In the electron transport chain, what is the final electron acceptor?

• A. Water
• B. Oxygen
• C. Carbon dioxide
• D. NAD+

Answer: (B)

Electrons move through a series of carrier proteins and their flow is used to pump protons across the mitochondrial inner membrane, creating a gradient that powers ATP synthase. The last step of the chain is the transfer of electrons to a molecule that can accept them all: molecular oxygen. Oxygen receives four electrons and combining with protons forms water (O2 + 4 e− + 4 H+ → 2 H2O). This final acceptance keeps the chain turning and maintains the proton motive force needed to generate most of the cell’s ATP.

If oxygen isn’t available, there’s nowhere for the electrons to go, the chain stalls, NADH and other carriers remain reduced, NAD+ isn’t regenerated, and oxidative phosphorylation halts. Cells may switch to fermentation to regenerate NAD+, but ATP production drops significantly. Water is a product, not the acceptor, and carbon dioxide is a downstream metabolic product, not involved in the final step of the electron transport chain. NAD+ is an oxidized carrier that accepts electrons earlier, not the terminal acceptor.