Polysaccharides

The term polysaccharide is usually employed for polymers containing at least 10 monosaccharide units. Polysaccharides are named after their monosaccharide monomers by changing the – ose ending of the monosaccharide to –an. Thus cellulose, a polysaccharide formed of glucose units, is called glucan.

Polysaccharides have been classified as homopolysaccharides (homoglycans) and heteropolysaccharides (heteroglycans). Homoglycans contain only one type of monosaccharide while heterglycands contain at least two types. Most polysaccharides contain one or two types of glycose units. Polysaccharides containing three types of glycose units are less common. Polysaccharides are formed by condensation of many molecules of monosaccharides with corresponding elimination of water molecules. The monosaccharide units are joined by glycosidic bonds.

A. Homopolysaccharides (homoglycans).

Some of the better known homoglycans are: -

Glucans – starch, glycogen, cellulose, chintin.

Galactans – agar, pectin, galactan from snails.

Mannas – yeast mannan.

Xylans – hemicellulose xylem.

Fructans – inulin

Starch – Starch is the reserve sustains in plant cells. It is a polymer of D-glucopyranose units. Amylose is linear while amylopectin is branched. Treating startch with hot water dissolves amylase, while amylopectin remains.

Glycogen is the major reserve carbohydrate in animals and is therefore also called animal starch. It is found mainly in the liver and in muscles. The glycogen in the liver supplies glucose to all tissues through the blood. Muscle glycogen on the other hand is available during contraction of muscle.

Cellulose is the most important structural component of the cell wall of plants. It is also found in a few microorganisms and lower organism. It is not digested by the digestive enzymes of man, and therefore forms the ‘bulk’ or ‘roughage’ of food. In contrast to starch and glycogen, cellulose is insoluble in ordinary solvents and is not hydrolysed by boiling dilute acids. It gives no color with iodine.

Chitin is and important polysaccharide of invertebrates. It is found in the hard exoskeleton of insects and crustaceans and in the cell walls of fungi.

Agar is a galactan consisting of both D – and and L – galactose. It is used as a bacteriological cultural medium.

Pectins are abundant in fruits, particularly in the rind of citrus fruits like oranges and lemons. They are present in the cell wall and in galacturonic acid. 

Xylan In addition to cellulose all plants contain xylan. Xylan is a hemicellulose and consists of D – xylose units linked through β – 1,4 glycosidic bonds.

B. Heteropolysacharides (heteroglycans)

There are two major groups of heteropolysaccharides. One group consists of neutral sugars and the other group includes mucopolysaccharides. The polysaccharides of the first group yield more than one type of sugar on hydrolysis, and sometimes nonsugar components also. This group includes some hemicelluloses, some gums, mucilages and pectic substances.


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