- ATP synthesis in the electron transport chain primarily occurs through oxidative phosphorylation.
- This process involves the transfer of electrons through a series of protein complexes located in the inner mitochondrial membrane, collectively known as the electron transport chain (ETC).
- As electrons move along the chain, energy is released and used to pump protons (H⁺ ions) across the membrane, creating a proton gradient.
- This gradient generates a form of potential energy called the proton motive force.

- ATP synthase, an enzyme embedded in the mitochondrial membrane, utilizes this proton motive force to synthesize ATP from ADP and inorganic phosphate.
- The movement of protons back into the mitochondrial matrix through ATP synthase drives the phosphorylation of ADP, producing ATP—a process known as chemiosmosis.

It is important to differentiate oxidative phosphorylation from other ATP-generating processes:
- Substrate-level phosphorylation occurs directly in metabolic pathways such as glycolysis and the Krebs cycle, without the involvement of the electron transport chain or proton gradients.
- Photophosphorylation is specific to photosynthetic organisms and involves light-driven electron transport to produce ATP.
- Fermentation produces ATP solely through substrate-level phosphorylation and does not involve the electron transport chain or oxidative phosphorylation.

Thus, the key mechanism by which the electron transport chain produces the majority of cellular ATP is through oxidative phosphorylation.

Reference: Lehninger Principles of Biochemistry, 7th Edition, Chapter 19: Oxidative Phosphorylation and Photophosphorylation