ATP synthase enzyme complex, illustration. The ATP synthase enzyme complex drives the synthesis of the energy-carrying molecule ATP (adenosine triphosphate). The complex is embedded in the inner mitochondrial membrane (blue). The upper part in the membrane (red) is a channel through which protons (H+, light blue dots) move. The lower part (orange/pink) is where ATP synthesis takes place. The protein complex rotates as ATP is synthesised and protons move through the channels. ATP is formed from a reaction between ADP (adenosine diphosphate) and inorganic phosphate (Pi). The proton gradient across the inner mitochondrial membrane is created during cellular respiration, and this drives the synthesis of ATP.