Glycogen molecule. Illustration of the polymeric nature of a glycogen molecule. Glucose molecules (C6. H12.O6, one magnified at top left) link up to form the glycogen polymer. The blue circles reveal the size of the glucose units within the polymer. Glycogen acts as a store of glucose. It is stored in the liver and muscles, ready to be broken down to glucose. Glucose is a simple sugar that is an important source of energy in the body.