In aerobic respiration, which step directly uses oxygen as the final electron acceptor to form water?

• A. Glycolysis.
• B. Krebs Cycle.
• C. Electron Transport Chain.
• D. Fermentation.

Answer: (C)

Oxygen serves as the final electron acceptor in aerobic respiration, and this happens directly in the electron transport chain. As electrons flow through the chain, the energy transferred pumps protons across the membrane, creating a strong gradient. At the end of the chain, molecular oxygen accepts those electrons and combines with protons to form water. This step is what enables the continued flow of electrons and drives ATP production through chemiosmosis, yielding most of the cell’s ATP.

Glycolysis and the Krebs cycle don’t use oxygen as the final acceptor themselves. Glycolysis occurs in the cytoplasm and can proceed without O2, producing ATP and NADH but not forming water from oxygen. The Krebs cycle generates reduced carriers (NADH and FADH2) that must be reoxidized by the electron transport chain; without oxygen, this recycling slows or stops. Fermentation bypasses the electron transport chain altogether and regenerates NAD+ by converting pyruvate into lactate or ethanol, not by using oxygen as the final acceptor.