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 ofeach of the four alcoholic groups in the molecule is identical with the corresponding groups in a fermentible hexose.
Oxidation by Bacteria.—The bacillusBacterium xylinumcontains 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 CH2OH 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.
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 (seeChapter 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 α-glucase to affect a β-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.
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 (C6H10O5)n. It should be noted that an exactly accurate formula should be (C6)n(H12O6)n-1; but since the value ofnis very high, the simpler formula is approximately correct. The value ofnhas 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.
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, "α and β 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.
Dextrinsmay occur in plants as transition products in the transformation of starch into sugars, orvice 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-dextrineasily soluble in water, neutral taste, red color with iodine; andachroo-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 inplants.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, andpara isodextranare 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.
Inulinreplaces 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 ofIris,Hyacinthus, andMuscari. 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 enzymeinulinaseyields fructose; in fact, inulin bears the same relation to fructose that starch does to glucose.
Graminin, irisin, phlein, sinistrin, and triticinare 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.
Mannanbears 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.
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.
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 (seepage 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 characteristicallyoilyinstead ofstarchyin 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 significanceof 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.
Abderhalden, E.—"Biochemisches Handlexikon, Band 2 ... Die Einfachen Zuckerarten, Inuline, Cellulosen, ...," 729 pages, Berlin, 1911, and "Band 8—1Ergänzungsband(same title as Band 2)—" 507 pages; Berlin, 1914.Armstrong, E. F.—"The Simple Carbohydrates and Glucosides," 233 pages.Monographson 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 derKohlenhydrate", 816 pages, 29 figs., Leipzig, 1914 (3d ed.).
Abderhalden, E.—"Biochemisches Handlexikon, Band 2 ... Die Einfachen Zuckerarten, Inuline, Cellulosen, ...," 729 pages, Berlin, 1911, and "Band 8—1Ergänzungsband(same title as Band 2)—" 507 pages; Berlin, 1914.
Armstrong, E. F.—"The Simple Carbohydrates and Glucosides," 233 pages.Monographson 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 derKohlenhydrate", 816 pages, 29 figs., Leipzig, 1914 (3d ed.).
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 (C5H8O4)n, or hexosans having the formula (C6H10O5)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, 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 C23H38O22, 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 C91H150O78. 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 interfereswith the process of sugar manufacture, is identical with gum arabic in composition and properties.
Gum tragacanthis the soluble portion of the natural gum which is found in several species ofAstragalus. 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 gumis 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.
Thepentosans, 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.
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.
Many fruits, such as currants, gooseberries, apples, pears, etc., and many fleshy roots of vegetables, such as carrots, parsnips, etc., contain substances known aspectins. 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.
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.
Thehemi-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 whenhydrolyzed 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.
Thenormal 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 (C6H12O5)n. When hydrolyzed under the influence of the enzymecytase, 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
(C6H12O5)n+ nH2O = 3nCO2+3nCH4.
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 intooxycellulose; while dilute sulfuric acid, under similar conditions, yieldshydro-cellulose, a substance having the formula C12H22O11, 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 NO3groups 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, orcollodion, 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, C12H14(NO3)6O4, is the violent explosive known asgun-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."
Thecompound cellulosescomprise 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-cellulosesare 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 termcuto-cellulosesis 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 (seeChapter 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.
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 onlyrare 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.
Abderhalden, E.—"Biochemisches Handlexikon, Band 2, Gummisubstanzen, Hemicellulosen, Pflanzenschleimen ..." 729 pages, Berlin, 1911; and "Band 8—1Ergänzungsband(same title as Band 2)—," 507 pages, Berlin, 1914.Schwalbe, C. G.—"Die Chemie der Cellulose," 665 pages, Berlin, 1911.
Abderhalden, E.—"Biochemisches Handlexikon, Band 2, Gummisubstanzen, Hemicellulosen, Pflanzenschleimen ..." 729 pages, Berlin, 1911; and "Band 8—1Ergänzungsband(same title as Band 2)—," 507 pages, Berlin, 1914.
Schwalbe, C. G.—"Die Chemie der Cellulose," 665 pages, Berlin, 1911.
Strictly speaking, the termglucosideshould 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·C6H11O5, or R·(CHOH)5CHO) 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 fromd-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 beendetermined.
FOOTNOTES:[3]The structural formula for benzene, C6H6,is one which it is 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, CH3, adjacent to an angle of the hexagon means that this group replaces the H atom in the compound which is being illustrated.
[3]The structural formula for benzene, C6H6,is one which it is 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, CH3, adjacent to an angle of the hexagon means that this group replaces the H atom in the compound which is being illustrated.
[3]
The structural formula for benzene, C6H6,is one which it is 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, CH3, adjacent to an angle of the hexagon means that this group replaces the H atom in the compound which is being illustrated.
All natural glucosides are hydrolyzed into a sugar and another organic residue by boiling with mineral acids; although they vary widely in the ease with which this hydrolysis is brought about.
In most cases, the glucoside is easily hydrolyzed by an enzyme which occurs in the same plant tissue, but in different cells than those which contain the glucoside. Injury to the tissues, germination processes, and perhaps other physiological activities of the cells, result in bringing the enzyme in contact with the glucoside and the hydrolysis of the latter takes place. A large number of such enzymes have been found in plants, many of which hydrolyze only a single glucoside. However, two enzymes, namely, the emulsin of almond kernels, andmyrosinof black mustard seeds, each hydrolyze a considerable number of glucosides. In general,emulsinwill aid in the hydrolysis of any glucoside which is a derivative of β-glucose, and myrosin will help to split up any sulfur-containing glucoside. Glucosides which are derivatives of rhamnose require a special enzyme, known asrhamnase, for their hydrolysis.
The following reactions for the hydrolysis of arbutin and of amygdalin are typical of this action, and will serve to illustrate the general structure of these compounds: