Glycogen is a branched polymer of glucose, consisting of main branches of glucose units joined in (1->4) linkages. Every 7-20 residues, (1->6) branches of glucose units are also present. Glycogen is a primary energy storage material in muscle. Individual glucose units are cleaved from glycogen in a phosphorolytic mechanism catalyzed by glycogen phosphorylase.

The storage polysaccharides, such as glycogen, are admirably designed to serve their function. Glucose and even maltose are small, rapidly diffusing molecules, which are difficult to store. Were such small molecules present in large quantities in a cell, they would give rise to a very large cell osmotic pressure, which would be deleterious in most cases. Therefore, most cells build the glucose into long polymers, so that large quantities can be stored in a semi-insoluble state. Whenever glucose is needed, it can be obtained by selective degradation of the polymers by specific enzymes.