The Chemistry of Plant Life
by Roscoe Wilfred Thatcher · English · Gutenberg #33394 · 666,165 chars · public domain (US)
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Title: The Chemistry of Plant Life
Author: Roscoe Wilfred Thatcher
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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 o
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