Carbohydrates

CARBOHYDRATES, organic compounds consisting of carbon, hydrogen, and oxygen, with the latter two usually present in the same proportion as in water. This definition, being based on a purely formal characteristic, is naturally insufficient, and it is more correct to characterize carbohydrates as aldehyde and ketone derivatives of polyhydric alcohols of varying degrees of complexity. The simplest of them, monoses (see), have 3, 4, 5, 6, or 7 carbon atoms and are such aldo-alcohols (see Aldoses) or keto-alcohols (see Ketoses), while more complex carbohydrates can be regarded as ethers or anhydrides formed by the combination of two or more monose particles with the elimination of water (see Polysaccharides).

In 1927, the International Commission for the Reform of Chemical and Biological Nomenclature proposed the following nomenclature for carbohydrates: glucids (carbohydrates); oses (monoses); osides: holosides (polysaccharides), heterosides (glycosides).

Carbohydrates are extremely widespread in the plant world, serving here, on the one hand, as skeletal substances (cellulose, hemicellulose, pectin), and on the other hand, taking part in metabolism (starch, inulin, sugars). In the animal organism, the number of various forms of carbohydrates is significantly smaller, but here too they play a huge role in metabolism (glycogen, sugars), participating in the construction of skeletal tissues only in tunicates (Tunicata). The animal organism is incapable of synthesizing carbohydrates from mineral substances and obtains them from the outside, either directly with plant food (herbivores) or through the agency of herbivores (carnivores).

In plants, carbohydrates are formed in the process of photosynthesis in green plants and chemosynthesis in prototrophic organisms (nitrifying and sulfur bacteria). At the present time, photosynthesis has been studied, and even then not in all its parts, only in terms of where carbon dioxide and water combine with a molecule of the green pigment chlorophyll; this compound absorbs solar rays, predominantly from the red-orange part of the spectrum, and at the expense of the energy of the latter, a chemical rearrangement occurs, seemingly leading to the formation of an organic peroxide compound. The latter is then split off from chlorophyll with the release of gaseous oxygen.

According to the theory first advanced by Baeyer (A. Baeyer, 1870), formaldehyde is formed in the process; six molecules of the latter condense of the aldol type and form a monose. This can be expressed by the following equations: CO₂ + H₂O = CH₂O + O₂, 6CH₂O = C₆H₁₂O₆. However, there is still no definitive proof of the formation of formaldehyde. The nature of the monose formed during photosynthesis has not been established, but there is a series of data suggesting that it is fructose.

The process of photosynthesis requires the absorption of significant amounts of solar energy (674 large calories for the formation of one gram-molecule) and is the main energy process of the entire organic world, since all life phenomena of plants and animals directly or indirectly depend on it. Fructose formed during photosynthesis easily rearranges into glucose and mannose, which are close to it in structure, and also, through relatively simple changes, can give rise to a whole series of other carbohydrates (see also Assimilation).

A large number of studies (E. Fischer, Kiliani, Tollens, Haworth, Irvine, and others) have been devoted to elucidating the structure of carbohydrates, and at present the view is accepted that monoses can occur in stable and unstable forms. The former (pyranoses) are derivatives of the six-membered heterocyclic pyran nucleus, while the latter (furanoses) are derivatives of the five-membered heterocyclic furan nucleus: pyran, furan.

Furan derivatives in their natural state exist only in bound form (fructofuranose in cane sugar), whereas in the free state they are unstable and rapidly convert into pyranoses. All monoses contain an aldehyde or ketone group and thanks to this, on the one hand, they are easily reduced, e.g., by the action of sodium amalgam on aqueous solutions, giving rise to polyhydric alcohols (arabinose to arabitol, xylose to xylitol, glucose and fructose to sorbitol, galactose to dulcitol), which are often found in plants in a free state.

On the other hand, they are very easily oxidized, and for aldoses three types of the latter can be distinguished: 1) upon oxidation of only the aldehyde group, gluconic acids are formed (gluconic, mannonic, galactonic, arabonic, xylonic); 2) upon oxidation of the primary alcohol group while keeping the aldehyde group unchanged, uronic acids (glucuronic); 3) upon oxidation of both the aldehyde and primary alcohol groups, sugar acids (saccharic acid from glucose, mannosaccharic acid from mannose, mucic acid from galactose). Upon oxidation of fructose, the carbon chain breaks at the site of the keto group, and mild oxidation yields a mixture of glycolic and d-erythronic acids, while more energetic oxidation yields a mixture of oxalic, tartaric, mesotartaric, and glycolic acids.

The ability of monoses to easily oxidize is used for their quantitative determination: most methods are based on the reduction of copper oxide to suboxide, and the Willstätter and Schudel method, in which the aldehyde group is oxidized by the action of iodine in an alkaline solution, which makes it possible to determine aldoses in the presence of ketoses, since the latter are not oxidized by iodine. Characteristic compounds serving to separate monoses from each other are hydrazones and osazones (see), i.e., compounds with one or two molecules of phenylhydrazine.

