In aerobic cellular respiration, what is the final electron acceptor in the electron transport chain?

• A. Water
• B. Carbon dioxide
• C. Glucose
• D. Oxygen

Answer: (D)

The final electron acceptor in the electron transport chain is oxygen. In aerobic respiration, electrons are passed along a series of protein complexes in the inner mitochondrial membrane, and their energy pumps protons across the membrane to create a gradient. At the end of this chain, there needs to be a place for those electrons to go to keep the chain moving. Oxygen acts as the ultimate receiver because it is highly electronegative and readily accepts electrons (and protons) to form water. This step completes the flow of electrons, allowing the electron carriers to be regenerated and the proton gradient to drive ATP synthase, producing ATP. Without oxygen, the chain can’t offload electrons, the gradient collapses, and oxidative phosphorylation stops.

Water is what forms when oxygen accepts the electrons and protons, not the recipient itself. Glucose is the fuel that feeds glycolysis, and carbon dioxide is a product released earlier in metabolism, not the end recipient of electrons.