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A term applied to a group of substances which include the sugars, starches, and cellulose, along with many other related substances. This group of compounds plays a vitally important part in the lives of plants and animals, both as structural elements and in the maintenance of functional activity. Plants are unique in that they alone in nature have the power to synthesize carbohydrates from carbon dioxide and water in the presence of the green plant chlorophyll through the energy derived from sunlight, by the process of photosynthesis. This process is responsible not only for the existence of plants but for the maintenance of animal life as well, since animals obtain their entire food supply directly or indirectly from the carbohydrates of plants. See Carbohydrate metabolism, Photosynthesis
The term carbohydrate originated in the belief that naturally occurring compounds of this class, for example, d -glucose (C6H12O6), sucrose (C12H22O11), and cellulose (C6H10O5)n, could be represented formally as hydrates of carbon, that is, Cx(H2O)y. Later it became evident that this definition for carbohydrates was not a satisfactory one. New substances were discovered whose properties clearly indicated that they had the characteristics of sugars and belonged in the carbohydrate class, but which nevertheless showed a deviation from the required hydrogen-to-oxygen ratio. Examples of these are the important deoxy sugars, d -deoxyribose, l -fucose, and l -rhamnose, the uronic acids, and such compounds as ascorbic acid (vitamin C). The retention of the term carbohydrate is therefore a matter of convenience rather than of exact definition. A carbohydrate is usually defined as either a polyhydroxy aldehyde (aldose) or ketone (ketose), or as a substance which yields one of these compounds on hydrolysis. However, included within this class of compounds are substances also containing nitrogen and sulfur. See Deoxyribose, Fructose
The properties of many carbohydrates differ enormously from one substance to another. The sugars, such as d -glucose or sucrose, are easily soluble, sweet-tasting, and crystalline; the starches are colloidal and paste-forming; and cellulose is completely insoluble. Yet chemical analysis shows that they have a common basis; the starches and cellulose may be degraded by different methods to the same crystalline sugar, d -glucose.
The carbohydrates usually are classified into three main groups according to complexity: monosaccharides, oligosaccharides, and polysaccharides. Monosaccharides are simple sugars that consist of a single carbohydrate unit which cannot be hydrolyzed into simpler substances. These are characterized, according to their length of carbon chain, as trioses (C3H6O3), tetroses (C4H8O4), pentoses (C5H10O5), hexoses (C6H12O6), heptoses (C7H14O7), and so on. Oligosaccharides are compound sugars that are condensation products of two to five molecules of simple sugars and are subclassified into disaccharides, trisaccharides, tetrasaccharides, and pentasaccharides, according to the number of monosaccharide molecules yielded upon hydrolysis. Polysaccharides comprise a heterogeneous group of compounds which represent large aggregates of monosaccharide units, joined through glycosidic bonds. They are tasteless, nonreducing, amorphous substances that yield a large and indefinite number of monosaccharide units on hydrolysis. Their molecular weight is usually very high, and many of them, like starch or glycogen, have molecular weights of several million. They form colloidal solutions, but some polysaccharides, of which cellulose is an example, are completely insoluble in water. On account of their heterogeneity they are difficult to classify. See Monosaccharide, Oligosaccharide, Polysaccharide
The sugars are also classified into two general groups, the reducing and nonreducing. The reducing sugars are distinguished by the fact that because of their free, or potentially free, aldehyde or ketone groups they possess the property of readily reducing alkaline solutions of many metallic salts, such as those of copper, silver, bismuth, mercury, and iron. The most widely used reagent for this purpose is Fehling's solution. The reducing sugars constitute by far the larger group. The monosaccharides and many of their derivatives reduce Fehling's solution. Most of the disaccharides, including maltose, lactose, and the rarer sugars cellobiose, gentiobiose, melibiose, and turanose, are also reducing sugars. The best-known nonreducing sugar is the disaccharide sucrose. Among other nonreducing sugars are the disaccharide trehalose, the trisaccharides raffinose and melezitose, the tetrasaccharide stachyose, and the pentasaccharide verbascose.
The sugars consist of chains of carbon atoms which are united to one another at a tetrahedral angle of 109°28′. A carbon atom to which are attached four different groups is called asymmetric. A sugar, or any other compound containing one or more asymmetric carbon atoms, possesses optical activity; that is, it rotates the plane of polarized light to the right or left.
any of an extensive group of organic compounds that are components of all living things. The first known representatives of this class of substances had compositions corresponding to the general formula CmH2nOn, that is, watered carbon, hence the name “carbohydrate.” Later, the term came to include the numerous derivatives of these compounds with different compositions formed by oxidation, reduction, or the introduction of substituents.
