ATP is called the energy currency of the cell, and of life, because it is the energy molecule that all cells need in order to do anything within the human body. The molecule is used like a battery within cells and allows the consumption of one of its phosphorous molecules.
Once the cell's phosphorous is used it becomes ADP, and in the mitochondria, the phosphorous is returned through a system called cellular respiration. During cellular respiration, if oxygen is available, the mitochondria can produce 36 to 38 ATP per reaction through aerobic respiration. If there is an oxygen debt when the muscles have used all of their supplies, only two ATP can be supplied through anaerobic respiration.