of either phase. It is apparent that the
molecules in the surface layer are exposed on the inner side to the
attraction and influence of similar molecules, while on the opposite, or
outer, side they are exposed to the influence of molecules of an entirely
different kind. This results in a state of tension, known as "surface
tension," with the development of resultant forces and energy which
profoundly affect the chemical reactivity of the molecules which are
present in this surface layer. The so-called "surface energy," which
results from this surface tension, produces marked increases in the
possibility of chemical reaction between the materials which are present at
the surface boundaries. In colloidal gels, this effect is so pronounced, in
many cases, as to completely overshadow other types of influences upon
reaction velocities. Also, the surface layer of a liquid is compressed by
its surface tension, to such an extent that the solubility of substances in
this surface layer is greatly increased over that of the same substances in
the interior of the liquid, which results in greatly increased
concentration of dissolved substances in the surface layer, and so
increases the rate of chemical changes which take place there, as
contrasted with the rate of the same reactions going on in the interior of
the solution. This latter consideration seems to be the factor of largest
influence in colloidal catalysis.
But in addition to the increased rate of reaction in the surface layer due
to the increased energy available there and to the increased concentration
of dissolved substances, there is the possibility that the act of
concentration itself bring into play molecular forces which give rise to a
resultant increase in chemical potential, or chemical affinity, of the
reacting materials, such as has been observed to result in other
concentrated solutions. A discussion of the theoretical and mathematical
considerations upon which this conception is based would be out of place
here, but there is ample experimental evidence to indicate its soundness.
Further, as has been pointed out, colloidal phenomena are essentially due,
in large part at least, to the electric charges on the dispersed particles.
Electric charges accumulate at the surface of any charged body. Hence, the
surface layers in any colloidal system carry its electric charges in
highest concentration, and all of the chemical changes which are stimulated
by electrical phenomena are most strongly influenced at the surface
boundaries between the different phases of the system. This latter
consideration affords a satisfactory explanation of the well-known
depressing, or stimulating, action of electrolytes, especially acids and
bases, upon the enzymic catalysis of protoplasmic reactions.
These few, brief statements are sufficient to indicate how extensively the
chemical activities of colloidal protoplasm are influenced by the phenomena
arising from the surface boundaries between different materials, which are
present in such enormous extent in a colloidal gel. Surface boundary
phenomena in a heterogeneous system, such as we have seen protoplasm to be,
provide the possibilities for many reactions which would otherwise take
place very slowly, if at all. Mere subdivision of the protoplasmic
materials into the film, or foam, structure brings into play energies which
may predominate over all other types of energy in the system. Here, too,
effects may be extraordinarily modified by slight changes in environment,
which effects could not be explained by any considerations which govern
ordinary chemical reactions. Here, we deal with adsorption and other
colloidal phenomena, rather than with ordinary stoichiometric combinations.
Indeed, it is not too much to say that the differences between the
chemical phenomena which are called "vital" and those which take place in
ordinary laboratory reactions are due to the fact that the former are
manifestations of the interchanges of energy between the different phases
of a heterogeneous colloidal system, while the latter are governed by the
laws of ordinary stoichiometric combinations.
ELECTRICAL PHENOMENA OF PROTOPLASM
The investigations of this phase of the physical chemistry of protoplasm
have dealt almost exclusively with animal tissues and reactions, and have
included the study of such phenomena as nerve impulses, muscular
contractions, heart-beats, glandular secretions, etc. Tissues which respond
to nerve, or brain, control are, of course, not found in plants. But there
is plenty of experimental evidence to show that plant protoplasm carries
electrical charges and exhibits electrical phenomena which are similar in
character to those of animal tissues. In fact, it has been shown that the
contraction of the lobes of the Venus' fly trap, when they close over an
imprisoned insect, are accompanied by electrical phenomena in the leaf
tissues which are precisely similar to those which take place in an animal
muscle when it contracts. It seems probable that many of the observations
and conclusions which have been derived from the study of the electrical
disturbances in animal tissues may later be found to have definite
applications to the vital phenomena of plant cells. Hence, it seems proper
to give some brief consideration to these matters here.
The statement has been made that "every active living cell is essentially
an electric battery," and it is believed that every activity of living
matter, such as the rhythmic contraction of the heart, the passage of a
nerve impulse, etc., is accompanied by an electric disturbance in the
protoplasm of the tissues in question. Experimental proof of this
electrical disturbance has been repeatedly obtained, by connecting a
delicate galvanometer in a circuit through the living tissue which is
undergoing different activities and obtaining widely varying readings of
the instrument as the different phenomena are in progress, or by connecting
the instrument with muscular tissue and observing its fluctuations with
either the irregular contractions of a voluntary muscle or with the
rhythmic contractions of a heart muscle.
By means of such investigations as those just mentioned, it has been found
that the part of the protoplasm which is most active is always
electro-negative to the part which is less so; that is, the electric
current flows from the more active to the less active portion of the
protoplasm.
Many different explanations of the origin of the electric current which
develops when the protoplasm is stimulated into activity have been
suggested; but none of them have, as yet, any experimental confirmation.
The most that can be said is that whenever any stimulus excites the
protoplasm into activity, there is instantly developed in it an electrical
disturbance, which continues as long as the action is in progress. Recent
investigations, which have shown that there is a direct relation between
many of the vital processes of protoplasm and the ratio of the electrolytes
which it contains, particularly the ratio of sodium and potassium to
calcium, would seem to indicate that the development of the electrical
disturbance is a direct result of variations in the proportions of the
salts of these metals, either brought about by, or themselves causing,
changes in the permeability of the protoplasm, following the stimulus which
determines the nature of the activity which it is to undergo. But there is
as yet no indication concerning the mechanism by which this stimulation,
with its resultant electrical phenomena, is transmitted to the protoplasm
and accomplishes its characteristic effects.
ACIDITY OR ALKALINITY OF PROTOPLASM
The preceding sections of this chapter have dealt almost exclusively with
the physical properties of protoplasm; including the phenomena of solution,
ionization, surface boundary effects, and electrical disturbances, and
their probable effects upon the chemical reactions which constitute its
biological activities. It is necessary now to consider another phase of the
physical chemistry of protoplasm, namely, its chemical reaction, whether
acid, alkaline, or neutral, the effects of variation of this condition upon
the activity of the protoplasm, and the mechanism by which it tends to
preserve its own proper reaction in this respect.
The earlier methods of investigation of the chemical reaction of
protoplasm were all based upon its color reactions to various staining
agents. These sometimes led to erroneous conclusions, because of the
effects of the staining agent itself upon the tissue; some stains are
poisonous and result in the death of the protoplasm, others do not easily
penetrate the semi-permeable colloidal mass, others are themselves changed
by the oxidizing or reducing action of the protoplasm, etc. Again,
colloidal adsorption effects often lead to the so-called "capillary
segregation" of added staining materials. So that this method of study must
be used with great care, or wholly erroneous conclusions will be reached,
and many of the earlier reports have subsequently been found to be
incorrect.
The recent improvements in the apparatus and methods for the determination
of hydrogen-ion concentration have afforded a much more trustworthy method
of determining the actual acidity or alkalinity of such materials than is
obtained by color reactions, and this method is now being extensively used
in the study of the reaction of active protoplasm.
It must be kept in mind that protoplasm is an heterogeneous mass and not an
homogeneous solution, so that it is not always possible to determine the
actual conditions as to neutrality of different parts of the protoplasm of
a single cell, for example. Hence, one of the best methods of determining
the reaction which is favorable to the life and activity of any given type
of protoplasm is to investigate the reaction of a liquid medium in which
the cells live and grow; this plan being based upon the assumption that a
cell is not likely to have a reaction different from that of the medium
which is favorable to its growth.
The results of all of the many investigations which have dealt with this
problem point to the conclusion that the normal reaction for living
protoplasm is either neutral or very faintly alkaline; but that it becomes
acid when the cell is working in the absence of sufficient oxygen, and
after the death of the cell.
The first effect of a change in the reaction toward acidity of the
protoplasm is a decrease in the rate of respiration of the tissue, while
increased alkalinity stimulates respiratory activity. Whet carried to the
point of actual acidity, the respiratory coefficient becomes negative, and
the cell actually gives off carbon dioxide because of the stoppage of the
synthetic processes.
A second effect of change in reaction of protoplasm is to alter the enzymic
activity of the cell. As has been pointed out, enzymes are extraordinarily
sensitive to minute changes in the reaction of the medium in which they are
working. A change toward acidity in protoplasm immediately results in the
stimulating of carbohydrate-splitting enzymes, which increases the supply
of easily oxidizable simple carbohydrates, thereby tending to compensate
for the decrease in respiratory activity. Further, increase in acidity
increases proteolysis, thereby liberating alkaline ammonia-derivatives
which tend to neutralize the rising acidity and so to restore normal
neutrality or alkalinity. Thus it will be seen that in the very great
sensitivity of its enzyme catalysts to slight changes in the reaction of
the medium, the protoplasm possesses a very efficient mechanism for
regulating changes and restoring equilibrium, if the latter be disturbed by
any abnormal conditions. It should also be noted, at this point, that the
almost universal presence in protoplasm of salts of carbonic and phosphoric
acids acts as an additional "buffer" against pronounced changes in reaction
of the material; the bicarbonates acting by means of their ready release or
absorption of carbon dioxide, and the phosphates by their easy change from
mono-sodium phosphate to di-sodium phosphate, and _vice versa_, the former
being slightly acid and the latter slightly alkaline in reaction.
A third effect of increasing acidity is that it induces increased
imbibition of water by the colloidal gel and causes swelling of the tissue.
After death, when the reaction of the protoplasm becomes pronouncedly acid,
this swelling often proceeds to the point of rupturing of the cell-wall, or
internal membranes of the protoplasm, thus permitting the entrance of the
putrefactive bacteria and hastening the decay of the tissue.
Finally, comparatively slight variations in the reaction of the protoplasm
produce enormous changes in its colloidal condition, affecting in a very
marked degree its permeability, its power of adsorption, etc.
It is clear, therefore, that variations in the chemical reaction of
protoplasm profoundly affect its colloidal condition, its enzymic activity,
and its respiratory processes. This necessarily brief survey is sufficient
to indicate how important to the activity of the protoplasm is the chemical
reaction of the material, and the mechanism with which it is provided for
maintaining the favorable condition of neutrality or slight alkalinity.
SUMMARY
It is evident that, within the limits of a single chapter, it has been
possible to give only a very brief and incomplete discussion of some of the
most important applications of the principles of physical chemistry to the
properties and activities of protoplasm. Therefore, it may be profitable to
summarize briefly these into a series of definite statements which may
serve as a review of the principles which have been discussed in the
preceding chapters, as applied to the activities of protoplasm.
