ATP synthase is an enzyme that directly generates adenosine triphosphate (ATP) during the process of cellular respiration. ATP is the main energy molecule used in cells. ATP synthase forms ATP from adenosine diphosphate (ADP) and an inorganic phosphate (Pi) through oxidative phosphorylation, which is a process in which enzymes oxidize nutrients to form ATP. ATP synthase is found in all lifeforms and powers all cellular activities.