In animals, ATP is a product of glycolysis in the mitochondria, while in plants, it generally comes from photophosphorylation during photosynthesis. When used, ATP converts to adenosine diphosphate, or ADP, a chemical that can then be recharged with energy to become ATP again. Because of this, an organism's supply of ATP + ADP is constant, although the relative balance of the two fluctuates according to the organism's energy level.
