What is the role of oxygen in the electron transport chain?

• A. Accepts electrons to form water
• B. Donates electrons to the chain
• C. Breaks down glucose directly
• D. Phosphorylates ADP directly

Answer: (A)

Oxygen serves as the final electron acceptor in the electron transport chain, pulling electrons through the chain and combining with protons to form water. This step keeps electrons flowing, maintains the proton gradient used by ATP synthase to convert ADP to ATP, and is essential for aerobic respiration. Oxygen does not donate electrons, break down glucose directly, or phosphorylate ADP itself—the phosphorylation of ADP is driven by the proton motive force as protons flow back through ATP synthase. Without oxygen, the chain backs up and oxidative phosphorylation halts.