Glycogen is a polysaccharide of glucose, which is branched in structure, and is the form in which food is stored in animals.

Starch and glycogen are made from alpha-glucose. This is an isomer of glucose in which the hydroxyl (-OH) group attached to carbon number 1 is below the plane of the ring.

Starch is itself composed of two types of polymer:amylose and amylopectin. In amylose, the glucose monomers are linked by 1,4 glycosidic bonds. This means that the bond connects carbon atom number 1 in one glucose to carbon atom number 4 in the other glucose. This produces an unbranched chain of glucose which then folds up to form a coil or helix.

In amylopectin there are two types of glycosidic bonds: 1,4 and 1,6. The 1,4 links are the same as in amylose. In addition some glucose molecules have a glycosidic link from carbon atom number 6 to carbon atom number 1 in a new glucose molecule. This produces a branch point in the amylopectin molecule. Amylopectin is therefore a branched polymer.

Glycogen is similar in structure to amylopectin, but branches more frequently.