Glycogen is the principal storage form of glucose in animal cells. In humans, the most glycogen is found in the liver, whereas muscles only contain a relatively low amount of glycogen. In addition, small amounts of glycogen are found in certain glial cells in the brain.
Sometimes called 'animal starch'. It is stored in liver and muscle cells and can be converted to glucose if needed. In the liver this conversion is regulated by the hormone glucagon. Under certain conditions, between meals for instance, liver glycogen is an important source of blood glucose. Muscle cell glycogen appears to be only for local use. Glycogen is the primary glucose (energy) storage mechanism.
Glycogen is a glucose polymer. It is generated from glucose by the enzyme glycogen synthase. Its breakdown into glucose, called glycogenolysis, is mediated by the enzyme glycogen phosphorylase.