Polysaccharides are formed by joining monosaccharides into polymeric carbohydrate structures. These monosaccharides are linked with glycosidic bonds. Polysaccharides have two main forms and use: structural and storage. Important examples of these are starch and glycogen (storage), and cellulose and chitin (structural). Polysaccharides must be more than about ten monosaccharides in length but have no real limit to their length, with some examples consisting of hundreds of sugar units. There is, however, a general formula that can be used to show the construction of the polysaccharide. As most polysaccharides are formed using a backbone of six-carbon monosaccharides, the general formula is (C6H10O5)n, where n is the unknown number and therefore the length of the chain.
Both starch and glycogen are polysaccharides of glucose. The glucose molecules in starch and glycogen are linked by α-1,4-glycosidic bond and α1,6-glycosidic bonds. Starch is mainly used as the energy store in plants. It is stored in the form of tubers, fruits and seeds. Glycogen is the main energy store in humans.
Glycosidic bonds are formed during condensation reactions. A small molecule, usually water, is released as a side product.