CARBOHYDRATE

class of naturally occurring compounds and derivatives formed from them. In the early part of the 19th century, substances such as wood, starch, and linen were found to be composed mainly of molecules containing atoms of carbon (C), hydrogen (H), and oxygen (O), and to have the general formula C6H12O6; other organic molecules with similar formulas were found to have a similar ratio of hydrogen to oxygen. The general formula Cx(H2O)x is commonly used to represent many carbohydrates, which means watered carbon. Carbohydrates are probably the most abundant and widespread organic substances in nature, and they are essential constituents of all living things. Carbohydrates are formed by green plants from carbon dioxide and water during the process of photosynthesis. Carbohydrates serve organisms as energy sources and as essential structural components; in addition, part of the structure of nucleic acids, which contain genetic information, consists of carbohydrate. any member of a very abundant and widespread class of natural organic substances that includes the sugars, starch, and cellulose. The first carbohydrates analyzed (in the early 19th century) were determined to be composed of carbon (C), along with hydrogen (H) and oxygen (O) in the proportions found in water (H2O). Many carbohydrates have the general formula Cx(H2O)x, but the class is so broad that no simple definition encompasses them all. Several classification schemes have been devised for carbohydrates. One of the most common divides them into four major groupsmonosaccharides, disaccharides, oligosaccharides, and polysaccharides. Molecules of monosaccharides, or simple sugars, contain from three to nine carbon atoms, most commonly five or six. Three of the most important simple sugars are glucose (also known as dextrose, grape sugar, or corn sugar), fructose (fruit sugar), and galactose. Two simple-sugar molecules are linked to each other in a disaccharide, or double sugar. The disaccharide sucrose (table sugar) consists of one molecule of glucose and one molecule of fructose. Lactose (milk sugar) and maltose are also disaccharides. Oligosaccharides, which consist of three to six monosaccharide units, are rare. Polysaccharides are large molecules, such as cellulose, starch, and glycogen, in which as many as 10,000 monosaccharide units are linked together. They include most of the structural and storage carbohydrates found in nature. Green plants utilize the energy of sunlight to convert carbon dioxide and water into carbohydrates. This process, called photosynthesis, releases oxygen into the atmosphere and transforms light energy into the chemical energy of carbohydrates. Plants convert simple carbohydrates into sucrose, which is the sugar in many fruits; cellulose, their principal structural component; starch, which is stored; and a wide variety of other polysaccharides that function as essential structural components. In most animals, carbohydrates provide a quickly accessible reservoir of energy. Glucose, the sugar circulating in the blood of higher animals, is absorbed by the cells, where its oxidation energizes the metabolic processes. Glycogen, which consists of branching chains of glucose molecules, is stored in the liver and muscles of higher animals, to be broken down into glucose under conditions of stress or muscular activity. In addition, polysaccharides function as structural components in certain animals. For example, chitin, which is similar to cellulose, makes up the exoskeleton of insects and other arthropods. The names of the monosaccharides combine a prefix that designates the number of carbon atoms in the molecule (e.g., pent- means five and hex- means six) and the generic suffix -ose. Thus, pentose denotes any monosaccharide containing five carbon atoms. When a monosaccharide molecule assumes a straight-chain form, its carbon atoms make up the backbone of the molecule. Attached to most of these carbon atoms are a hydroxyl group (OH) and one hydrogen atom (two if it is a terminal carbon in the chain). The hydroxyl groups account for the great solubility of sugars in water. One carbon atom in the chain differs from the others, however, in that it is linked by means of a double bond to an atom of oxygen. This carbon and its double-bond oxygen form either an aldehyde group (if it occurs at a terminal carbon in the chain) or a ketone group (if it occurs at an internal carbon in the chain). Therefore, monosaccharides are either aldehydes or ketones; these are identified by the prefixes aldo- and keto-, as, for example, aldopentoses and ketohexoses. Glucose is an aldohexose, with six carbon atoms and an aldehyde group. Many carbohydrates are isomersthat is, they have the same atomic composition but different structures. Glucose, fructose, and galactose, for example, are all isomers with a formula of C6H12O6. The most common naturally occurring monosaccharides are glucose, mannose, fructose, and galactose among the hexoses, and xylose and arabinose among the pentoses. Two other monosaccharidesribose and deoxyriboseare found in all cells, where they form the carbohydrate component of ribonucleic acid (RNA) and deoxyribonucleic acid (DNA), respectively. Xylose is prepared from cottonseed hulls, corncobs, or straw by the chemical breakdown of xylan, a polysaccharide. Galactose is a common constituent of oligosaccharides and polysaccharides, such as agar and carrageenan. It also occurs in carbohydrate-containing lipids, called glycolipids, present in the brain and other nerve tissues of most animals. Galactose is generally prepared from lactose, a disaccharide composed of galactose and glucose. Glucose is found in fruits, honey, blood, and, under abnormal conditions, urine. It is also a constituent of the two most common naturally occurring disaccharides, sucrose and lactose, and it is the sole structural unit of the polysaccharides cellulose, starch, and glycogen. Glucose is produced commercially in large amounts by hydrolysis of cornstarch; the largest amount is sold as corn syrup, although its crystalline form is sometimes sold under the name dextrose. Fructose is one of the constituents of the disaccharide sucrose and is also present in uncombined form in honey, apples, and tomatoes. It is produced from sucrose and is used by the food industry. Arabinose can be obtained from plant gums (its name derives from gum arabic) and is a component of pectins. Mannose is obtainable from polysaccharides known as mannans. Most monosaccharides can be oxidized easily. When a terminal group (CH2OH) of a monosaccharide is oxidized chemically or biologically, a uronic acid is formed. Glucuronic acid (formed from glucose) is a major component of the polysaccharides in connective tissue; it and other uronic acids are involved in the detoxification of many poisons and their excretion in the urine (hence their name). Sugars may be reduced (by the addition of hydrogen) to alditols, or sugar alcohols. The product thus formed from glucose is sorbitol, commonly used as a sweetening agent. Sugars in solution (including those inside the cell) assume a ring structure in which a hydroxyl group is attached to the carbon that carries the aldehyde or ketone group. This hydroxyl group is highly reactive, enabling the sugars to react with each other or with other classes of compounds to form derivatives called glycosides; glycosides formed from glucose are called glucosides. Among the wide variety of natural glycosides are plant pigments (e.g., indican), heart-muscle stimulants (e.g., digitalis), antibiotics (e.g., streptomycin), and precursors of flavourings (e.g., vanillin). In some monosaccharides, one hydroxyl group is replaced by an amino group (NH2); these compounds, called amino sugars, are widely distributed in nature. In deoxy sugars, one hydroxyl group is replaced by a hydrogen atom; by far the most important of these is deoxyribose, derived from ribose and present in deoxyribonucleic acid (DNA), the carrier of genetic information. Disaccharides are specialized glycosides in which a molecule of one sugar has combined with a molecule of a second. Among the few disaccharides of commercial or biological significance, the most important are sucrose, trehalose, lactose, and maltose. Sucrose is a disaccharide composed of glucose and fructose. It is the familiar table sugar of commerce and food preparation; more than 100 million metric tons are produced annually worldwide. Sucrose is obtained commercially by treatment of sugarcane or sugar beet by crushing and extraction with water, purification, and crystallization. Less-pure crystals deposited from the concentrated liquor are known as brown sugar. The residual syrupy material is called cane final molasses, or blackstrap molasses. Trehalose (composed of two glucose molecules) is similar in many respects to sucrose but is much less widely distributed. Lactose (composed of glucose and galactose) is another of the sugars found most commonly in human diets; it composes about 5 percent of the milk of all mammals. Although not found uncombined in nature, maltose is biologically important as a product of the enzymatic breakdown of starches during digestion. Maltose consists of two molecules of glucose, but it differs structurally from trehalose. Polysaccharides, or glycans, may be classified in a number of ways. Homopolysaccharides are polysaccharides formed from only one monosaccharide. These may be further subdivided into straight-chain, branched-chain, and cyclic representatives, depending on the arrangement of the monosaccharide units. Heteropolysaccharides contain two or more different monosaccharides; their molecules also may be either linear or branched. The two best-known polysaccharides, both homopolysaccharides, are cellulose and starch. Cellulose, the basic structural component of most plants, is a large, linear molecule composed of 3,000 or more glucose molecules. Cellulose is used for a wide variety of commercial purposes, including the manufacture of paper and cloth, and is prepared by treating plant material with hot alkali. Similar to cellulose is xylan, which contains xylose units and is also found in plant-cell walls. The term starch denotes a group of plant polysaccharides consisting of glucose units; most starches are composed of a mixture of two componentsa linear component (amylose) and a branched component (amylopectin). In humans, utilization of starch requires breaking it into individual glucose units; this process is initiated by enzymes called amylases, which are present in saliva, and continues in the intestinal tract. The product of amylase action is maltose, which is hydrolyzed to glucose as it is absorbed through the walls of the intestine. Another important homopolysaccharide is glycogen. Found in all animal tissues, it is the primary animal storage form of carbohydrate. Glycogen is made of glucose units; it resembles starch but is more highly branched. Polysaccharides composed of galactose or galacturonic acide.g., pectins and agarsare important because they can form gels. Pectins, obtainable from citrus fruit rinds, are used commercially in the preparation of jellies and jams. Agar is widely used in biological laboratories in growth media for microorganisms and in the bakery industry as a gelling agent. Additional reading Comprehensive works are Henry R. Mahler and Eugene H. Cordes, Basic Biological Chemistry (1968); Abraham White et al., Principles of Biochemistry, 6th ed. (1978); Albert L. Lehninger, David L. Nelson, and Michael L. Cox, Principles of Biochemistry, 2nd ed. (1993); Thomas Briggs and Albert M. Chandler (eds.), Biochemistry, 2nd ed. (1992); John W. Hill, Dorothy M. Feigl, and Stuart J. Baum, Chemistry and Life, 4th ed. (1993); Lubert Stryer, Biochemistry, 3rd ed. (1988); Donald Voet and Judith G. Voet, Biochemistry (1990); Geoffrey Zubay, Biochemistry, 3rd ed. (1993); and Laurence A. Moran et al., Biochemistry, 2nd ed. (1994). David J. Holme and Hazel Peck, Analytical Biochemistry, 2nd ed. (1993), covers newer methods of analysis. The Editors of the Encyclopdia BritannicaStudies focusing specifically on carbohydrates include S.F. Dyke, The Carbohydrates (1960), an introduction to basic reactions in the carbohydrate field, written almost in outline, schematic form; R.D. Guthrie and John Honeyman, An Introduction to the Chemistry of Carbohydrates, 3rd ed. (1968), a short monograph on basic carbohydrate structure and reactions; R.J. McIlroy, Introduction to Carbohydrate Chemistry (1967), a short introduction to carbohydrate chemistry recognizing the importance of stereochemistry and conformational factors; E.G.V. Percival, Structural Carbohydrate Chemistry, 2nd ed., rev. by Elizabeth Percival (1962), a standard work detailing methods of structural determination, particularly of sugar ring size and polysaccharide structure, still useful as a summary of structural methods even though many modern techniques are not discussed; Ward Pigman, Derek Horton, and Anthony Herp (eds.), The Carbohydrates: Chemistry and Biochemistry, 2nd ed., 2 vol. in 4 (197080), a standard reference work; Jaroslav Stanek, The Monosaccharides, trans. from Czech (1963); and Jaroslav Stanek, Miloslav Cern, and Josef Pacak, The Oligosaccharides, trans. from Czech (1965), standard reference works detailing structure, reaction, and properties of monosaccharides and oligosaccharides; Roy L. Whistler and Charles Louis Smart, Polysaccharide Chemistry (1953), a summary of chemical information on polysaccharides up to 1952, with emphasis on homopolymers such as starch and cellulose since few detailed structures were known at that time; Eugene A. Davidson, Carbohydrate Chemistry (1967), an intermediate college-level text emphasizing stereochemistry, conformation, and modern organic reaction mechanisms as applied to carbohydrates, including both physical and chemical methods for structural determination; Marcel Florkin and Elmer H. Stotz (eds.), Comprehensive Biochemistry, vol. 5, Carbohydrates (1963), a detailed volume covering all aspects of monosaccharide and many areas of polysaccharide structure and chemistry, although physical methods do not receive sufficient attention; M. Stacey and S.A. Barker, Carbohydrates of Living Tissues (1962), a detailed account of the chemistry and biochemistry of polysaccharide substances, primarily those found in animal tissues; and Karla L. Roehrig, Carbohydrate Biochemistry and Metabolism (1984).Works primarily of historical interest are Frederick J. Bates et al., Polarimetry, Saccarimetry and the Sugars (1942), practical information on sucrose and other common sugars together with methods for analysis and preparation of simple derivativesmany of the data tables provide useful reference information; and Walter Norman Haworth, The Constitution of the Sugars (1929), a classic description of the knowledge of sugar chemistry at that time, particularly Haworth's work in defining the ring structure of the carbohydrates.Methods of analysis are described in Roy L. Whistler and M.L. Wolfrom (eds.), Methods in Carbohydrate Chemistry (1962 ), a necessary reference work for research workers in the field, detailing laboratory procedures for monosaccharide and polysaccharide preparations; M.F. Chaplin and John F. Kennedy (eds.), Carbohydrate Analysis: A Practical Approach (1986); and two works dealing with carbohydrate analysis using nuclear magnetic resonance, gas chromatography, and mass spectrometry: Christopher