Polysaccharides

Polysaccharides are the third and largest class of carbohydrate defined in section 14.2.1: they form when a very large number of monosaccharide units -- hundreds to many thousands -- are linked together in sequence by glycosidic linkages, essentially the same kind of ether-oxide bridging bond used in disaccharides, just repeated on a much larger scale to build a genuine polymer. Starch, cellulose and glycogen, examined individually in the three lettered sub-sections below, are the three most important natural polysaccharides, and between them illustrate three quite different biological roles for a polysaccharide: starch is the carbohydrate energy-storage reserve of PLANTS (concentrated in grains, tubers, etc.) and is correspondingly an important dietary energy source for humans and other animals; cellulose is not a storage molecule at all but a structural one, forming the rigid main constituent of plant (and bacterial) cell walls, and of wood and cotton fibre; and glycogen is the carbohydrate …