ATP synthase is a complex transmembrane protein that utilizes potential energy stored via the electrochemical gradient to synthesize ATP. This protein acts as a tiny generator, rotated by the force of the hydrogen ions diffusing through it via chemiosmosis (see figure below). The motion of the protons makes the stalk subunit of ATP synthase rotate, causing the active site of the enzyme domain to change shape. This ultimately results in the phosphorylation of ADP, turning it into ATP in a process called oxidative phosphorylation.

A cylindrical structure forms the rotor portion of ATP synthase. This structure is embedded in the bilayer inner mitochondrial membrane. Protons enter the rotor from the intermembrane space and cross through to exit in the mitochondrial matrix. The protons are traveling down an electrochemical gradient, with a higher charge in the intermembrane space and a lower charge in the mitochondria membrane, so their movement through the rotor generates energy and cause the rotor to turn. The active site of ATP synthase is represented by overlapping oval shaped structures that extend into the mitochondrial matrix. Here, arrows show that ADP and a phosphate group enter the active site, and a molecule of ATP is released from the active site.

ATP synthase is a protein complex that uses the proton gradient in the inner mitochondrial membrane to produce ATP.