Adenosine triphosphate, or ATP, is the energy currency of life, the way that individual cells store and use chemical energy. Any food or other source of energy a cell takes in is converted to ATP, in which form the mechanisms of the cell can easily use it. It does this by shedding a phosphate group, becoming adenosine diphosphate, or ADP, a highly energetic reaction that powers all of a cell's molecular machinery.
Adenosine triphosphate's chemical structure contains a relatively complex carbon-based molecule, including cyclic carbon subgroups, but the main function comes from the phosphate groups, or rather, the last phosphate group, which is the one shed when ATP provides energy to the cell. The molecule contains three phosphate groups in a chain. Each phosphate is bonded to four oxygen atoms. Three of these oxygen atoms are shared either by two phosphates or the first phosphate and the carbon-based group.
In animals and other eukaryotic heterotrophs, organelles known as mitochondria use the energy from food to convert depleted adenosine diphosphate back into adenosine triphosphate. This process largely uses glucose in a process known as the Krebs cycle. In plants, chloroplasts, the organelles responsible for converting light, water and carbon dioxide into carbohydrates, also change ADP into ATP.