Conversion of ADP to ATP by the electron transport system in aerobic respiration is called oxidative phosphorylation. During this process the phosphorylation occurs in the inner mitochondrial membrane when hydrogen protons pass through ATP synthetase complex. The energy required for phosphorylation comes from oxidation-reduction process in respiration and therefore, the process is called oxidative phosphorylation.

Oxidative phosphorylation is a process in which electrons are transferred from electron donors to oxygen, which acts as electron acceptor. The oxidation-reduction reactions are involved in the formation of proton gradient. The main role in oxidative phosphorylation is played by the enzyme ATP synthase (complex V). This enzyme complex consists of `F_(0)` and `F_(1)` components. The `F_(1)` headpiece is a peripheral membrane protein complex and contains the site for ATP synthesis from ADP and inorganic phosphate. `F_(0)` component is a part of membrane protein complex, which acts as a channel for crossing of the protons from inner mitochondrial membrane to the mitochondrial matrix. For every two protons passing through `F_(0)–F_(1)` complex, synthesis of one ATP molecule takes place.