Glycogen is the chief carbohydrate used to store energy in animals (its counterpart in plants is starch). It is a polysaccharide consisting of a long, highly-branched chain of up to 100,000 glucose molecules.
Glycogen is particularly abundant in the liver and to a lesser extent in muscle. It can be broken down into glucose with glucagon, while insulin converts glucose to glycogen (a process called glycogenesis). In the electron microscope, glycogen has a characteristic asterisk or star appearance.