ATP provides a chemical bond for universal energy storage in living cells. The bond is between ADP (Adenosine DiPhosphate, which as its name implies, has two phosphate groups attached to a molecule of adenosine) and the third phosphate group, which makes ADP into Adenosine TriPhosphate. The energy which the cell uses to attach the third phosphate group is stored in ATP until it is needed to power a chemical reaction elsewhere in the cell. Other molecules are used for energy storage in certain cells in certain situations, but ATP is used to store energy in almost all living cells.
