ATP is a high-energy chemical substance. ATP is produced by the addition of ADP and Pi by the action of the ATP-ase enzyme. The process of making ATP is called phosphorylation. ATP is produced in the part of the thylakoid that faces the surface stroma. ATP is the source of chemical energy in living cells. ATP stores a lot of energy. ATP provides the energy required for various cell reactions. So ATP is called biological coin or energy coin or molecular currency. British biochemist Peter Michel proposed oxidative phosphorylation as the process of making ATP.