J. Biermann and Gary D. McGinnis (eds.), Analysis of Carbohydrates by GLC and MS (1989); and Lawrence J. Berliner and Jacques Reuben (eds.), Carbohydrates and Nucleic Acids (1992). John F. Kennedy (ed.), Carbohydrate Chemistry (1988); and J. Thiem (ed.), Carbohydrate Chemistry (1990), include information on synthesis.Ongoing research is published in Advances in Carbohydrate Chemistry and Biochemistry (annual), multiauthored volumes reviewing specific areas of carbohydrate chemistry and biology. Eugene A. Davidson The Editors of the Encyclopdia Britannica Classes of carbohydrates Monosaccharides Sources The most common naturally occurring monosaccharides are D-glucose, D-mannose, D-fructose, and D-galactose among the hexoses, and D-xylose and L-arabinose among the pentoses. In a special sense, D-ribose and 2-deoxy-D-ribose are ubiquitous because they form the carbohydrate component of ribonucleic acid (RNA) and deoxyribonucleic acid (DNA), respectively; these sugars are present in all cells as components of nucleic acids. Sources of some of the naturally occurring monosaccharides are listed in Table 2. D-xylose, found in most plants in the form of a polysaccharide called xylan, is prepared from corncobs, cottonseed hulls, or straw by chemical breakdown of xylan. D-galactose, a common constituent of both oligosaccharides and polysaccharides, also occurs in carbohydrate-containing lipids, called glycolipids, which are found in the brain and other nervous tissues of most animals. Galactose is generally prepared by acid hydrolysis (breakdown involving water) of lactose, which is composed of galactose and glucose. Since the biosynthesis of galactose in animals occurs through intermediate compounds derived directly from glucose, animals do not require galactose in the diet. In fact, in most human populations (Caucasoid peoples being the major exception) the majority of people do not retain the ability to manufacture the enzyme necessary to metabolize galactose after they reach the age of four, and many individuals possess a hereditary defect known as galactosemia and never have the ability to metabolize galactose. D-glucose (from the Greek word glykys, meaning sweet), the naturally occurring form, is found in fruits, honey, blood, and, under abnormal conditions, in urine. It is also a constituent of the two most common naturally found disaccharides, sucrose and lactose, as well as the exclusive structural unit of the polysaccharides cellulose, starch, and glycogen. Generally, D-glucose is prepared from either potato starch or cornstarch. D-fructose, a ketohexose, is one of the constituents of the disaccharide sucrose and is also found in uncombined form in honey, apples, and tomatoes. Fructose, generally considered the sweetest monosaccharide, is prepared by sucrose hydrolysis and is metabolized by man. Chemical reactions The reactions of the monosaccharides can be conveniently subdivided into those associated with the aldehydo or keto group and those associated with the hydroxyl groups. The relative ease with which sugars containing a free or potentially free aldehydo or keto group can be oxidized to form products has been known for a considerable time and once was the basis for the detection of these so-called reducing sugars in a variety of sources. For many years, analyses of blood glucose and urinary glucose were carried out by a procedure involving the use of an alkaline copper compound. Because the reaction has undesirable featuresextensive destruction of carbohydrate structure occurs, and the reaction is not very specific (i.e., sugars other than glucose give similar results) and does not result in the formation of readily identifiable productsblood and urinary glucose now are analyzed by using the enzyme glucose oxidase, which catalyzes the oxidation of glucose to products that include hydrogen peroxide. The hydrogen peroxide then is used to oxidize a dye present in the reaction mixture; the intensity of the colour is directly proportional to the amount of glucose initially present. The enzyme, glucose oxidase, is highly specific for b-D-glucose. In another reaction, the aldehydo group of glucose reacts with alkaline iodine to form a class of compounds called aldonic acids. One important aldonic acid is ascorbic acid (vitamin C, see structure), an essential dietary component for man and guinea pigs. The formation of similar acid derivatives does not occur with the keto sugars. Either the aldehydo or the keto group of a sugar may be reduced (i.e., hydrogen added) to form an alcohol; compounds formed in this way are called alditols, or sugar alcohols. The product formed as a result of the reduction of the aldehydo carbon of D-glucose is called sorbitol (D-glucitol). D-glucitol also is formed when L-sorbose is reduced. The reduction of mannose results in mannitol, that of galactose in dulcitol. Sugar alcohols that are of commercial importance include sorbitol (D-glucitol), which is commonly used as a sweetening agent, and D-mannitol, which is also used as a sweetener, particularly in chewing gums, because it has a limited water solubility and remains powdery and granular on long storage.