24-12 Polysaccharides and Other Sugars in Nature lose and starch are unbranched polymers. -0 5o89eme hg2gaon2agem2cmtchainsacaountorth g8aanareoy20o25 Glycogen is a source of energy. ndmt 85ea The enzyme glycogen phos Se2ce0ttg6e2 ●中年年年中◆g 884666 4…。 ●e●0 99 24-12 Polysaccharides and Other Sugars in Nature Cellulose and starch are unbranched polymers. Cellulose is a poly-β-glucopyranoside linked at C4, containing about 3000 monomeric units. Its molecular weight is about 500,000 and it is largely linear. Hydrogen bonding between adjacent chains accounts for the highly rigid structure of cellulose. Cellulose is abundant in trees and other plants. Cotton fiber is almost pure cellulose. Wood and straw contain about 50% of the polysaccharide. Starch is also a polyglucose, but the monomeric units are connected by α linkages rather than β linkages, as in cellulose. Cellulose serves as a major food reserve in plants and (like cellulose) can by cleaved by aqueous acid into glucose. Major sources are corn, potatoes, wheat and rice. Granular starch swells in hot water and can then be separated into two components, amylose (~20%) and amylopectin (~80%). Amylose is less soluble in cold water. Amylose contains a few hundred glucose units (MW, 150,000- 600,000). It tends to form helical structures rather than linear structures (cellulose). Amylopectin is branched, mainly at C6, about once every 20 to 25 glucose units. Its molecular weight is in the millions. Glycogen is a source of energy. Glycogen is similar to amylopectin, but with a higher frequency of branching (1 per 10 glucose units). Glycogen is the major energy storage polysaccharide in humans and animals. Glycogen is stored in the liver and is the immediate source of blood glucose between meals. Glycogen is also stored in the skeletal muscle and provides a source of energy during strenuous physical activity. Glycogen is utilized in the following way: The enzyme glycogen phosphorylase removes a molecule of glucose from a non-reducing end of the glycogen molecule as α- D-glucopyranosyl 1 phosphate. Because of the high degree of branching (non-reducing ends) many molecules of glycogen phosphorylase can be active at any one time. Glycogen phosphorylase cannot break α-1,6-glycosidic bonds. Two additional enzymes circumvent this problem, a transferase and an α-1,6-glucosidase