ATP production in a mitochondrion. Illustration of the enzyme complex that drives the synthesis of the energy-carrying molecule ATP (adenosine triphosphate, red). Known as ATP synthase, the enzyme complex (two shown) is embedded in the mitochondrial inner membrane (orange, across bottom). The lower part is a channel through which protons (yellow dots) move. The upper part is where ATP synthesis takes place. ATP is formed from a reaction between ADP (adenosine diphosphate) and inorganic phosphate (both shown in orange). The proton gradient (yellow dots inside and outside the mitochondrion) across the mitochondrial membrane is created during cellular respiration. For the enzyme complex in isolation, see image F021/9623.