What step in cellular respiration forms ATP?

The story behind the answer

Oxidative phosphorylation is where most ATP is made in cellular respiration. Electrons from NADH and FADH2 flow through the electron transport chain in the inner mitochondrial membrane, pumping protons to create a gradient. ATP synthase then uses this proton motive force to phosphorylate ADP to ATP—chemiosmosis in action. While glycolysis makes a small amount of ATP directly (substrate-level phosphorylation), and pyruvate oxidation makes none, oxidative phosphorylation generates the big payoff.

Context tip: Calvin cycle is part of photosynthesis, not respiration. Memory aid: “OP = Output of Power”—oxidative phosphorylation is the final step that outputs the bulk of ATP, powered by oxygen as the final electron acceptor, forming water.