Which energy-yielding pathway uses oxygen as the final electron acceptor?

• A. Fermentation
• B. Oxidative Phosphorylation
• C. Glycolysis
• D. Citric Acid Cycle

Answer: (B)

The key energy-yielding process that uses oxygen as the final electron acceptor is oxidative phosphorylation. In this pathway, electrons are passed along the electron transport chain located in the inner mitochondrial membrane. NADH and FADH2 deliver their electrons to the chain, and as electrons move through the chain, protons are pumped across the membrane to create a proton gradient. Oxygen sits at the end of the chain and accepts electrons, combining with protons to form water. That flow of protons back through ATP synthase powers the production of ATP. Without oxygen, the chain backs up, energy production drops dramatically, and cells may switch to fermentation to regenerate NAD+, which yields far less ATP.

Glycolysis, while necessary for breaking down glucose to pyruvate, does not involve an electron transport chain or use oxygen as the final electron acceptor. The citric acid cycle generates electron carriers (NADH and FADH2) but the substantial ATP harvested comes mainly through oxidative phosphorylation, which requires oxygen. Fermentation occurs in the absence of oxygen and bypasses the electron transport chain entirely, yielding only a small amount of ATP.