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Polysaccharides are polymers of monosaccharides. Starch, glycogen, and cellulose are examples of polysaccharides. Polysaccharides can be homogeneous in terms of its fundamental unit (monosaccharide). If more than one type of monosaccharide is present in a polysaccharide, then it is called a heteropolysaccharide.


Starch is composed of two types of polymer structures derived from α-glucose - a linear polymer form called amylose, and a branched polymer form called amylopectin. The above Figure shows the two types of glucose polymers involved in the formation of starch. The amylose has α(1->4) linkages of glucose subunits, and the amylopectin has α(1->6) and α(1->4) linkages.


Glycogen is a polysaccharide found in most animals as a storage medium of carbohydrate. It is stored mainly in the skeletal muscles and the liver. We can say that starch is to plants as glycogen is to animals. In other words, the animal carbohydrate storage medium is glycogen, and the plant carbohydrate storage medium is starch. The structure of glycogen is mostly like that of the amylopectin structures found in starch, only with more branching.


Cellulose is a polysaccharide found in plants. But unlike starch and glycogen, the monosaccharide units that build up the cellulose are β-glucose units. Also the polysaccharide chains found in the cellulose structure are unbranched. In addition, the linkages in cellulose units are β-linkages instead of β-linkages. This type of β-linkage is the reason why humans cannot digest cellulose. To be more precise, the enzymes that we have do not work on such linkages. Some herbivores such as cattle have cellulose digesting bacteria in their stomachs which digest the foods containing cellulose. Once the bacterial enzymes break the cellulose structures into simple sugars, then the animal can use them for its metabolic needs.


Figure: The glucose polymers that are involved in the formation of starch

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