Glycogen is a large, branched polysaccharide molecule made up of multiple linked glucose subunits. Single molecules of glycogen have been found to hold 1,700 to 600,000 units of glucose. Glycogen is the storage form of glucose in animal cells, directly analogous to starch in plant cells. In humans, glycogen is stored in the liver and skeletal muscles until it is broken down under signals from glucagon in a process called glycogenolysis. The total amount of glycogen stored in the human body is enough to maintain blood glucose levels for several hours of heavy activity in the absence of dietary glucose input. As an example, typical marathon runners who do not ingest glucose during a race will entirely deplete their body stores of glycogen before the end of the marathon.