Protoplasm is a complex hydrogel, composed of an heterogeneous mixture of
proteins, fats, and carbohydrates, arranged in a foamlike structure, the
compartments of the gel being filled with an aqueous solution of the
soluble organic products of synthesis and of varying proportions of mineral
salts which are of the same general nature as those of sea-water.
The gel is not uniform throughout the volume of any given cell, but is
differentiated in different parts into what are known as the nucleus, the
chloroplasts, the plasma of the cell, etc.
The vital activities of the cell consist in chemical reactions which are
controlled by comparatively slight changes in the electrolyte distribution,
or other environmental changes which affect the colloidal condition of the
mass and, generally speaking, result in changes of the water content of the
plasma, most such chemical changes being essentially reversible hydrolytic
reactions.
The components of active protoplasm are in a condition most favorable to
chemical reactions by reason of the enormous surface area of the colloidal
material, resulting in abundance of available energy, intimate contact of
the reacting materials, and the nearest possible approach to the condition
of true solution which can be obtained without the loss of stable form and
structure.
The reactions which take place in cell protoplasm, as a result of the
action of either physical or chemical stimuli, are accompanied by
electrical disturbances, which may be either caused by, or the result of,
changes in the electrical charges of the mineral salts which are present in
the gel. Such changes, like the chemical reactions which they accompany,
may be regarded as reversible and mutually self-regulatory; so that the
protoplasm has not only the possibilities of enormous chemical reactivity,
but also the mechanism for self-regulation of its actions, the products or
results from any given series of changes generally tending to reverse the
process by which they are proceeding and so to restore the condition of
normal equilibrium.
Finally, the most characteristic difference between the reactions which go
to make up the vital activities of a living cell and those of the same
chemical substances when in inanimate form in the laboratory lies in the
presence in the colloidal mass of the accelerating catalysts known as
enzymes, which are produced by the protoplasm itself in some way which is
as yet wholly unknown; and which not only add to the possibilities of rapid
chemical change which are afforded by the colloidal nature of the material,
but also, because of their extreme sensitiveness to minute changes in
environmental conditions, serve to govern both the rate and the direction
of the individual chemical reactions which constitute the vital activities
of the protoplasmic mass. These enzymes are not distributed uniformly
through any given cell, or organism, but are localized in different parts
of the cell or tissue and so give to its different parts the ability to
perform their various different functions.
References
ATKINS, W. R. G.--"Some Recent Researches in Plant Physiology," 328 pages,
28 figs., London, 1916.
CZAPEK, F.--"Chemical Phenomena of Life," 152 pages, New York, 1911.
CZAPEK, F.--"Ueber eine Methode zur direkten Bestimmung der
Oberflächenspannung der Plasmahaut von Pflanzen," 86 pages, 3 figs., Jena,
1912.
HÖBER, M. R.--"Physikalische Chemie der Zelle und der Gewebe," 671 pages,
55 figs., Leipzig, 1911.
LIVINGSTON, B. E.--"The Rôle of Diffusion and Osmotic Pressure in Plants,"
149 pages, Chicago, 1903.
MCCLENDON, J. F.--"Physical Chemistry of Vital Phenomena," 248 pages,
Princeton University Press, 1917.
MACDOUGAL, D. T.--"Hydration and Growth," Publication No. 297, Carnegie
Institution of Washington, 176 pages, 52 figs., Washington, D. C., 1920.
SPEIGEL, L., trans. by LUEDEKING, C. and BOYLSTON, A. C.--"Chemical
Constitution and Physiological Action," 155 pages, New York, 1915.
THOMPSON, D'A. W.--"On Growth and Form," 793 pages, 408 figs., Cambridge,
1917.
WILLOWS, R. S. and HATSCHEK, E.--"Surface Tension and Surface Energy and
their Influence on Chemical Phenomena," 116 pages, 21 figs., New York,
1919, (2d ed.).
CHAPTER XVII
HORMONES, AUXIMONES, VITAMINES, AND TOXINS
Reference has frequently been made, in preceding chapters, to the effect of
various stimulating or inhibiting agencies upon the physiological
activities of plant protoplasm. In the main, these agencies are _external_
to the plant and are either physical, such as changes of temperature,
amount of light received, etc.; or chemical, such as variations in the
salts received from the soil, or common anæsthetics applied to the plants
by man. A plant grows normally under certain conditions to which it has
become adjusted by hereditary acquirements. When these conditions are
altered, the effect upon the functioning of the plant protoplasm may be
either stimulating or depressing. Extreme changes in environmental
conditions generally result in the death of the plant; but changes which do
not result in the lethal condition affect the plant by either stimulating
it to more rapid physiological activity or by depressing its normal growth
or functions. As has been pointed out, the same external influence, either
chemical or physical, which acts as a stimulant if it differs only slightly
from normal conditions, may become depressing, or positively toxic, if
present to a larger extent.
There is also the possibility of the elaboration by the plant itself of
_internal_ agents, or substances, which may have a definite stimulating or
inhibitory effect upon its metabolism and growth. The study which has been
given to these matters has practically all been carried on within very
recent years and is still in progress. Most of it is still in the
experimental stage, in which no definite conclusions are as yet possible.
Hence, the most that can be done at present is to give a brief review of
the suggestions which have been made thus far, as indicative of the
uncertainty of our present knowledge of these matters and of the general
trend of the investigations which are now in progress.
Substances which are elaborated by plants and which are supposed to have a
definite stimulating or beneficial effect upon the activities of the plant
which produces them, or to influence the physiological activities of other
plants with which these substances come in contact through either the
parasitic or the symbiotic relation, have been variously discussed under
the names "hormones," "auximones," and "vitamines"; while injurious
substances are generally known as "toxins." Whether these different terms
actually represent different definite types of substances, or whether there
are actually different groups of stimulating or inhibitory agents produced
in plants, is uncertain; but the following brief statements will serve to
indicate the general nature of the suggestions which have been put forward
and of the experimental work which is now in progress.
HORMONES
The term "hormone" was first used to designate certain stimulating
substances which are supposed to exist in the intestinal tracts of animals
and to cause the glands to elaborate and secrete their characteristic
enzymes. The supposed "hormones" are not themselves active in performing
the digestive functions of the glandular secretions, but are the exciting,
or stimulating, agents which cause the glands to secrete their active
enzymes.
The same term has been used, by certain plant physiologists, to designate
any agency, either external or internal, which stimulates plant protoplasm
to abnormal activity. It has been pointed out that there are a variety of
substances, which are themselves chemically neutral, that are powerful
stimulants of vital activity if used in only minute proportions, but are
powerful poisons if present in larger amounts. Many of the alkaloids act in
this way upon the animal organism; while chloroform, toluene, and even some
of the more complex hydrocarbons, act similarly upon the tissues of plants,
and ether vapor is known to be a powerful stimulant in accelerating the
flowering of plants and the ripening of fruits. It has been shown that the
vapors of all such substances readily penetrate the protoplasm of leaves,
seeds, etc., even when the same parts are impermeable to most mineral
salts, sugars, etc.; and that upon entrance to the protoplasm of a leaf, or
a seed, they tremendously stimulate its metabolic activity. These
hormones, as a class, are chemical substances which have very little
attraction for, or power of combination with water; and it has been
suggested that the ease with which they penetrate the protoplasm is due to
the fact that they are not held at the surface by combination with the
active water molecules which are present in the surface layer.
The principal effect which is supposed to be produced by these "hormones"
is the stimulation of the enzymic activity, particularly that of the
degenerative processes which take place late in the plant's life, at the
flowering or ripening periods. Many of the changes which take place
normally at ripening time, such as the change in color from green to yellow
or red and finally to brown or black, when the fruit or vegetable is fully
ripe, can be greatly accelerated by treatment with these substances.
Hormones are similar in type to the ethereal salts, or esters, which
constitute the natural essential oils that develop in many plants at this
stage of their growth. Hence, it seems probable that these changes in
plants which are maturing naturally may be hastened by the hormone action
of the esters and similar bodies which are developed in largest quantities
at that stage. It has been pointed out that the characteristic group which
is present in many natural glucosides is of the same general type as the
"hormone" substances which are used in the artificial stimulation of the
flowering or ripening changes. This fact, together with the possibility of
the liberation of greater percentages of these aromatic compounds from
their glucoside combinations at the later periods of plant growth, is
assumed, by some plant physiologists, to account for the change from
synthetic to degenerative processes at this stage of the plant's
development.
Further, it has been suggested that the autumnal coloration of leaves, and
their dropping from the stems of the plant, as well as the ripening of
seeds, is probably determined by the liberation in the plant, at that stage
of its growth, or as a result of changed climatic conditions at that
particular season of the year, of the hormones which either initiate or
hasten the special enzymic changes which distinguish the degenerative from
the synthetic processes of the plant.
Similarly, it has been suggested that parasitic fungi are able to penetrate
the host plant by first excreting "hormones" which bring about degenerative
changes in the tissues of the host plant and so make it more easily
penetrable by the hyphae of the parasite.
It will be seen that, in general, "hormones" are a type of substances
(possibly often present in plants in the form of glucosides) which are
supposed to stimulate the degenerative (or katabolic) vital processes in
contrast to the synthetic (or anabolic) changes. It has been suggested that
they do this in either one of two ways; namely, by favoring the
introduction of water into the protoplasm and so diluting the cell
contents, changing the osmotic pressure, etc.; or by bringing about a
separation of the colloidal layers, or films, of the protoplasmic complex,
producing a result similar to that produced by freezing the tissues. These
ideas have been suggested by studies of the changes in the equilibrium of
protoplasm when foreign substances are introduced into it. These studies
have not as yet been brought to the stage of final conclusions, and the
ideas presented must be considered as suggestive rather than as conclusive.
VITAMINES
"Vitamines," as contrasted with "hormones," are supposed stimulants of
synthetic metabolic processes, or accelerators of growth, rather than of
degenerative processes.
The term "vitamine" was first used to designate the substance, or
substances, which must be present in the diet of animals in order that the
animal organism may grow. Absence of these substances from the food of the
animal results in the stoppage of growth of young animals and in various
so-called "deficiency diseases" (such as beri-beri, scurvy, polyneuritis,
etc.) of adults. This means that the animal organism is altogether unable
to elaborate its own vitamines, and extended investigations have indicated
that the vitamines necessary for animal uses are wholly of plant origin.
