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ATP synthase is a molecular motor that couples downhill proton movement across a cell membrane to formation of ATP from ADP and inorganic phosphate. In oxidative phosphorylation, this is the machinery that makes chemiosmosis productive: an electrochemical gradient becomes reusable chemical driving capacity.^[ref]

Its usefulness is a conversion between currencies. A membrane-spanning proton-motive force can power a local machine, while newly formed ATP can participate in energy coupling elsewhere. The enzyme supplies a route between these forms of free energy; it cannot make an energetically uphill total cycle favourable merely by accelerating its chemistry.

A molecular motor couples an energy supply to directed mechanical motion. ATP synthase provides a rotary example.