What are the two stages of oxidative phosphorylation?

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Oxidative phosphorylation consists of two linked steps: the electron transport chain and chemiosmosis. In the electron transport chain, electrons are passed along a series of protein complexes in the inner mitochondrial membrane, and their transfer drives the pumping of protons across the membrane, creating a proton gradient. Chemiosmosis then uses that gradient as protons flow back through ATP synthase, providing the energy to convert ADP and inorganic phosphate into ATP. This whole process relies on reducing equivalents like NADH and FADH2 produced earlier in pathways such as glycolysis and the citric acid cycle, and it requires oxygen as the final electron acceptor to keep the chain moving. Substrate-level phosphorylation, fermentation, and the separate glycolysis/citric acid cycle steps are not the oxidative phosphorylation stages themselves, making the combination of electron transport chain and chemiosmosis the correct pairing.