Certain transformations undergone by carbohydrates have been known since antiquity because they underlie processes of fermentation, wood treatment, and the production of paper and fabrics from plant fiber. Cane sugar (sucrose) can be regarded as the first organic compound isolated in chemically pure form. The chemistry of carbohydrates arose and developed with organic chemistry. In 1861, the formulator of the structural theory of organic compounds, A. M. Butlerov, carried out the first synthesis of a sugar-like compound, from formaldehyde. The structures of the simplest sugars were established at the end of the 19th century as a result of the basic research of the German scientists H. Kiliani and. E. Fischer. The research was guided by the stereochemical concepts of J. H. van’t Hoff, and the findings brilliantly confirmed these concepts. In the 1920’s, the English chemist W. N. Haworth laid the basis for the structural chemistry of poly-saccharides. Owing to the biological importance of carbohydrates, the years since 1950 have witnessed a rapid development of the chemistry and biochemistry of carbohydrates. Research has been based on the modern theory of organic chemistry and has been carried out with the aid of the most modern experimental techniques.
Classification and distribution. The three main groups of carbohydrates are the monosaccharides, oligosaccharides, and polysaccharides. The ordinary monosaccharides are polyhydroxyalde-hydes (aldoses) or polyhydroxyketones (ketoses) having a linear chain of carbon atoms (m = 3–9), each of which, with the exception of the carbonyl carbon, is linked to a hydroxyl group. The simplest monosaccharide, glyceraldehyde, has one asymmetric carbon atom and exists in the form of both optical antipodes (D and L). Other monosaccharides have several asymmetric carbon atoms; they may be regarded as derivatives of D- or L-glyceraldehyde. These monosaccharides are assigned on the basis of the absolute configuration at the (m - 1)th carbon atom to the D- or L-series. The differences between the monosaccharides in each series result from the relative configuration of the other asymmetric centers. A characteristic property of monosaccharides in solution is the ability to undergo mutarotation, that is, to establish a tautomeric equilibrium between the acyclic alde-hydo and keto forms and the two five-member (furanose) and two six-member (pyranose) cyclic hemiacetal forms. The pyra-noses and furanoses formed are distinguished by the (α or β) configuration of the asymmetric center arising upon cyclization at the carbonyl carbon atom (indicated here by an asterisk).
The relative amounts of the tautomeric forms in equilibrium are determined by the forms’ thermodynamic stability (with pyranose forms predominating in ordinary sugars). The hemiacetal
hydroxyl differs markedly from the other hydroxyl groups of a monosaccharide in that it tends to participate in nucleophilic substitution reactions. Reactions of this type with various alcohols result in the formation of glycosides; the alcohol residue in a glycoside is called an aglycon. Oligosaccharides and polysaccharides are formed when monosaccharide molecules serve as aglycons. Here, each monosaccharide residue may have a pyranose or furanose structure, as well as an α or β configuration of the glycosidic linkage; the residue may also be bonded to any of the hydroxyl groups of the adjacent monosaccharide. Thus, the number of polymeric molecules differing in structure that can be constructed from a single monosaccharide is enormous.
Among the more typical monosaccharides are D-glucose, D-mannose, D-galactose, D-fructose, D-xylose, and L-arabinose. The monosaccharides also include the deoxy sugars, in which one or more hydroxyls have been replaced by hydrogen atoms (L-rhamnose, L-fucose, 2-deoxy-D-ribose), and the amino sugars, in which one or more hydroxyls are replaced by amino groups (D-glucosamine, D-galactosamine). Polyhydroxy alcohols, or alditols, formed by the reduction of the carbonyl groups of monosaccharides (sorbitol, mannitol), are themselves considered monosaccharides, as are uronic acids, that is, monosaccharides in which the primary alcohol group is oxidized to a carboxyl group. Other monosaccharides include branched-chain sugars, which contain a nonlinear chain of carbon atoms (apiose, L-streptose), and the higher sugars, with chain lengths of more than six carbon atoms (sedoheptulose, sialic acids). With the exception of D-glucose and D-fructose, free monosaccharides are rarely encountered in nature. They usually occur as components of various glycosides, oligosaccharides, and polysaccharides, and they can be obtained from these compounds by acid-catalyzed hydrolysis. Methods of chemical synthesis have been developed for obtaining rare monosaccharides that involve the use of more available monosaccharides as starting materials.
Oligosaccharides contain from two to ten monosaccharides joined by glycosidic linkages. Most common in nature are the di-saccharides sucrose, trehalose, and lactose. Numerous glycosides of the oligosaccharides are known, a category encompassing such various physiologically active substances as flavonoids, cardiac glycosides, saponins, many antibiotics, and glycolipids.
Polysaccharides are high-molecular-weight linear or branched-chain compounds, the molecules of which are made up of monosaccharides joined by glycosidic bonds. Polysaccharides may also include noncarbohydrate substituents (residues of phosphoric, sulfuric, and fatty acids). In turn, polysaccharide chains may be linked to proteins to form glycoproteins. A separate group of polysaccharides is formed by the biopolymers, whose molecules contain residues of monosaccharides or oligosaccharides that are joined to one another not by glycosidic linkages but by phospho-diester bonds; this group includes the teichoic acids from the cell walls of gram-positive bacteria, certain polysaccharides of yeast, and nucleic acids, the main structural component of which is the polyribose-phosphate (RNA) or poly-2’-deoxyribose-phosphate (DNA) chain.