Two molecules of monoses, combining with each other with the elimination of one particle of water, give rise to disaccharides (see) or bioses, the best known of which are sucrose, maltose, and lactose. Disaccharides can be formed either by two identical monoses (two glucose molecules form maltose and trehalose when combined of the α-glucoside type, gentiobiose and cellobiose when combined of the β-glucoside type, etc.) or hexoses may belong to different types (sucrose consists of glucopyranose and fructofuranose, melibiose of galactose and glucose, lactose also of galactose and glucose). The same is observed for trisaccharides: amylotriose consists of three glucose particles, and raffinose of glucose, fructose, and galactose. Both monoses, disaccharides, and trisaccharides crystallize well, possess characteristic optical rotation, occurring in dextrorotatory, levorotatory, and inactive modifications. Spatial isomerism is also characteristic of them. Thus, for example, glucose occurs in two isomers—α and β—and gives two series of derivatives: α- and β-glucosides, characterized, among other things, by their attitude towards enzymes: α-glucosides are cleaved by enzymes of the yeast maltase type (α-glucosidases), β-glucosides by enzymes of the emulsin type (β-glucosidases).

0-glucopyranose If one takes into account all possible cases of optical isomerism, spatial isomerism, and the fact that each monose can occur in both pyranose and furanose forms, it is easy to understand that the number of possible sugars can be enormous and that at present only a part of them are known. Monoses enter into glycosidic bonds not only with other monoses but also with compounds that do not possess the properties of the latter: fatty and aromatic alcohols, aldehydes, ketones, flavones, anthocyanins, purine bases, mustard oils, etc. All these compounds are called glycosides and, with respect to enzymes, behave as indicated above. Molecules of monoses, combining with each other, can give rise not only to di-, tri-, and tetrasaccharides but also to substances of much greater complexity, such as starch (see), inulin (see), glycogen (see), hemicelluloses, cellulose, pectic substances, mucilages, gums, which are grouped under the name of polysaccharides or polyoses. Common characteristics of all these compounds are their amorphous nature and insolubility in water (inulin is an exception in this respect). Starch and glycogen form colloidal solutions with water. In this process, more or less profound changes usually occur in the form of partial hydrolysis, the breakdown of complex aggregates into simpler ones. When heated with acids or under the action of specific enzymes, polysaccharides break down with the formation of monoses, most often glucose (cellulose, starch, glycogen, some hemicelluloses), and in some cases mannose, galactose, fructose (inulin), arabinose, xylose. Sometimes hydrolysis produces a complex mixture of monoses and their derivatives, as for example in the hydrolysis of pectic substances, where arabinose, galacturonic acid, and galactose are formed. The exact structural formulas are unknown for any colloidal polysaccharide. The same must be said regarding their molecular weight. One can only suppose that individual monoses are connected pairwise within the molecules of polysaccharides: this is evidenced by the formation of maltose during diastatic breakdown of starch, cellobiose during the breakdown of cellulose. Some sugars have been synthesized artificially; for the first time, a sugar-like substance was obtained by Butlerov in 1861 by the condensation of formaldehyde with the action of lime water; the similarity of the substance he obtained to sugars was established by E. Fischer, who obtained a similar substance by the action of barium water on the dibromide of acrolein and named it α-acrose. In recent years, many works have appeared devoted to the artificial photochemical synthesis of carbohydrates, however, the results of these works are very contradictory. The nutritional significance of carbohydrates is extremely great, since they are one of the main sources of energy in the body. 1 g of carbohydrates gives upon combustion about 4.1 large calories. The human body obtains the necessary amount of carbohydrates partly from meat food (glycogen), from milk (lactose), but mainly from plant food in the form of starch, sucrose, various monoses, etc. The significance of starch is particularly great, being the main constituent part of many food products (see). Thus, for example, wheat grains contain 65% of it, corn-65%, rye-69%, rice-75%, sorghum and barley-60%, oats-53%, potato tubers-from 14% to 25% (of raw weight), sweet potato tubers-from 10% to 20%. Second place belongs to sucrose, the content of which in sugar cane stems reaches 20% of raw weight, in sweet sorghum stems-up to 15%, and in sugar beet roots-up to 20-22%. In banana fruits, the sugar content reaches 23% of raw weight, in grapes-up to 22%, etc. Besides the carbohydrates themselves, material for their formation in the body is provided by proteins. Carbohydrates in turn provide material for the synthesis of fats in the body. See also Metabolism, Glycolysis, Muscles. Substances of the carbohydrate group also have great technical and industrial importance, for example, cellulose is used for the production of paper, collodion, artificial silk. A derivative of sugars-furfural, obtained as a by-product in paper production, has found wide application in the production of artificial resins, bakelite, etc. Carbohydrates serve as material for obtaining ethyl alcohol, acetone, lactic acid, and other technically important products.