The name "vitamine" was first used because it was supposed that these
substances are chemical compounds of the amine type and, since they are
necessary to normal life processes of animals, the name "vitamine" seemed
to represent both their chemical character and their functions. Later
investigations have caused doubt as to the accuracy of the first belief as
to their chemical nature, and various other names have been suggested for
the general group of substances which have the observed beneficial
effects; while such specific names as "fat-soluble A," "water-soluble B,"
etc., have been used to designate individual types of these accessory food
substances. However, the term _vitamine_ is such a convenient one and is so
generally recognized and accepted that it will probably continue to be
used, at least until some more definite knowledge of the nature and
composition of these growth-promoting, disease-preventing, and
reproduction-stimulating food constituents is obtained.
The following definition of the term "vitamines" gives a satisfactory
conception of the nature and functions of these substances, so far as they
are yet known. "Vitamines; constitute a class of substances the individuals
of which are necessary to the normal metabolism of certain living
organisms, but which do not contribute to the mineral, nitrogen, or energy
factors of the nutrition of those organisms." As sub-groups of the
vitamines, there have already been recognized the growth-promoting,
fat-soluble A; the antineuritic B, and the antiscorbutic C.
Until very recently, the investigations of vitamines have dealt exclusively
with their relation to human nutrition; although it has been generally
believed that the vitamines themselves are elaborated only by plants. It
was generally recognized, however, that those plants, or parts of plants,
which are capable of very rapid growth or metabolic changes, such as germs,
spores, leaves, etc., are generally the richest source for vitamines for
animal needs. Hence, there seemed to be considerable basis for the
assumption that the elaboration of these substances by plants is definitely
connected with their own metabolic needs. Recently, investigations of the
functions of vitamines in the growth of plants have been begun. These are
still in progress, but the following conclusions seem to be justified at
the present time: (_a_) Potato tubers appear to contain growth-promoting
substances which are essential to the proper growth of the sprouts. Whether
these are the same substances which are efficient in the prevention of
scurvy in men has not yet been investigated. (_b_) Baker's yeast is
probably dependent upon a supply of vitamines in the medium in which it is
to grow. Yeast itself, after having grown in barley wort, is one of the
most important sources of vitamines for animal uses or for purposes of
investigations of vitamine activity. But it has been reported that a yeast
cell will not grow in an artificial medium which contains all the essential
nutrients for yeast but has no vitamines of other plant origin in it. The
addition of barley wort, containing the vitamines from barley germs, or any
other similar supply of vitamines, induces rapid growth and the storage of
vitamines in the growing yeast masses. (_c_) The growth of many bacteria is
either wholly dependent upon or greatly stimulated by the presence of
vitamine-like substances in the medium upon which the microorganisms grow.
(_d_) _Sclerotinia cinerea_, the brown rot fungus of peaches and plums,
will grow only in a medium which contains, in addition to the essential
sugar, salts, and nitrogenous material, vitamines derived from either the
natural host plant tissues or other plant sources. These may be of two
types (namely, a vegetative factor and a reproductive factor) or two
different manifestations of activity of the same vitamine substance. But
both of these factors must be provided before the fungus can make its
characteristic growth.
There is, as yet, no conclusive evidence on many of the matters concerning
the relation of vitamines to plant growth. But it seems that these
substances are of almost universal occurrence in the organic world; that
they are not of the same general type as other substances which are
essential to the nutrition of plants or animals, but have specific
stimulating or regulating effects upon the physiological activities of the
organism; that the vitamines which are essential to animal life are
elaborated by plant tissues, but that in the case of the bacilli of certain
human diseases there seems to be some indication that the affected tissues
of the animal host produce vitamines which are essential, or favorable, to
the growth of the parasitic organism. There seems, therefore, to be
evidence of a mutual relation between plants and animals with respect to
their nutritional needs for the so-called "vitamines." But the evidence
concerning the function of these substances in the tissues of the organism
which elaborates them is, as yet, inadequate to provide any clear
conception of the reason for their development or of the mechanism by which
they are elaborated. Neither is there, as yet, any conclusive evidence
concerning the chemical nature of the substances themselves.
AUXIMONES
Certain investigations have indicated that bacteria, at least, develop
exogenous vitamines which are beneficial to the growth of other plants.
These are the so-called "auximones." For example, bacterized peat seems to
contain auximones which may be isolated from the peat and exert a
beneficial effect upon the growth of various seed-plants, including common
farm crops. Neither the original experimental data, nor the theories which
have been advanced to account for the observed beneficial effects of the
supposed "auximones" have, as yet, sufficient confirmatory evidence
definitely to establish their soundness. But it seems that there is a
probability that some plants, at least, do elaborate vitamines, or
auximones, which are useful to other plants.
TOXINS
Toxins are substances which affect injuriously the normal activities of the
organism. As has been pointed out, they may be the same substances which,
in lesser concentrations, exert a stimulating effect upon the same
organism. Hence, it is probably inaccurate to discuss the toxins as a
distinct group of substances.
There are, however, a large number of water-soluble chemical substances
which are injurious to all living protoplasm, even at concentrations
considerably less than the point of osmotic equilibrium in the juices of
the protoplasm. These substances may act either directly or indirectly upon
the protoplasm, but at certain concentrations they always affect it
injuriously. In the main, these toxins are _external_ agents of other than
plant origin; although chemical substances developed by one plant may be
toxic to other plants, or even to other organs of the same plant than those
in which they are elaborated.
Toxins may be either _general_ (i.e., injurious to all types of plants), or
_specific_ (i.e., injurious to only certain species) in their action.
Examples of specific toxicity are of only minor importance in plant
studies. They seem to be generally explainable on the basis of some unusual
lack of resistance or failure of the susceptible plants to be able to
exclude the entrance of these injurious substances into the protoplasm by
"selective adsorption," or to convert the injurious substances into
insoluble and non-injurious forms, as is done by other plants which are not
susceptible to injury by these "specific" poisons. Hence, particular
attention need not be given to this type of toxins.
Toxic substances may act injuriously upon plant tissues in a variety of
ways. Many electrolytes, especially the salts of the heavy metals of high
valency, coagulate protein material and the entrance of such substances
into the protoplasm causes disturbances in the colloidal condition which
cannot be otherwise than injurious to its normal activities. Similarly,
formaldehyde and many other organic compounds may affect the colloidal
properties of the protoplasmic gel in such a way as to injure the plant
tissues.
The same substance is sometimes much more injurious to the tissues of one
part of a plant than it is to those of another part of the same plant.
Thus, the rootlets of a young growing plant are much more susceptible to
injury by many mineral salts than are the vegetative parts of the same
plants; while anæsthetics of various kinds generally exhibit their greatest
injurious effects upon the leaves, or synthetizing cells. Again, the
mycelia of fungi are much more easily killed by toxic agents used as
fungicides than are the spores of the same fungi. Some of these observed
differences in toxicity may be due to differences in the physiological
effect of the substance upon the protoplasm of the tissues which it enters,
and others may be due to differences in the resistance of the protoplasm,
or of its protective coverings, to penetration by the toxic material.
Indeed, the possibilities of different types of toxic action, and of
resistance to it by individual plants and species, are so varied that it is
not possible to divide toxic agents into specific groups according to the
nature of their injurious action upon the plant cell. They are, therefore,
more commonly grouped into classes according to their chemical nature and
economic significance as fungicides, as follows: inorganic and organic
acids; caustic alkalies; salts of the heavy metals; hydrocarbon gases;
formaldehyde; alcohols and anæsthetics; nitrogenous organic compounds; and
miscellaneous decomposition productions of organic origin. The following
brief review of some of the results of the experimental studies of the
toxicity of different compounds belonging to these several groups will
serve to indicate the general trend of the investigations of these matters
which have thus far been made.
=Acids.=--The common inorganic acids (hydrochloric, nitric, and sulfuric)
kill the rootlets of common farm crops when the latter are immersed for
twenty to twenty-four hours in solutions of these acids containing from
three to five parts per million of free acid. Acetic acid must be about
five times as concentrated as this, and other organic acids may be much
more concentrated still before they produce the same injurious effects. The
toxic effect of all these acids is greatly reduced in soil cultures, or if
particles of sand, graphite, clay, filter paper, etc., are suspended in the
solutions containing the acids, the reduction in toxic effect being
probably due to the adsorption of the acids upon the solid particles.
Hence, the concentrations which limit the toxic effects of these acids in
water solutions cannot be taken as representing the condition with which
the same plant will have to contend when growing under normal cultural
conditions.
=Alkalies.=--The caustic alkalies must usually be present in from five to
ten times as great concentrations as those of the mineral acids, in order
to produce the same injurious effects upon the rootlets of common plants.
The so-called "alkali" of soils is not alkali at all, but is neutral
soluble salts present in sufficient concentration to exert a toxic effect.
=Salts of the heavy metals= are especially toxic to rootlets of plants.
Salts of copper, mercury, and silver, have been found to kill the roots of
seedlings immersed in them for twenty-four hours when present in
proportions of less than three parts per ten million, while salts of many
other heavy metals are toxic when present in concentrations of less than
one part per million. The salts of the alkali metals are considerable less
injurious than are those of the heavy metals, but even these exert their
familiar injurious effect if present in concentrations which, measured by
the ordinary standards, would still be regarded as very dilute solutions.
=Illuminating gas=, and similar hydrocarbon gases, kill plants when present
in the atmosphere in as little as one part per million. Leaves, buds, and
roots are all alike sensitive to this toxic effect, the nature of which is
not yet understood.
=Formalin=, or formaldehyde, is a penetrating toxic agent for nearly all
plant cells, and is commonly used as a fungicide for the destruction of
parasitic fungi. It probably affects the colloidal condition in some way
similar to its hardening effect upon gelatin, etc.
The toxic effect of many different =organic compounds= is so varied in its
nature and extent that it is impossible to give any satisfactory brief
review of its manifestations. Recent investigations appear to indicate that
organic products of decomposition of plant residues in the soil may exert
powerfully toxic effects upon succeeding generations of the same, or of
different, plants growing on the land. But the experimental data and
conclusions concerning these matters are not yet accepted without question
by all students of plant science or of the problems of the productivity of
the soil. In fact, it is yet an open question whether toxic soil
constituents are really an important factor in the so-called
"unproductivity" of certain soils.
Alkaloids, and even the amino-acids which are produced in the tissues of
some species of plants, while not toxic to the plants or organs which
elaborate them, sometimes exhibit strikingly toxic action upon other plant
organs with which they are brought into contact. There is, as yet, no
satisfactory explanation of this difference in behavior between plant
tissues toward various organic toxic substances.
In fact, the whole subject of the toxic action of various substances upon
plants needs much more study before it is brought to the point where it
will afford definite knowledge of either the physiological problems
involved or of their practical applications in questions of soil
productivity, etc.