Physicochemical properties. Because of the large number of polar groups (hydroxyl, carbonyl) in the molecules of monosaccharides, carbohydrates are readily soluble in water and insoluble in nonpolar organic solvents (benzene, petroleum ether). The tendency of monosaccharides to undergo tautomeric transformations usually hinders crystallization. If transformations are not possible, as with glycosides and oligosaccharides of the sucrose type, the substances crystallize easily. Many glycosides with aglycons of low polarity, for example, saponins, exhibit the properties of surfactants. Polysaccharides are hydrophilic polymers, the molecules of which are able to form associations, in this case highly viscous solutions (mucilage, hyaluronic acid); when the free and associated segments of the molecules are present in certain proportions, polysaccharides give strong gels (agar, pectic substances). In some cases, polysaccharide molecules form highly ordered supramolecular structures that are insoluble in water (cellulose, chitin).
Biological role. The role of carbohydrates in living things is extremely varied. In plants, monosaccharides are the primary products of photosynthesis, and they serve as the starting materials for the biosynthesis of various glycosides and polysaccharides, as well as of other classes of substances (amino acids, fatty acids, polyphenols). These transformations are realized through the corresponding enzyme systems, the substrates for which are, as a rule, energy-rich phosphorylated derivatives of sugars, mainly nucleoside diphosphate sugars. Carbohydrates are stored as starch in higher plants and as glycogen in animals, bacteria, and fungi; they serve as an energy reserve for the vital activities of an organism. In plants and animals the transport of various products of metabolism is carried out with the products in glycoside form. Numerous polysaccharides and more complex carbohydrate-containing polymers function as structural components. The rigid cell walls of higher plants are built with cellulose and hemicellu-lose, while those in bacteria are built with peptidoglycan. Chitin is found in the cell walls of fungi and in the shells of arthropods. In humans and animals, sulfated mucopolysaccharides of connective tissue serve as structural components. These compounds simultaneously preserve the form of the body and allow movement of the individual parts. Polysaccharides of this type also help to maintain the water balance and the selective permeability of cells to cations. Similar functions in multicellular marine algae are performed by sulfated galactans (red algae) and sulfated heteropoly-saccharides (brown and green algae), which are more complex. Pectic substances perform an analogous function in the proliferating, juicy tissues of higher plants. Complex carbohydrates play an especially important role, a role not yet fully studied, in the formation of specific cell surfaces and membranes. Thus, glycoli-pids are the most important components of nerve cell membranes, and lipopolysaccharides constitute the capsule of gram-negative bacteria. The carbohydrates of cell surfaces often account for immunological specificity; this role has been conclusively established in the case of blood group substances and a number of bacterial antigens. There are indications that carbohydrate structures also participate in such highly specific cell interactions as fertilization and cell “recognition” in tissue differentiation and in the rejection of foreign tissue.
Practical importance. Carbohydrates constitute a large part, often the major part, of the human diet. In this regard, they are widely used in the food-processing and confectionery industry (starch, sucrose, pectic substances, agar). Carbohydrate transformations in alcohol fermentation underlie processes for obtaining ethyl alcohol as well as for beer brewing and bread-making; other types of fermentation make possible the production of glycerol, lactic acid, citric acid, gluconic acid, and other substances. Glucose, ascorbic acid, cardiac glycosides, carbohydrate-containing antibiotics, and heparin are used in medicine. Cellulose is the basic material of the textile industry and of processes for producing artificial cellulose fibers, paper, plastics and explosives.
Research on the chemistry and biochemistry of carbohydrates is presently geared toward improving methods for establishing the structure of carbohydrates and synthesizing natural carbohydrates. Work is also under way on clarifying both the relationship between carbohydrate structure and function in organisms and the paths followed in carbohydrate biosynthesis. This work, which is conducted at chemical and biochemical research centers, is being carried out concurrently with research on major problems in organic chemistry, biochemistry, and molecular biology.
International publications devoted to carbohydrate research include the yearbook Advances in Carbohydrate Chemistry and Biochemistry (published since 1945) and the journal Carbohydrate Research (since 1965).
REFERENCESKhimiia uglevodov. Moscow, 1967.
Metody khimii uglevodov. Moscow, 1967. (Translated from English.)
Glikoproteiny [vols. 1–2]. Moscow, 1969. (Translated from English.)
Carbohydrates. Edited by G. O. Aspinall. London-Baltimore .
Industrial Gums, 2nd ed. Edited by R. L. Whistler and J. N. BeMiller. New York-London, 1973.
A. I. USOV