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Glycogen is a polysaccharide that serves as the primary form of glucose storage in animals, fungi, and bacteria. In humans, it is primarily stored in the liver and muscles. This branched-chain glucose polymer is synthesized and stored when glucose levels in the blood are high, such as after a meal. When blood glucose levels drop, the body breaks down glycogen through a process called glycogenolysis to release glucose.

Liver glycogen plays a crucial role in maintaining blood glucose homeostasis. When blood glucose falls below normal levels, the liver releases glucose into the bloodstream by breaking down its stored glycogen. This ensures that the brain and other vital organs have a continuous supply of energy. Muscle glycogen, on the other hand, is primarily used as an energy source for muscle contraction during physical activity. Muscle cells have their own glycogen stores and can break them down for their immediate energy needs, but they cannot directly release glucose into the bloodstream.

The synthesis of glycogen, known as glycogenesis, occurs when glucose is abundant. Enzymes like glycogen synthase are involved in this process, adding glucose units to a growing glycogen chain. The breakdown of glycogen, glycogenolysis, is catalyzed by enzymes such as glycogen phosphorylase. Both processes are tightly regulated by hormones like insulin and glucagon, which respond to changes in blood glucose levels. Defects in glycogen metabolism can lead to a group of genetic disorders known as glycogen storage diseases.