ATP Synthase is the terminal step of oxidative phosphorylation (cellular respiration). Complexes I, II, III, and IV take electrons from NADH or FADH(2) and transfer them to O2 to form H2O. During this process protons (H+) are pumped out of the mitochondrial matrix into the outer mitochondrial space. The proton gradient that is formed as a result of this pumping action is what drives ATP synthase to favor the formation of ATP from ADP plus phosphate. The transfer of electrons to O2 is coupled to the formation of ATP such that if there is no ADP and phosphate, the rate of respiration dramatically decreases.