CHAPTER XVIII
ADAPTATIONS
Most of the discussions which have been presented in the preceding chapters
have dealt with the types of compounds, the kinds of reactions, and the
mechanism for the control of these, which are exhibited by plants under
their normal conditions for development. The results of the evolutionary
process have produced in the different species of plants certain fixed
habits of growth and metabolism. So definitely fixed are these that in each
particular species of plants each individual differs from other
individuals, which are of the same age and have had the same nutritional
advantages and environmental opportunities for growth, by scarcely
perceptible variations, if at all. Indeed, this fixed habit of development
makes possible the classification of plants into genera, species, etc.
While _different species_ of plants, given the same conditions of nutrition
and environment, produce organs of the widest conceivable variety in form,
color, and function; within the _same species_, the form and size of
leaves, the position and branching of the stem, the color, size, and shape
of the flower, the coloration and markings of the fruit, etc., are
relatively constant and subject to only very slight modifications.
It is unnecessary to say that the mechanism, or the impulses, which govern
the morphological characters of the tissues which any given species of
plants will elaborate out of the crude food material which it receives from
the soil and atmosphere, are wholly unknown to science. It is the commonly
accepted assumption that the fixed habit of growth of the species is
transmitted from generation to generation through the chromosomes of the
germ cells. But the nature of the elements, or substances, which may be
present in the chromosomes, which influence the character of the organs
which will develop months later, after the plant which grows from the germ
cell has gone through its various stages of vegetative growth, is still
altogether unknown. There can be no question, however, that some influence
produces a fixity of habit of growth and development which is almost
inevitable in its operation.
But while this unvarying habit of growth is one of the fixed laws of plant
life, there are occasional deviations from it. A plant which, under normal
conditions of growth, develops in a certain fixed way, when exposed to
unusual environmental conditions, may, and often does, alter its habit of
growth in what may metaphorically be said to be an attempt to adjust itself
to the new conditions. Numerous examples of this phenomenon might be cited.
Certain algæ, which grow normally in water at a temperature of 20° to 30°
and which are killed if the temperature rises above 45°, have been grown
for successive generations in water the temperature of which has been
gradually raised, until they produce apparently normal growth in water the
temperature of which is as high as 78°; also, certain types of algæ
normally grow in the water of hot springs at temperatures of 85° to 90°,
and others in arctic sea-water the temperature of which sometimes falls to
-1.8° and never rises above 0° C. This phenomenon of the adjustment of a
species of plants to new conditions, which in the case of farm crops is
sometimes called "acclimatization," is of common occurrence and is often
utilized to economic advantage in the introduction of new strains of crops
into new agricultural districts. Again, the normal development of plants
may be altered as the result of injury or mutilation. Thus, if the ear is
removed from the stalk of Indian corn, at any time after flowering, there
always results an abnormal storage of sucrose in the stalk, instead of the
normal storage of starch in the kernels. Similarly, midsummer pruning of
fruit trees generally results in the production of abnormally large number
of fruit buds on the remaining limbs. Many other familiar examples of
alteration of normal development in response to, or as the result of,
abnormal conditions of growth might be cited.
TYPES OF ADAPTATIONS
To designate these different alterations of normal growth, several
different terms have been used. Among these, "adaptation," "accommodation,"
and "adjustment" have been commonly used by different biologists. Sometimes
these are used interchangeably, and sometimes different terms are used to
designate different types of response to altered conditions of growth.
Inasmuch as there seems to be no generally accepted usage of these
different terms, only one of them, namely, the word "adaptation" will be
used here; and different manifestations of this phenomenon will be
distinguished by using appropriate adjectives, as "physiological
adaptations," "chromatic adaptations," "morphological adaptations," etc.
Two markedly different types of responses to altered conditions, or of
adjustment to environment, may be recognized. In the first of these, for
which we will use the term "physiological adaptation," the species of plant
simply acquires the ability to exist and grow normally under conditions
which formerly inhibited its growth. Thus, we may speak of the phenomena
mentioned above as "acclimatization" as the _physiological adaptation_ of
the crop to the new conditions of growth. In general, physiological
adaptations include such variations in the characters or habits of growth
of plants as results in differences in resistance to heat or to cold,
relations to water, aggressiveness in competition with other plants, etc.
In such cases, no modification of the morphological characters of the plant
can be observed, the changes which take place in the structure of the plant
(if, indeed, there be any such changes) must be only minor adjustments of
the protoplasm to meet the new environmental needs.
In the second type of adaptations, for which we will use the term
"morphological adaptations," the structure, or color, or some other
morphological character of the plant is actually changed in some easily
recognizable way, in order that the plant may be better adjusted to its
environment. As examples of _morphological adaptations_, there may be cited
the change in color of sea-weeds with increasing depth in the sea, and
other examples of chromatic adaptation which are discussed below; the
development of fewer, or a larger number, of buds on the above-ground stems
of plants, in response to decreases, or increases, in the available supply
of food; the alteration in the size and shape of the leaves of many plants
when they are grown in shade; the dwarfing of plants at high altitudes, or
under conditions of severe drought; the development of underground storage
organs for certain species of shrubs and trees which grow in regions that
are subject to periodical burning-over, in such a way as to destroy the
above-ground storage stems, etc.
Hence, the two terms, as we will use them here, may be defined as follows:
_morphological adaptation_ is a change in the structural character of the
species in order that it may be better fitted to meet the needs of the new
conditions of growth; while _physiological adaptation_ is an acquired power
to survive and develop under abnormal conditions, which is not accompanied
by any visible change in the characteristic structure of the species.
Both of these types of adjustment may be either hereditary (or
evolutionary), or spontaneous in their origin and development. Changes
which are evolutionary are fixed by heredity and become definite habits of
growth in the species. Their origin may be explained in either one of two
ways; namely, the so-called "increase by use," and "the survival of the
fittest." The hypothesis of "increase by use," as an explanation of
adaptations, is based upon the well-known observation that, in animals,
muscles and other organs increase in volume as they are extensively used;
and the assumption of the application of this principle to the phenomenon
of adaptation supposes that the modification of any given structure or
composition is the result of the hereditary accumulations of increased size
resulting from use, or of atrophy from disuse. The "survival of the
fittest" theory supposes that individuals of a species differ from each
other by spontaneous variations, and that in the competitive struggle for
existence those forms which are best adapted to the environmental
conditions survive while the others perish. The contrast between these two
views is that the first holds that adaptation proceeds by development, and
the second that it proceeds by variation and elimination; the first
presupposes the existence in the organism of a mechanism for response to
changing conditions, and the second assumes that there are chance
variations followed by the death through competition of the forms which are
not able to meet the needs of the environment.
Confusion arises whenever an attempt is made to apply either of these
theories to all kinds of adaptations. The idea of increase by use can be
applied with some satisfaction to certain morphological adaptations in
animal structure; and to such phenomena as the increase in strength of the
branches of fruit trees, either with or without corresponding increase in
size, as the load of fruit increases. But it certainly cannot apply to
color change in surface pigmentation of either animals or plants, which is
one of the most common forms of adaptation. Furthermore, it is difficult
to conceive the general application of this idea to alterations of habits
of growth of plants, since a plant cannot have any such thing as a
voluntary control over the amount of "use" which it makes of its different
organs in response to changes of environment. The common form of statement
that a plant develops an organ, or a process to meet a certain need, or
modifies its habits of growth to meet a change of environment are, of
course, purely metaphorical, and can only be taken to mean that such
processes are mechanical responses to changes in external conditions.
The nature of the mechanism by which these responses are accomplished is,
as yet, wholly unknown. There is accumulating a large mass of experimental
evidence which goes to show that, while both temperature and light are very
important factors in determining the type of changes which will take place
in a living organism, the so-called "photochemical action of light" is by
far the most potent of all the climatic factors which influence the course
of development of a plant. But we have, as yet, no inkling of how the
protoplasm of the plant adjusts or controls its responses to variations in
any of these external factors.
With these general considerations in mind, we may now proceed to the
consideration of certain particular types of adaptations.
CHROMATIC ADAPTATIONS
Adaptations have been observed in both the energy-absorbing pigments of the
general tissues and in the ornamental epidermis pigments of plants. The
former are by far the most important from the physiological point of view;
while the latter may have interesting biological significance.
Under nearly all conditions of growth of land plants, the supply of the
chlorophylls and their associated pigments provides for the absorption of
solar energy far in excess of the amount necessary for the photosynthetic
assimilation of all the carbon dioxide which is available to the plant. It
has been shown that an active green leaf, on an August day, can absorb
eight times as much radiant energy as would be required to assimilate all
the carbon dioxide present in the air over its surface. No land plant,
under normal conditions, develops supplementary pigments in order to
utilize other than the parts of the spectrum which are absorbed by
chlorophyll and its associated pigments.
But deep-sea plants show quite a different phenomenon of pigment
development. Water is a blue liquid. At depths of 40 feet or more, the
light which penetrates is devoid of red rays, feeble in yellow, and is
characteristically green or blue in color. Now, the red rays of the
spectrum are the ones which are most efficient for photosynthesis. Sea
weeds which grow at these depths are brilliantly red in color, at
intermediate depths they are brown, and at the surface they are green, in
the same latitudes. While it is possible that the temperature of the water
at these different depths may have something to do with the chemical
synthesis of the pigments, it appears plain that this color change at
increasing depths is a definite adaptation to provide for the absorption of
the solar energy which is available at these depths. It has been shown that
these pigments of deep-sea plants are additional to, and not substitutes
for, the chlorophylls, etc. The latter pigments are present in normal
amounts, but are supplemented by those which absorb the green and blue
portion of the spectrum. Hence, this type of adaptation might be conceived
to be a "survival of the fittest," resulting in the "natural selection" of
individuals of the highest total pigmentation. But, on the other hand,
there is experimental evidence to show that plants possess some means of
varying their pigmentation in response to the character of the light which
comes to them. For, it has been found that a complete change in color of
certain highly colored plants can be produced in a single generation, by
growing the plants in boxes or chambers whose walls are composed entirely
of differently colored glass, so that the plants within receive light of
only a particular part of the spectrum. In such cases, the plant, starting
with an initial "natural" color, changes through a succession of colors
until it finally reaches equilibrium at one which provides for the proper
absorption of the right kind of light from the new supply which is
available to it. Hence, it seems proper to conclude that chromatic
adaptation is not a process of "natural selection," but a definite result
of an actual mechanism for adaptation to changed environmental conditions
of supply of radiant energy.
STRUCTURAL ADAPTATIONS
Changes in structure to meet special conditions of growth may be of several
different types.
