Bioenergetics
In living organisms, the molecule that is the source of the stored energy used for cell functions comes in the form of Adenosine Triphosphate, or ATP. The energy is stored in the bonds between the phosphates in the ATP. When energy is needed to do work, for instance moving materials across the cell membrane or for chemical reactions, the bonds are broken and energy is released. Cells therefore have to produce the ATP through the processes of photosynthesis and cellular respiration.