11.12 Polysaccharides
Polysaccharides are carbohydrates that contain many monosaccharide units joined by glycosidic bonds. All the anomeric carbon atoms of polysaccharides are involved in acetal formation. So, polysaccharides do not react with Tollen's reagent, and they do not mutarotate.
Polysaccharides that are polymers of a single monosaccharide are called homopolysaccharides. If they made by more than one type of monosaccharide are called heteropolysaccharides. Example, a glucan is made by glucose units and galactan, which is made by galactose units. There are three important polysaccharides, which are starch, glycogen and cellulose.
Starch is a glucose polymer that is the principal food storage carbohydrate in plants. It is a mixture of two components that can be separated on the basis of water solubility.
Amylose is a linear polymer of D-glucose units joined by α -1,4'-glycosidic linkages.
Amylopectin is a branched polymer of D-glucose units hooked by α -1,4'-glycosidic linkages and the branches are created by α-1,6'-glycosidic linkages.
Glycogen functions as a carbohydrate storage form for animals. Like amylopectin, it is non-liner polymer of D-glucose units joined by α -1,4'-glycosidic linkages and α-1,6' -glycosidic linkages at branches. The structure of glycogen is similar to that amylopectin, but it has more branches. The highly branched structure of glycogen provides many available glucose end groups for immediate hydrolysis to provide glucose needed for metabolism.
Cellulose serves as structural material in plants, providing structural strength and rigidity to plants. It is a linear polymer of D-glucose units joined by β -1,4'-glycoside bonds. Humans and other mammals do not have the β -glucosidase enzyme needed to hydrolyze cellulose, so they cannot obtain glucose directly from cellulose.
