Final answer:
Glycogen is the complex carbohydrate used to store carbohydrates in the liver and muscle cells. When energy is needed, glycogen is broken down into glucose for use by cells. This allows ATP to be produced for a longer period of time during exercise.
Step-by-step explanation:
Glycogen is the complex carbohydrate used to store carbohydrates in the liver and muscle cells. It is a polymer of glucose and is found as granules in liver and muscle cells. When energy is needed, glycogen is broken down into glucose for use by cells. This allows ATP to be produced for a longer period of time during exercise.