Glycogen is a polysaccharide that is the storage form of glucose in the human body. Glucose is an important biomolecule that provides energy to cells throughout the entire human body. Humans derive glucose from the foods that they eat. When they are running low on glucose, glycogen can be utilized as a glucose source.
In humans, glycogen is stored and produced by the hepatocytes in the liver. The main function of glycogen is as a secondary long-term energy-storage molecule. The primary energy-storage molecules are adipose cells. Glycogen is also stored in muscle cells. Muscle glycogen is converted into glucose by the muscle cells whenever muscles are overworked and tired. Glycogen from the liver is converted into glucose to be used mainly by the central nervous system, which includes the brain and spinal cord.
In the liver, blood glucose from the foods that humans eat reaches the liver via the portal vein. There, insulin stimulates the liver cells, which stimulates glycogen synthase. This enzyme stimulates the synthesis of glycogen in the liver; therefore, glycogen in the liver is formed from the food that humans eat. Muscle-cell glycogen is chemically identical to liver glycogen. However, it functions as an immediate source of glucose for muscle cells. When muscles are tired, they may convert glycogen to glucose to continue to function properly. However, liver glycogen does not convert into glucose unless the body is deprived of food.