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,49figs., 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,BulletinNo. 45, 70 pages, Washington, D. C., 1903.Russell, E. J.—"Soil Conditions and Plant Growth," 243 pages, 13 figs.,Monographson 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,BulletinNo. 57, 127 pages, 24 figs., Washington, D. C., 1909.
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,49figs., 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,BulletinNo. 45, 70 pages, Washington, D. C., 1903.
Russell, E. J.—"Soil Conditions and Plant Growth," 243 pages, 13 figs.,Monographson 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,BulletinNo. 57, 127 pages, 24 figs., Washington, D. C., 1909.
From the standpoint of their ability to synthetize synergic foods (seepage 2) from inorganic raw materials, plants may be divided into two types; namely, theautotrophic, or self-nourishing, plants, and theheterotrophicplants.
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 eachparticular 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 (seeChapter XIV) which are always present in active plant protoplasm. Proper conditions for rapid chemical action are further assured by the colloidal nature (seeChapter XV) of the protoplasm itself.
The whole cycle of chemical changes which is involved in plant growth represents the net result of two opposite processes; thefirst 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.
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 thesegroups 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.
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 whenphysiological usesare discussed, reference is being made to the plant's own internal needs; while the phrasebiological significancewill be understood to refer to the relation of the plant to other living organisms.
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.
Theframework 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.
Thesynergic 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.
Thesecretionsmay 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.
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.
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 decompositionof 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.
The simplest carbohydrates known to occur commonly in plant tissues are the hexoses (seeChapter IV) having the formula C6H12O6, which is just six times that of formaldehyde, CH2O. Also, it is known that formaldehyde easily, and even spontaneously, polymerizes into more complex forms having the general formula (CH2O)n; trioxymethylene, C3H6O3, 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 (seeChapter 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 duplicatedin vitroin 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
H2O + CO2= CH2O + O2.
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.
As has been mentioned, it is easily possible to cause either formaldehyde, or trioxymethylene, to condense into C6H12O6, 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 (seeChapter 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 (seeChapter XIV), whose natureand 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
6H2O + 6CO2= C6H12O6+ 6O2;
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.
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:
1.CO2= CO + OH2O = H2+ O2.H2+ CO = CH2O3.6(CH2O) = C6H12O6
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 whenTropaeolumplants were surrounded with an atmosphere in which therewas 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 CO2is 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 withTropaeolummentioned above, usually acts as a powerfulanæsthetictowards 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.H2CO3= H2CO2+ O2.H2CO2= CH2O + 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
CO2+ 3H2O = CH2O + 2H2O2.
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 inChapter VIIIthat 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 aqueoussolution 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 next step in the process, the conversion of formaldehyde into sugars and starches, is not necessarily aphotosynthetic 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.
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. byGibson, R. J. H.—"Plant Physiology," 564 pages, 172 figs., Oxford, 1907.Marchlewski, L.—"Die Chemie desChlorophylls," 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," inBiochemical Journal, Vol. 6, pages 1 to 47, 1912.Pfeffer, W., trans. byEwart, A. J.—"Physiology of Plants." Vol. I, 632 pages, 70 figs., Oxford, 1900.
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. byGibson, R. J. H.—"Plant Physiology," 564 pages, 172 figs., Oxford, 1907.
Marchlewski, L.—"Die Chemie desChlorophylls," 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," inBiochemical Journal, Vol. 6, pages 1 to 47, 1912.
Pfeffer, W., trans. byEwart, A. J.—"Physiology of Plants." Vol. I, 632 pages, 70 figs., Oxford, 1900.
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., C6H12O6, C12H22O11, C18H32O16, 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 otherpolyatomicopen-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 endingoseis 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., C5H10O5, pentose, C6H12O6, 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.
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, C6H12O6, are known asmono-saccharides. The group of next greater complexity, those which have the formula C12H22O11and 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 asdi-saccharides. Compounds having the formula C18H32O16(i.e., three molecules of C6H12O6minus two molecules of H2O) aretri-saccharides; and the still more complex groups, having the general formula (C6H10O5)n, are called thepoly-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 thesugars; while the polysaccharides are generally insoluble in water and of a neutral taste, and are calledstarches. 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, C5H10O5, and their condensation products, the pentosans (C5H8O4)n, and their methyl derivatives, C6H12O5; certain polyhydric alcohols having the formula C6H8(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.
Four sugars having the formula C6H12O6, 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 isstructuralisomerism, and the other isspace- orstereo-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,and the sugar is of the type known as "aldoses"; whereas, if the oxygen is attached to any other carbon in the chain, the ketone arrangement,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:
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 moreasymmetric carbon atomsand 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]
Comparisons of the above formulas will show that the difference between the formulas ford- andl-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 thed- andl-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 onpage 37for 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: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 α and β modification ofd-glucose (seepage 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.
[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.
[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: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 α and β modification ofd-glucose (seepage 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.
[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:
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 α and β modification ofd-glucose (seepage 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.
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: