What are Carbohydrates?

Carbohydrates, or saccharides, polyhydroxy aldehydes or ketones which have the empirical formula Cm(H2O)n. The number of carbon atoms can vary from three upwards, and the saccharides are named trioses, tetroses, pentoses, hexoses, accordingly. Saccharides exist as ring compounds: in the case of a hexose the ring consists of five carbon atoms and one oxygen, and the other carbon atom is connected to one of the five carbon atoms but remains outside the plane of the ring. Most saccharides have at least one asymmetric carbon atom and thus exist as stereoisomers.

Monosaccharides, such as glucose, consist of one unit, or ring, but the units can polymerise to give oligosaccharides (up to 10 units) and polysacchar-ides. The polymers can be hydrolysed by hot dilute acid to give monosaccharide units which are not broken down further by the acid. On the other hand, treatment with concentrated acid will lead to complete dehydration.

Hydroxy groups in saccharides can undergo a series of reactions. They can be acylated, methylated or attacked with phenylhydrazme. They will also produce amino sugars in which the hydroxy group at a certain position in the ring will be replaced by an amino group. Examples are glucosamine and galactosamine, which are major constituents of chitin and cartilage.

Oxidation of the aldehyde group of an aldose-type sugar will produce sugar acids, such as gluconic acid. Vitamin C, or ascorbic acid, is an important compound of this type.

Deoxy sugars are pentose compounds (five carbon atoms) which are found in the very important nucleic acid, DNA. Disaccharides undergo the same hydroxy reactions as monosaccharides, and consist of, as the name implies, two monosaccharide units. The best examples are maltose, lactose, and sucrose.

Polysaccharides are polymers of high molecular weight, which can be divided into two groups, homo- and hetero-polysaccharides, depending on whether the constituent units are the same. Examples of the former are starch, glycogen and cellulose, and of the latter hyaluronic acid. Polysaccharides are also classified on the basis of function, as storage or structural types. Thus starch and glycogen, which consist of glucose units, act as a store of metabolic energy in the form of glucose, while cellulose, which also contains glucose, is a structural material. It exists in the cell wall, preserving shape and rigidity, and is the major component of wood.

Some polysaccharides can link with amino acids forming peptidoglycans, important compounds in bacterial cell walls. They are examples of complex structural polysaccharides. Animal cell walls consist of these acid muco-polysaccharides, as they are sometimes called, as well as glycoproteins and glycolipids, while plant cells consist mainly of cellulose.

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