One of these, which is often cited as an example of adaptation (in this
case, the term is used with a significance quite different than that in
which it is being used here) is that of the development of unusual and
often fantastic shapes of flowers, which are so related to the anatomy of
certain species of insects that visit these flowers in search of nectar,
that provision for the cross-fertilization of the plants is insured, in
that the pollen from the anthers of one flower becomes lodged on the body
of the insect as it is withdrawing from the flower in such a way that it
comes in contact with the pistil of a second flower as the insect enters
it. Such flowers often have such peculiar shapes and lengths of nectar
tubes, etc., that only a single species of insect, whose anatomical shape
is "adapted" to that particular blossom shape can enter the flower in its
search for nectar. It is clear that this form of "morphological adaptation"
is a highly specialized one, which can only be the result of a long process
of evolutionary development. It is obvious that the plant cannot possibly
possess a mechanism, or ability, to alter its flower form in order to make
it conform to the shape and length of the proboscis, or other body parts,
of a particular species of insect. Either the insect or the plant, or both,
must go through a process of evolutionary development in order to arrive at
this form of mutual "adaptation."
A form of true morphological adaptation (in the sense in which we have been
using the term) is exhibited by many species of plants, which are provided
with many more buds, or growing points, than ever actually begin to grow.
For example, the single plumule which develops from a germinating wheat
embryo has at its upper end a hundred or more tiny growing points. At the
proper stage of its growth, several of these tiny buds begin to grow into
individual separate stems, and the new wheat plant thus produces several
stems from one seed and root system, a process known as the "stooling." The
number of stems in a single "stool" depends upon the number of the
potential growing points which are stimulated into growth. It varies from
only two or three up to as many as thirty or forty, and is apparently
controlled by the favorable or unfavorable conditions of climate or
nutrition at the time when the "stooling" takes place. The plant is thus
provided with a mechanism for adapting its possibilities of growth to the
supply of growth-promoting material which is available to it.
Many other plants produce far more buds than ever develop into growing
tissues, and buds which, under normal conditions, remain dormant, under
altered conditions start into growth and so provide for an "adaptation" of
the total mass of the growing plant to correspond with the altered
conditions of growth. The actual means by which certain buds are stimulated
into growth while others remain dormant, or are inhibited from growing, are
as yet unknown. Two theories have been advanced. One is that the growing
buds absorb all available nutrition and the others remain dormant by reason
of lack of growth-promoting material. The other is that the vegetating
(growing) tissue elaborates and sends to other parts of the organism one or
more substances, which actually inhibit growth of the other parts, as
dormant buds, etc. The experimental evidence which has been presented thus
far is inconclusive, but seems to favor the distribution of nutritional
material as the governing factor, although there is some evidence which
seems to indicate that a supposed growth-inhibiting substance is actually
translocated from rapidly-vegetating tissues to other parts of the plant.
There is, however, no explanation of how the buds, or other tissues, which
do grow get their initial stimulus, while the dormant buds do not. After
growth has once started, the changes in osmotic pressure due to the
accumulation and translocation of synthetized materials can account for the
movement of new nutritional material for the synthetic processes into the
growing organ; but this would not account for the selective stimulation of
only a part of the buds, or possible growing points, of a plant, or for an
adaptational development of others under altered conditions of growth.
The form of morphological adaptation which has been discovered in the
course of the study of the native vegetation of the campos of Brazil (which
have a very dry season and have been regularly burned over by the natives
for many generations) in which the papilionaceous shrubs have developed
underground trunks, or stems, and seem actually to profit in luxuriance of
growth when the rainy season comes on by reason of this morphological
adaptation to the unusual environmental conditions, is wholly inexplicable
by any present knowledge of the science of plant growth.
PHYSIOLOGICAL ADAPTATIONS
The type of adjustment to environmental conditions which does not result in
any recognizable alteration in the structure of the plant, but simply
permits it to grow under new conditions, manifests itself in many ways.
These adjustments are usually associated with differences in temperature
during the growing season, and for this reason, most such examples of
adaptation have been studied in connection with possible temperature
reactions upon the growing organism.
However, recent investigations seem to point strongly to the conclusion
that the amount of _light_ rather than the _temperature_ of the new
surroundings is the most important influence in determining the
physiological processes known as the "acclimatization" of plants. For
example, a very elaborate series of investigations has shown that the
flowering stage in the development of plants is determined by the length of
the daylight period per day, irrespective of the actual amount of
vegetative growth which the plant has made. Thus, tobacco plants, which
during a period of long days grow to the height of 8 or 10 feet before
blossoming, if grown at the same temperature in periods of short days (or
if kept in the dark during a portion of the longer days) will blossom when
less than 3 feet in height and when the total mass of vegetative material
which has been produced is less than one-third of that of the "gigantic"
plants of the same variety grown with longer periods of illumination per
day. This same principle has been found to hold good for many widely
different types of plants. In some species, however, flowering is favored
by long days, and vegetative growth by short daylight illumination. But in
all species which have been studied, there seems to be a direct relation
between the length of day, or the total illumination per day, and the
normal or abnormal functioning of the plant. It is apparent that at least
the physiological function of sexual reproduction (flowering and
seed-production) is determined by the length of daylight illumination. The
duration of daylight per day which is necessary to induce the blossoming of
the plants varies for different species, but it is constant for individuals
of the same species. This adaptation of stage of growth to duration of
daily illumination must, therefore, be an evolutionary character of the
species.
Hence, it appears that in many cases physiological adaptation may be a
direct response of the life-processes of the plant to the daily length of
photochemical stimulation which it receives from solar light. But there is,
as yet, no explanation of how this (or any other) influence actually
changes the vital processes of the plant protoplasm so as to bring about
either a morphological adaptation of structure or a physiological
adaptation of functions to altered conditions of growth.
CONCLUDING STATEMENTS
Enough has been said to show how very inconclusive and unsatisfactory is
our knowledge of the phenomena known as "adaptation." Even the nomenclature
used by different scientists to describe its various manifestations is
confused and misleading. For example, certain crops are said to be
"adapted" (i.e., suited) to certain types of soils, and _vice versa_; crops
are said to be "adapted" to given agricultural districts, etc.
In this chapter, an attempt has been made to arrange in some semblance of
order some of the known manifestations of alteration of fixed habits of
growth of plants in response to changes of environment, and to point out
some of the suggestions of possible explanations of these phenomena which
have been presented by different investigators.
This presentation cannot be considered as anything other than an
introduction to a field of study which is as yet almost entirely
unexplored, and, like all other unexplored territory, is full of mysteries.
If the study of this chapter serves to stimulate interest in these
mysteries and wonders of plant life, its purpose will have been
accomplished.
INDEX
Bold-face figures indicate main references
Accelerators, 196.
Accessory substances, 19.
Achroo-dextrin, 61.
Acid, acetic, 125, 126, =128=, 132, 133, 166.
arabic, 68.
arachidic, 133.
aspartic, 168, 177.
brassic, 133.
butyric, 126, 133.
capric, 133.
caprylic, 133.
carnaubic, 140.
cerotic, 133, 140.
citric, 125, 127.
convolvulinic, 81.
crotonic, 133.
diamino-oxysebacic, 169.
diamino-trioxydodecanic, 169.
digallic, 96.
ellagic, =96=.
euxanthic, 84.
formic, 25, =126=, 128, 132.
galactonic, 42.
gallic, =96=.
geddic, 69.
gluconic, 42.
glucuronic, 42, 43.
glutamic, 168, =177=.
glycero-phosphoric, 142.
hydrocyanic, 77.
jalapinic, 81.
lauric, 133.
lignoceric, 133.
linoleic, 133.
linolenic, 133.
malic, 124, =127=.
malonic, 124.
mannonic, 42.
melissic, 133.
meta-pectic, 68, =70=.
mucic, 68.
myristic, 133.
nitric, 125.
nucleic, =162=.
oleic, 133.
oxalic, 68, 124, 125, =126=, 128.
palmitic, 133, 140.
parapectic, 70.
pectic, =31=, 70.
phosphoric, 141, 142, =162=.
propionic, 126, 166.
pyrocatechuic, 96.
quercitannic, =98=.
racemic, 54.
ricinoleic, 133.
ruberythric, =83=.
saccharic, 42, 68.
salicylic, 81.
sarco-lactic, =128=.
stearic, 131, =133=.
succinic, =127=, 128.
sulfuric, 125.
sylvinic, 149.
talonic, 42.
tannic, 97, =127=.
tartaric, =127=.
uric, =160=.
xanthoproteic, 173.
Acid amides, 151.
Acidity of protoplasm, =234=.
Acid glucosides, 81.
Acid potassium oxalate, 125.
Acid potassium sulfate, 88.
Acid salts, =124=.
Acids as toxins, =246=.
Acid sodium sulfate, 125.
Acrolein, 135.
Acrose, 28.
Activators, 196.
Adamkiewicz's reaction, 173.
Adaptations, =249=.
Adenase, 190.
Adenine, =160=, 162.
Adipo-celluloses, =74=.
Adsorption, =214=.
Æsculetin, 81, 82.
Æsculin, =81=, 82.
Ætiophyllin, 106, 107, 109.
Ætioporphyrin, 108, 109, 110.
Alanine, 168, 177.
Albumins, 175, 176.
Albuminoids, 175, 176.
Alcogel, 205.
Alcohol, ethyl, 40, 125.
benzyl, 80.
carnaubyl, 135.
ceryl, 135, 140.
cetyl, 129, 135.
coniferyl, 80.
melissyl, 135.
myricyl, 129, 140.
phytyl, 104, =105=.
polyhydric, 31.
Alcohol glucosides, 80.
Alcosol, 205.
Aldehyde, benzoic, 148.
cinnamic, 148.
formic (_see_ formaldehyde).
glyceric, 35.
Aldehyde glucosides, 80.
Aldehydrol, 46.
Aldonic acids, 42, 44.
Aldose, 32.
Alizarin glucosides, 78.
Alkalinity of protoplasm, 234.
Alkalies as toxins, =247=.
"Alkali salts," 10, =247=.
"Alkali soils," 10, =14=.
Alkaloidal reagents, 154, 172.
Alkaloids, 18, 20, 151, =153=, 248.
Allose, 36, 37.
Allyl isosulfocyanide, 88, 89, 148.
Allyl sulfide, 148.
[alpha]-glucose, 46.
[alpha]-glucosides, 55.
[alpha]-methyl glucoside, =47=.
Altrose, 36, 37.
Aluminium, =4=.
Amandin, 170, 176.
Amines, =151=.
Amino-acids, 6, 151, =166=, 179, 248.
Ammonia, 152.
Ammonium hydroxide, 142, 152.
Ammonium salts, 6.
Amorphous chlorophyll, 104, 105
Amphoteric electrolytes, 172.
