Which stage of cellular respiration directly uses oxygen in its terminal step?

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

Answer: (A)

Oxygen is used directly in the final step of cellular respiration by the electron transport chain. In this stage, electrons are passed through a series of protein complexes embedded in the inner mitochondrial membrane. As electrons move, protons are pumped across the membrane to create a gradient. Oxygen serves as the final electron acceptor, pulling electrons through the chain and combining with protons to form water. This removal of electrons enables the chain to keep functioning and powers ATP synthase to make most of the cell’s ATP.

Glycolysis happens in the cytosol and does not require oxygen; it breaks glucose into pyruvate and yields a small amount of ATP and NADH. The Krebs cycle occurs in the mitochondrial matrix and generates NADH and FADH2, but it does not use oxygen directly. Fermentation can occur when oxygen is scarce; it regenerates NAD+ to allow glycolysis to continue without oxygen, but it does not involve the electron transport chain.