Carbohydrates1

Carbohydrates may be defined as polyhydroxy aldehydes or ketones or compounds which produce them on hydrolysis.

Carbohydrates of physiological importance are classified as

Monosaccharides are the simplest of carbohydrates and they cannot be further hydrolyzed into smaller units. The most common monosaccharides of biological importance are glucose, fructose, galactose and mannose. The monosaccharides are classified into different categories, based on the functional group and the number of carbon atoms.

Oligosaccharides contain 2 to 10 monosaccharide units. Based on the number of monosaccharide units present, the oligosaccharides are further classified into:

Disaccharides are formed by the union of two monosaccharide units (glycosidic linkage) with the elimination of one molecule of water. Disaccharides may be reducing or non-reducing depending upon the availability of free (potential) aldehyde/keto group in their molecular structure. The most common disaccharides of biological importance are maltose, lactose and sucrose.

Maltose: Maltose or malt sugar occurs in germinating seeds and also formed during digestion of starch by enzymes or by dilute acids. It is formed by the union of two glucose units linked by α, 1-4 glycosidic linkage. Maltose is a reducing disaccharide, since it contains one free aldehyde group in its molecular structure. It can be hydrolyzed either by acid or by the enzyme maltase of the intestinal juice.

Lactose: Lactose is present in the milk (milksugar). Lactose is formed by one molecule of galactose and one molecule of glucose linked by β, 1-4 glycosidic linkage. It is a reducing disaccharide, since it contains one free aldehyde group in its molecular structure. 3It can be hydrolyzed either by acid or by the enzyme lactase of the intestinal juice.

Sucrose: It is the sweetening agent. Present in the sugar cane. Sucrose is formed by one molecule of glucose and one molecule of fructose linked by α β, 1-2 glycosidic linkage. Sucrose is a nonreducing disaccharide, since the reducing groups of glucose (aldehyde group) and fructose (keto group) are involved in the formation of glycosidic bond. It can be hydrolyzed either by acid or by the enzyme sucrase (invertase) of the intestinal juice.

They are the macromolecular polymers of monosaccharide units linked by glycosidic linkage with high molecular weight.

Homopolysaccharides: They are made up of monosaccharide units of similar type.

For example: Starch, glycogen, inulin, cellulose, dextrins.

Starch: It is a storage polysaccharide found in plants, potatoes, rice, wheat etc. Starch is a polymer of glucose units linked by α glycosidic bonds. Starch consists of two components.

Heteropolysaccharides: They are made up of two or more different types of monosaccharide units or their derivatives.

For example: Mucopolysaccharides such as,