Amygdalase, 87.
Amygdalin, 81, =86=.
Amyl acetate, 148.
Amylase, 186, 189, =191=.
Amylo-cellulose, 60.
Amylo-dextrin, 61.
Amylo-pectin, 60.
Amylose, 60.
Anergic food, =2=, 17.
Animal nucleic acids, =162=.
Antagonism, 14.
Anthocyans, 83, 102, =115=, 121.
Anthocyanidins, 116.
Anthocyanin, 102.
Anthoxanthins, =117=.
Anthraquinone, 83.
Antienzymes, 120, =197=, 198.
Antioxidase, 120.
Antiscorbutic C, 243.
Apigenin, 84, 118.
Apiin, =84=.
Apiose, 84.
Araban, =69=.
Arabinose, 35, 44, 68, 69, 88.
Arabinosides, 56.
Arbutin, 77, =79=.
Arginine, 169, 171, 177.
Arsenic, 13.
Asymmetric carbon atom, 33.
Atropine, 155, =156=.
Autotrophic plants, 16, 18.
Auximones, 239, 240, =244=.
Available plant food, =4=.
Avenalin, 176.
Baptigenin, 79.
Baptisin, =79=.
Beeswax, 133.
Beet sugar (_see_ sucrose).
Berberine, 155.
Betaine, =152=.
[beta]-glucase, 55.
[beta]-glucose, 46.
[beta]-glucosides, 55.
[beta]-methyl glucoside, 47.
Biogens, 223.
Biological significance, =19=.
Biuret reaction, 173.
Borneol, 148.
Boron, 13.
Bromelin, 189.
Brucine, 155, =157=.
Buffers, 236.
Butter fat, 133.
Butyric acid ferment, 190.
Cadaverine, 152.
Caffeine, =160=.
Calcifuges, 9.
Calciphiles, 9.
Calcium, 3, 5, =9=, 10, 14, 68.
Calcium oxalate, 126.
Campferitrin, 118.
Campferol, 118.
Camphene, 147.
Camphor, 148.
Cane sugar (_see_ sucrose).
Caoutchouc, =147=.
Capillary segregation, 235.
Carbohydrases, 189.
Carbohydrates, 18, 20, =21=, =30=, 163, 234.
Carbon dioxide, 2, 3, 18, =21=, 22, 23, 24, 40, 222.
Carbonic acid, 227.
Carbon monoxide, 24.
Carboxyl, 124.
Carboxylases, 186, 190.
Carnauba wax, 133, 140.
Carotin, =112=, 113, 121.
Carotinoids, 102, =111=.
Carvacrol, =148=.
Casein, 165.
Castanin, 176.
Castor oil, 130.
Catalases, 190, 193.
Catalysis, 182.
Catalysts, 17, 25, 183.
Catechol tannins, 97.
Catechin, =97=.
Catechu tannins, =97=.
Cellobiose, 52.
Cell structure, 221.
Cellulase, =71=, 186, 189.
Celluloid, 73.
Cellulose, 20, 45, 63, 67, =72=.
Cell-wall, 9, 12, 222.
Cerebrosides, 141, =144=.
Chemical resistance, 216.
Cherry gum, 68.
Chinovose, 35.
Chlorine, =12=.
Chlorophyll, 10, 11, 21, 27, =102=, 105, 110, 111, 113, 122, 254.
Chlorophyll _a_, 103, 106, 107, =111=.
Chlorophyll _b_, 103, 106, 108, =111=.
Chlorophyllase, 104.
Chlorophyllin _a_, 106, 107.
Chlorophyllin _b_, 106, 107.
Cholesterol, 129, =136=.
Choline, 89, 103, 141, 142, =152=.
Chromatic adaptations, 251, =253=.
Chromogens, 92, 119.
Chromo-proteins, 175.
Chrysin, 117.
Cinchonine, 155, =157=.
Coagulated proteins, 175.
Coagulation enzymes, 190.
Cocaine, 155, =157=.
Cocoanut oil, 133.
Codeine, 155, =158=.
Coenzymes, =197=.
"Cold-drawn oils," 137.
Collodion, 73.
Colloidal phenomena, 17, =202=.
Colloidal solutions, 204.
Colloids, 202.
Colophene, 147.
Colophony, 149.
Compound celluloses, 71, =73=.
Conglutin, 176.
Coniferin, =80=.
Coniine, 155, =156=.
Conjugated proteins, 165, 174, =175=.
Continuous phase, 203.
Convolvulin, =81=.
Copper, 13, 247.
Cork tissue, 99, 101.
Corn oil, 130.
Corylin, 176.
Cottonseed oil, 130.
Critical elements, 4.
"Crude fat," 141.
Crystalline chlorophyll, 104, 105.
Crystalloids, 202.
Cumarin, 81, 148.
Curarine, 157.
Cuto-celluloses, =74=.
Cyanidin, 85, 116.
Cyanin, =85=.
Cyanophore glucosides, 86.
Cyanophyllin, 107, 108.
Cyanoporphyrin, 108.
Cymarigenin, 90.
Cymarin, =90=.
Cymarose, 90.
Cystine, 168, 171.
Cytase, 72, 189.
Cytosine, 161, 162.
Daphnetin, 81, 82.
Daphnin, 81.
Deaminases, 186, 190.
Delphinidin, 85, 116.
Delphinin, =85=.
Derived proteins, 173, =175=, 177.
Dextrin, 59, =61=.
Dextrinase, 189.
_d_-galactose, 33.
_d_-glucose, 33.
Dextrosans, 59.
Dextrose (_see_ glucose).
Dhurrin, =87=.
Diastase (_see_ amylase).
Diastase of secretion, =191=.
Digitaligenin, 89.
Digitalin, =89=.
Digitogenin, 89.
Digitonin, =89=, 90.
Digito-saponin, 90.
Digitoxigenin, 89.
Digitoxin, =89=.
Digitoxose, 89.
Diglycerides, 131.
Diose, 30.
Dioxyacetone, 35.
Dipeptides, 167.
Disaccharides, 31, =48=.
Dispersed phase, 203.
Dispersion medium, 203.
Dispersion phenomena, 203.
Drying oils, 132.
Dulcitol series, 36.
Edestin, 170, 176.
Egg-albumin, 165.
Electrical phenomena of protoplasm, =233=.
Electrolytes, 213, 227.
Emulsoids, 206, 214.
Emulsions, 206.
Emulsin, 55, 77, 87, 184, 189.
Enol, =44=, 56.
Enzymes, 17, 18, 19, 20, 23, 26, 120, 121, =181=, 183, 194, 199, 224.
Erepsin, 189.
Erythro-dextrin, 61.
Erythrophyllin, 107.
Erythrose, 35.
Essential elements, 4.
Essential oils, 18, 146, =147=, 224.
Esterases, 186, 189.
Esters, 124, 125, 129.
"Ether extract," 141.
Etherial salts (_see_ esters).
Ethersol, 205.
Ethyl acetate, 125.
Ethyl nitrate, 125.
Excelsin, 176.
Extracellular enzymes, 184.
Fats, 18, 20, =129=, 224, 227.
Fat-soluble A, =243=.
Fatty acids, 132, 142.
Fehling's solution, 39, =47=.
Fermentability, =40=.
Ferments (_see_ enzymes).
Ferric salts, 11.
Ferrous salts, 11.
Fisetin, 118.
Flavone, 82, 83, 102.
Flavonol, 84.
Food, =1=.
Formaldehyde, =22=, 23, 25, 26, 27, 247.
Frame-work material, 20, 67.
Fraxetin, 82.
Fraxin, =82=.
Fructose, 23, 28, 32, 36, 38, 41, 44, 45, =47=, 57, 162.
Fructosides, 41, 42.
Fruit sugar (_see_ fructose).
Fucose, 35.
Fucoxanthin, 102, 112, 114.
Galactans, 47, 59, =63=, 72.
Galactoheptose, 36.
Galactooctose, 36.
Galactose, 32, 36, 38, 45, =47=, 57, 72, 77.
Galactosides, 41, 42.
Gaultherin, 81.
Gel, 172, 205, 208.
Gelation, 210.
Gel-formation, =208=, 211.
Gentianose, 52, =53=.
Gentiobiose, 49, =52=, 53.
Gentisin, 119.
Gitaligenin, 89.
Gitalin, =89=.
Gitogenin, 89.
Gitonin, =89=.
Glaucophyllin, 107.
Gliadin, 165, 170, 176.
Globulins, 170, 175, 176.
Glucase, 186.
Glucodecose, 44.
Glucoheptose, 36, 44.
Glucononose, 36.
Glucooctose, 36.
Glucoproteins, 175.
Glucose, 23, 28, 32, 36, =37=, 40, 41, 42, 43, 44, 45, 46, 57, 77.
Glucosidases, 189.
Glucosides, 18, 20, 41, 48, 55, =76=, 91, 93.
Glue, 210.
Glutelins, 175, 176.
Glutenin, 176.
Glycerine (_see_ glycerol).
Glycerol, 129, 131, =134=, 142.
Glycine, 166, 168, 177.
Glycinin, 176.
Glycogen, 59, =61=.
Glycyphyllin, =79=.
Graminin, 59, =62=.
Granulose, 60.
Grape sugar (_see_ glucose).
Guanase, 190.
Guanine, =160=, 162.
Gulose, 36, 37.
Gum arabic, =68=.
Gums, 62, 67, =68=.
Gum tragacanth, =69=.
Gun-cotton, 73.
Hæmatin, 110.
Hæmatinic acid imide, 109.
Hæmatoporphyrin, 110.
Hæmoglobin, 110.
Hæmopyrrole, 109.
Helicin, 81.
Hemi-celluloses, 63, =71=.
Hemi-terpenes, 147.
Heptoses, 30.
Hesperidin, =79=.
Hesperitin, 79, 80.
Heterotrophic plants, 16.
Hexosans, 59, 67.
Hexoses, 22, 28, 30.
Histidine, 169, =177=.
Histones, 175, 176.
Honey sugar (_see_ fructose).
Hordein, 153, 170, 176.
Hormones, 92, 239, =240=.
"Hot-drawn oils," 137.
Humins, 67.
Hydrastine, 155.
Hydrazones, 40, 49.
Hydrocellulose, 73.
Hydrogen peroxide, 26, 27, 190.
Hydrogel, 205.
Hydrolases, 186, 189.
Hydroquinone, 77, 79.
Hydrosol, 205.
Hydroxy-phenyl ethyl amine, 153.
Hygrine, 155, 156.
Hyoscine, 155.
Hyoscyamine, =156=.
Hypoxanthine, =160=.
Idain, =85=.
Idose, 36, 37.
