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Carbohydrate Nomenclature
The predominant carbohydrates encountered in the body are structurally related to the aldotriose glyceraldehyde and to the ketotriose dihydroxyacetone. All carbohydrates contain at least one asymmetrical (chiral) carbon and are, therefore, optically active. In addition, carbohydrates can exist in either of two conformations, as determined by the orientation of the hydroxyl group about the asymmetric carbon farthest from the carbonyl. With a few exceptions, those carbohydrates that are of physiological significance exist in the D-conformation. The mirror-image conformations, called enantiomers, are in the L-conformation.


Structures of Glyceraldehyde Enantiomers

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Monosaccharides
The monosaccharides commonly found in humans are classified according to the number of carbons they contain in their backbone structures. The major monosaccharides contain four to six carbon atoms.
Carbohydrate Classifications
# Carbons Category Name Relevant examples
3 Triose Glyceraldehyde, Dihydroxyacetone
4 Tetrose Erythrose
5 Pentose Ribose, Ribulose, Xylulose
6 Hexose Glucose, Galactose, Mannose, Fructose

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3 Triose Glyceraldehyde, Dihydroxyacetone
4 Tetrose Erythrose
5 Pentose Ribose, Ribulose, Xylulose
6 Hexose Glucose, Galactose, Mannose, Fructose

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Disaccharides
Covalent bonds between the anomeric hydroxyl of a cyclic sugar and the hydroxyl of a second sugar (or another alcohol containing compound) are termed glycosidic bonds, and the resultant molecules are glycosides. The linkage of two monosaccharides to form disaccharides involves a glycosidic bond. Several physiogically important disaccharides are sucrose, lactose and maltose.
Sucrose: prevalent in sugar cane and sugar beets, is composed of glucose and fructose through an α–(1,2)–β-glycosidic bond.

Sucrose
Lactose: is found exclusively in the milk of mammals and consists of galactose and glucose in a β–(1,4) glycosidic bond.

Lactose
Maltose: the major degradation product of starch, is composed of 2 glucose monomers in an α–(1,4) glycosidic bond.

Maltose

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Polysaccharides
Most of the carbohydrates found in nature occur in the form of high molecular weight polymers called polysaccharides. The monomeric building blocks used to generate polysaccharides can be varied; in all cases, however, the predominant monosaccharide found in polysaccharides is D-glucose. When polysaccharides are composed of a single monosaccharide building block, they are termed homopolysaccharides. Polysaccharides composed of more than one type of monosaccharide are termed heteropolysaccharides.
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Glycogen
Glycogen is the major form of stored carbohydrate in animals. This crucial molecule is a homopolymer of glucose in α–(1,4) linkage; it is also highly branched, with α–(1,6) branch linkages occurring every 8-10 residues. Glycogen is a very compact structure that results from the coiling of the polymer chains. This compactness allows large amounts of carbon energy to be stored in a small volume, with little effect on cellular osmolarity.

Section of Glycogen Showing α–1,4– and α–1,6–Glycosidic Linkages

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Starch
Starch is the major form of stored carbohydrate in plant cells. Its structure is identical to glycogen, except for a much lower degree of branching (about every 20–30 residues). Unbranched starch is called amylose; branched starch is called amylopectin.

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