In cellular respiration, which stage yields the majority of ATP and occurs in the mitochondria using oxygen?

• A. Electron transport chain
• B. Glycolysis
• C. Krebs cycle
• D. Fermentation

Answer: (A)

The stage that yields the most ATP and happens in the mitochondria with oxygen is oxidative phosphorylation, the part of the electron transport chain. In this phase, NADH and FADH2 donate electrons to a series of proteins embedded in the inner mitochondrial membrane. As electrons flow along this chain, protons are pumped across the membrane, creating a gradient. ATP synthase then uses this proton motive force to synthesize ATP from ADP and inorganic phosphate. Oxygen is essential here because it is the final electron acceptor; without it, electrons back up, the chain stops, and ATP production drops dramatically. This is why this stage produces the bulk of ATP—roughly 26–28 per glucose in eukaryotic cells—while earlier steps like glycolysis (occurring in the cytosol) and the Krebs cycle contribute smaller amounts by feeding electrons into the chain. Fermentation can occur when oxygen is absent, but it yields far less ATP and does not operate in the mitochondria.