Illuminating gas as a toxin, 247.
Imbibition, 209.
Impermeable membranes, 228.
Indian yellow, =84=.
Indican, 78, =85=.
Indigo, 78, 84.
Indigotin, 85.
Indole, 158.
Indoxyl, 85.
Inhibitors, 196.
Intracellular enzymes, 184.
Inulin, 59.
Inulinase, 62, 189.
Invertase, 50, 189, =191=.
Invert sugar, 47, 50.
Iodine number, 138.
Ionization phenomena, 226.
Iridin, =79=.
Irigenin, 79, 80.
Iron, 3, 5, 11, 110.
Isochlorophyllin _a_, 106, 107, 108.
Isochlorophyllin _b_, 106, 107, 108.
Isohæmopyrrole, 109.
Isoleucine, 168.
Isomaltose, 51.
Isomerism, 32.
Isoprene, 147.
Isoquercitrin, 84.
Isoquinoline, 155.
Jalapin, =81=.
Japan wax, 129.
Juglansin, 176.
Ketose, 32.
Lactam, 104.
Lactase, =56=.
Lactic acid ferment, 190.
Lactone, 104.
Lactose, 45, 49, 52.
Laudanosine, 158.
Laudanum, 158.
Lecithin, 7, =141=, 142, 143.
Lecithoproteins, 175.
Legumelin, 176.
Legumin, 170, 176.
Leucine, 115, 168, =177=.
Leucomaines, 152.
Leucosin, 176.
_l_-galactose, 33.
_l_-glucose, 33.
Levulosans, 59, =62=.
Levulose (_see_ fructose).
Lichenin, =62=.
Light, 21, 253, 257.
Lignocelluloses, =74=.
Lignose, 31.
Limettin, =82=.
Limonene, 147.
Linalyl acetate, 148.
Linseed oil, 133.
Lipases, 186, 189.
Lipins (_see_ lipoids).
Lipoids, 129, =140=.
Lipoproteins, 175.
Lupinine, 155.
Lycopersicin, 102, 122, =114=.
Lysine, 169, 171, 177.
Lyxose, 35.
Magnesium, 3, 5, 9, =10=, 11, 13, 14, 68.
Maltase, 55, 184, 189.
Maltose, 45, 49, =51=, 52.
Malvidin, 85.
Malvin, =85=.
Mandelo-nitrile, 78, 88.
Mandelo-nitrile glucoside, 77, 87.
Manganese, =4=, 13.
"Manna," 47.
Mannans, 59, 62, =63=, 72.
Mannite, 47.
Mannitol, 47.
Mannitol series, 36.
Mannoheptose, 36, 44.
Mannononose, 36.
Mannooctose, 36.
Mannosans (_see_ mannans).
Mannose, 32, 36, 37, 41, 44, 45, =47=, 57, 72.
Mannosides, 42.
Maple sugar (_see_ sucrose).
Maysin, 176.
Melibiose, 49, =52=.
Melizitose, =52=.
Menthol, 148.
"Mercerizing" cotton, =73=.
Mercury, 247.
Merosinigrin, 88.
Metallic salts, 13, 224, 227, 237, 247.
"Metal proteids," 14.
Meta-pectin, 70.
Metaproteins, 175.
Methylethylmalein imide, 108.
Methyl glucosides, 42.
Methyl pentoses, 35.
Methyl salicylate, 81.
Middle lamella, 67, 70.
Millon's reaction, 173.
Molisch's reaction, 174.
Monoglycerides, 131.
Monohydric alcohols, 135.
Monosaccharides, 31, =35=, 45.
Morin, 119.
Morphine, 155, =158=.
Morphological adaptations, 251, 252, =255=.
Mucilages, 67, =70=.
Muco-celluloses, 74.
Muscarine, =152=.
Mustard oils, 88, 148.
Mustard oil glucosides, 88.
Mutarotation, 46, 49.
Myrosin, 77, 88, 149, 189.
Myrtillidin, 85.
Myrtillin, =85=.
Narceine, 158.
Narcotine, 158.
"Natural selection," 254.
Neurine, 152.
Nicotine, 155, =156=.
Nitrates, 6.
Nitrile reaction, =43=.
Nitriles, 43, 44.
Nitrogen, 3, 5, 6, 151, =164=.
Non-drying oils, 132.
Non-essential elements, 4.
Non-reducing sugars, 39, 49.
Nonoses, 31.
Normal celluloses, =72=.
Nuclease, 189.
Nucleoproteins, 162, 175.
Nutrients, 1.
Octoses, 31.
[OE]nidin, 85, 116.
[OE]nin, =85=.
Oils, =129=.
Oil of bergamot, 148.
Oil of bitter almonds, 86, 148.
Oil of cassia, 148.
Oil of cinnamon, 148.
Oil of garlic, 148.
Oil of lavender, 148.
Oil of mustard, 148.
Olive oil, 130.
Opium, 158.
Organic acids, 18, =124=, 248.
Organized ferments, 183.
Ornamental pigments, =102=, 123.
Ornithine, 169.
Oryzenin, 176.
Osazones, 40, 41, 49.
Osmotic pressure, 213, 228.
Osones, 41.
Oxidases, 186, 190, 193.
Oxime, 44.
Oxycellulose, 73.
Oxycumarin glucosides, 81.
Oxygenated oils, =147=.
Oxyhydroquinone, 95.
Oxyproline, 169.
Pæonidin, 85.
Pæonin, =85=.
Palm oil, 133.
Papain, 189.
Papaverine, 155, =158=.
Para-dextran, 62.
Para-isodextran, 62.
Paralyzers, 196.
Para-pectin, =70=.
Parasites, 16.
Peanut oil, =133=.
Pectase, 71.
Pectinase, 189.
Pectins, 20, 31, 67, =70=.
Pecto-celluloses, =74=.
Pectose, 31, 70.
Pelargonidin, 85, =116=.
Pelargonin, =85=.
Pentosans, 31, 67, 68, 72.
Pentoses, 30, 162.
Pepsin, 167.
Peptids, =166=, 167, 176.
Peptones, 176.
Permeable membranes, 228.
Peroxidases, 190.
Persimmons, 100.
Persuelose, 36.
Phæophytin, 107, 108.
Phaselin, 176.
Phaseolin, 176.
Phenol, 95.
Phenol glucosides, =79=.
Phenyl alanine, 168, =177=.
Phenyl hydrazine, 40.
Phlein, =62=.
Phloretin, 79.
Phloridzin, =79=.
Phloroglucinol, 95.
Phosphates, 7.
Phosphatides, 141, =143=.
Phosphoproteins, 175.
Phosphorus, 3, 5, =7=.
Photo-chemical action of light, 253, 257.
Photolysis, 26.
Photosynthesis, 7, 8, 18, 21, 22, =24=, 254.
Phycoerythrin, 102, =115=.
Phycophæin, 102, =115=.
Phyllins, 106, 107.
Phyllophyllin, 107.
Phyllopyrrole, 109.
Physiological adaptations, 252, =257=.
Physiological use, =19=.
Phytase, 189.
Phytochlorin, 108.
Phytorhodin, 108.
Pigment glucosides, =82=.
Pigments, 18, =102=, 224, 254.
Pinene, 147.
Piperidine, 154.
Piperine, =155=.
Plant amines, =151=, 152, 163.
Plant food, =1=.
Plant nucleic acids, =162=.
Polybasic acids, 124.
Polyhydric alcohols, 31.
Polypeptides, 167.
Polysaccharides, =59=.
Polyterpenes, 147.
Poppy wax, 140.
Populin, =80=.
Porphyrins, 108.
Potassium, 3, 5, 8, 10, 13, 14.
Primary amines, 152.
Proenzymes, 198.
Proinulase, 199.
Proinvertase, 199.
Prolamins, 175, 176.
Proline, 169, 177.
Prolipase, 199.
Prooxidase, 199.
Protamines, 175, 176.
Proteans, 175.
Proteases, 186, 189, =192=.
Protective colloids, 209.
Proteins, 7, 18, 20, 151, 162, 163, =164=, 224.
Proteoses, 175.
Protoplasm, 17, 26, 221.
Prulaurasin, 87.
Prunase, 87.
Prunasin, 87.
Ptomaines, 152.
Purine, =159=.
Purine bases, 151, =159=, 162.
Purpurin, 83.
Putrescine, 152.
Pyrimidine, 161.
Pyrimidine bases, =161=, 162.
Pyrocatechol, 95.
Pyrogallol, 95.
Pyrogallol tannins, =97=.
Pyroxylin, 73.
Pyrrophyllin, 107.
Pyrridine, 154.
Pyrrolidine, 154.
Quaternary amines, =152=.
Quercetin, 84, 118.
Quercitrin, =84=.
Quinine, 155, =157=.
Quinoline, 155, 158.
Radiant energy, 19.
Raffinose, 45, 52, =53=.
Rape-seed oil, 130.
Reducing sugars, 39, 49.
Reductases, 186, 190.
Reserve food, 21.
Resenes, 149.
Resins, 18, 146, =149=.
Resorcinol, 95.
Respiration, 18, 121, 235, 236.
Rhamnase, =77=, 189.
Rhamnetin, 84.
Rhamnose, 35, 52, 77, 79.
Rhodeose, 35, 81.
Rhodophyllin, 107.
Ribose, 35.
Ricin, 176.
Rubiadin, 83.
Rubiphyllin, 107.
Saccharide, =31=.
Salicin, =80=.
Saligenin, 80.
Salinigrin, =81=.
Salts, 224, =227=, 237.
Sambunigrin, 87.
Sapogenins, 90.
Saponification, 134.
Saponification value, 138.
Saponins, =90=.
Sapotoxins, 90.
Saprophytes, 16.
"Saturated" acids, 132.
Scopolin, =82=.
Secalin, =63=.
Secondary amines, 152.
Secretions, 20.
Sedoheptose, =36=.
Semipermeable membranes, =228=.
Sensitizers, 27.
Serine, 168.
Silicates, 12.
Silicon, 4, =12=.
Silver, 247.
Simple proteins, 165, 174, =175=.
Sinalbin, 89.
Sinalbin mustard oil, 89.
Sinapin acid sulfate, 89.
Sinigrin, =88=.
Sinistrin, =62=.
Sitosterol, 136.
Skimmetin, =81=, 82.
Skimmin, =81=.
Soaps, 134, 208.
Sodium, 4, 9, =12=, 13, 14.
Sodium stearate, 133.
Sol, 205.
"Soluble starch," 60.
Sorbitol, 48.
Sorbose, 36, 38, 45, =48=.
Specific rotatory power, =38=, 39.
, of fructose, 39, 47.
