Glucose is a readily available form of energy in organisms, but it is not always broken down immediately. Excess glucose can be stored in cells and then mobilized—that is, broken down—when necessary. Glucose can be stored in two major forms: as glycogen in animals and as starch in plants (Fig. 7.16). Both these molecules are large branched polymers of glucose.

Carbohydrates that are consumed by animals are broken down into simple sugars and circulate in the blood. The level of glucose in the blood is tightly regulated. When the blood glucose level is high, as it is after a meal, glucose molecules that are not consumed by glycolysis are linked together to form glycogen in liver and muscle. Glycogen stored in muscle is used to provide ATP for muscle contraction. By contrast, the liver does not store glycogen primarily for its own use, but is a central glycogen storehouse for the whole body, able to release glucose into the bloodstream when it is needed elsewhere. Glycogen provides a source of glucose 6-phosphate to feed glycolysis when the level of blood glucose is low. Glucose molecules located at the end of glycogen chains can be cleaved one by one, and they are released in the form of glucose 1-phosphate. Glucose 1-phosphate is then converted into glucose 6-phosphate, an intermediate in glycolysis. One glucose molecule cleaved off a glycogen chain produces three and not two molecules of ATP by glycolysis because the ATP-consuming step 1 of glycolysis is bypassed.