Glycogen is a carbohydrate similar to amylopectin, but larger. It can contain up to 500000 glucose monomers. The structure is a complicated branching structure, starting with the basic component of amylose, a series of 100 to 1000 glucose monomers linked by α(1->4) linkages (similar to maltose). These molecules are lined up in a branched structure, like the extedending tendrils of a tree. The difference between the branched form of amylopectin and that of glycogen is that glycogen's branches are shorter and more frequent.

As for biological functions, 10% of the liver's mass is made of glycogen, and 2% of muscle mass.