, of galactose, 49.
, of glucose, 39, 47.
, of maltose, 51.
, of raffinose, 53.
, of sucrose, 39.
Spermaceti, 129, 133.
Stachydrine, 155.
Stachyose, 54.
Starch, 8, 22, 28, 30, 31, 45, =59=, 64.
"Starch paste," 60.
Stearin, 131, 134.
Stereo-isomerism, =32=.
Stigmasterol, 136.
Structural adaptations, =255=.
Structural isomerism, =32=.
Strychnine, 155, =157=.
Substrate, 186.
Sucrase (_see_ invertase).
Sucrose, 28, =49=, 64.
Sugars, 8, 18, 22, 28, 30, 31.
Sulfur, 3, 5, =11=, 148.
Sulfuretted oils, 147, =148=.
Sulfur test, 174.
Sunflower-seed oil, 130.
Surface boundary phenomena, =231=.
Surface energy, 231.
Surface tension, 231.
"Survival of the fittest," 254.
Suspensoids, 206, 214.
Suspensions, 206.
Synergic foods, 2, 20.
Synthesis, 18.
Tagatose, 36, 38, 57.
Talose, 36, 38, 42, 57.
Tannins, 18, =94=, 97, 99, 100, 127, 208, 224.
Tannon group, 96.
Terpenes, =147=.
Tertiary amines, 152.
Tetrapeptids, 167.
Tetrasaccharides, =54=.
Tetrose, 30, 35.
Theobromine, =160=.
Theophylline, =160=.
Thioglucose, 88.
Threose, 35.
Thymine, 161, 162.
Thymol, 148.
Toxins, 13, 239, 240, =245=.
Translocation diastase, 191.
Trehalase, 51.
Trehalose, 49, =50=.
Triglycerides, 131.
Trimethyl amine, =152=.
Trimethyl glycocoll, 143.
Triose, 30, 35.
Trioxymethylene, 22, 23.
Tripeptids, 167.
Trisaccharides, 31, =52=.
Triticin, 59, =62=.
Tryptophane, 169, 171, 177.
Tuberin, 176.
Turanose, =49=, =53=.
Tyndall phenomena, 212.
Tyrosine, 115, 168, 177.
Unavailable plant food, 4.
Ultrafilter, 215.
Ultramicroscope, 203, 204, 205, =211=.
Unorganized ferments, 183.
"Unsaturated" acids, 132, 138.
Uracil, 161, 162.
Urease, 190.
Valine, 168.
Vanillin, 80, 148.
Vegetable bases, 18, =151=.
Vicianin, 88.
Vicilin, 176.
Vignin, 176.
Vitamines, 239, 240, 242.
Volatile oils, 20, 147.
Water, 3, 21, 22, 23, =224=.
Water-soluble B, 243.
Waxes, 18, 129, =140=.
Weathering, 4.
Wood gum, 68.
Wool fat, 129.
Wound gum, 68, =69=.
Xanthine, =160=.
Xanthone, 82, =83=, 102.
Xanthophyll, 112, =113=, 121.
Xanthopurpurin, 83.
Xanthorhamnin, 52, =84=.
Xylan, =69=.
Xylose, 35, 68, 69.
Xylosides, 56.
Yeast, 61.
Zein, 165, 170, 176, 177.
Zinc, 13.
Zymase, 51, 56, 190, =192=.
Zymogens, =198=.
Transcriber's Notes:
The original spelling and minor inconsistencies in the spelling and
formatting have been maintained.
The following words have been variably hyphenated in the original:
oxy(-)cumarin, tri(-)saccharides, sugar(-)like, mono(-)saccharides,
sea(-)weeds, di(-)sodium, foam(-)like, di(-)basic, aldo(-)hexoses,
chromo(-)proteins, galacto(-)octose, gluco(-)octose, keto(-)hexoses,
ligno(-)celluloses, manno(-)octose, para(-)pectic, di(-)saccharides,
poly(-)saccharides. The variable hyphenation has been retained in this
version.
Formatting:
Text in italics was marked using underscores (_text_) and bold text using
equals (=text=).
The greek characters alpha and beta were marked as [alpha] and [beta]
Triple bonds were marked as [trb] in order to keep the lt1-text.
On page 120 the illustration of the decomposition of chlorophyll a and
chlorophyll b could not be displayed due to its complex structure.
On page 147 the incorrect formulas of sitosterol, C_{27}H_{43}OH and
stigmasterol, C_{30}H_{49}OH were not changed due to the fact that the pure
compounds were first isolated in 1926 after this book was published.
The table below lists all corrections applied to the original text.
p 3: Abderhalden's "Biochemische Handlexicon" -> Handlexikon
p 10: with the blood pigment, haematin -> hæmatin
p 15: special type, or unusual origin. -> [period added]
p 17: that the photo-synthesis -> photosynthesis
p 20: Experiments with algae -> algæ
p 21: that when algae -> algæ
p 26: Experiments," 294 pages, 49 -> [period deleted]
p 36: acts as a powerful anaesthetic -> anæsthetic
p 40: Die Chemie der Chlorophyll -> Die Chemie des Chlorophylls
p 41: many other poly-atomic -> polyatomic
p 54: Left parenthesis at C6 of Aldopentose deleted
p 55: glucose yeilds -> yields
p 60: see pages -> see page
p 61: when heated to 82 C° -> 82° C
p 65: power of +148.° -> +148°.
p 67: attack other dissacharides -> disaccharides
p 74: Secalan -> Secalin
p 77: Erganzungsband -> Ergänzungsband
p 77: Handbuch der Kohlenhydrate," -> [closing quotes added]
p 86: Erganzungsband -> Ergänzungsband
p 88: has not yet been determined. -> [period added]
p 87: by a plane hexagon, thus [Illustration: hexagon]. -> [period added]
p 95: C_{21}C_{20}O_{12} -> C_{21}H_{20}O_{12}
p 96: blue grapes, _oenin_ -> _[oe]nin_
p 96: glucose + oenidin -> [oe]nidin
p 97: reactions of amydalin -> amygdalin
p 98: reactions of amgdalin -> amygdalin
p 107: Gallic acid, -> [period changed to comma]
p 112: Diospyrus -> Diospyros
p 114: the following formulas: -> [semicolon changed to colon]
p 121: obtained from aetioporphyrin -> ætioporphyrin
p 121: that from ætioporphryin -> ætioporphyrin
p 122: chlorophylls are fluorescent, -> [period changed to comma]
p 122: and red by reflected -> [comma deleted]
p 122: properties of these pigments, -> [period changed to comma]
p 123: roots of carrots, etc., -> [second period changed to comma]
p 123: due to these pigments. -> [period added]
p 124: OH groups an addition -> OH groups in addition
p 124: Buchner funnel -> Büchner funnel
p 126: that it contains leucin -> leucine
p 126: leucine and tyrosin -> tyrosine
p 134: Willsttäter -> Willstätter
p 135: malonic acid -> [space added]
p 137: C_{3}H_{5}•COOH -> C_{3}H_{7}•COOH
p 138: commercially as a bye-product -> by-product
p 146: of the experiment. Microörganisms -> Microorganisms
p 154: or through its basic (OH) group -> or through its basic (N) group
p 158: oxygenated and sulphuretted -> sulfuretted
p 161: Aetherische Oele, Harze, -> [comma added]
p 161: F. trans. -> [period added]
p 162: HEUSLER, F., [additional comma added]
p 164: ergot, and hordeine -> hordein
p 171: C_{5}H_{4}N_{4}O_{2}, -> [comma added]
p 171: gaunine -> guanine
p 177: CH_{3}•CHNH•COOH -> CH_{3}•CHNH _{2}COOH
p 179: monoamino-monocarboxylic acids: -> [period changed to colon]
p 181: Hystidine -> Histidine
p 181: Amadinn -> Amandin
p 184: which contains tyrosin -> tyrosine
p 184: or a tyrosin-containing -> tyrosine-containing
p 185: nitric acid on tyrosin -> tyrosine
p 186: acids, or alkalies. -> [period added]
p 187: Juglansin, -> [comma added]
p 187: of walnut and butternut. -> [period added]
p 187: barley. -> [period added]
p 192: reactions when caried -> carried
p 197: which accelerates the hydyrolysis -> hydrolysis
p 198: are further sub-divided -> subdivided
p 200: Butryic acid ferment -> Butyric acid ferment
p 202: and other microörganisms -> microorganisms
p 203: alcohol and carbon dixoide -> dioxide
p 205: especially tyrosin -> tyrosine
p 206: masses under the ultra-microscope -> ultramicroscope
p 212: J. trans -> J., trans. -> [comma added]
p 218: chemical nature and dialectric -> dielectric
p 231: Dresden and Leipsig -> Leipzig
p 231: Leisegang -> Liesegang
p 231: Beitrage -> Beiträge
p 233: observed under the ultra-microscope -> ultramicroscope
p 236: is undoubtedly tetravalent, [period changed to comma]
p 237: has a higher dialectric -> dielectric
p 239: As in well known -> As is well known
p 241: the organism to amother -> another
p 242: having a foam-like -> foamlike
p 249: Bestimmung der Oberflachenspannung -> Oberflächenspannung
p 249: The Role of Diffusion -> The Rôle of Diffusion
p 250: SPEIGEL, L., [additional comma added]
p 252: of combination with. water -> of combination with water
p 255: upon which the micro-organisms -> microorganisms
p 258: heavy metals; hydro-carbon -> hydrocarbon
p 261: Campferitirin -> Campferitrin
p 261: Certain algae -> algæ
p 261: certain types of algae -> algæ
p 262: mentioned above as "acclimitization"->"acclimatization"
p 270: succinic, 127, 128. -> [period added]
p 270: metapectic -> meta-pectic
p 271: mellisyl -> melissyl
p 271: myriscyl -> myricyl
p 271: Asymetric -> Asymmetric
p 272: Carbon dioxide, 2, 3, 18, 21, 22, 23, -> [comma added]
p 273: Curarin -> Curarine
p 274: Hesperitin, 79, -> [period changed to comma]
p 275: Linayl -> Linalyl
p 275: Leucosine -> Leucosin
p 275: Lupenine -> Lupinine
p 276: Methylethylmallein imide -> Methylethylmalein imide
p 276: Mryosin -> Myrosin
p 276: Oil of lavendar -> lavender
p 276: Oxy-hydroquinone -> Oxyhydroquinone
p 277: Photo-chemical action of light -> Photochemical action of light
p 277: Proteases, 186, 189, 102 -> Proteases, 186, 189, 192
p 278: Sorbose, -> [period changed to comma]
*** End of this LibraryBlog Digital Book "The Chemistry of Plant Life" ***
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