MCAT Biochemistry Question 169: Answer and Explanation

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Question: 169

4. The reduction half-reaction in the last step of the electron transport chain is:

A. O₂ + 4 e^– + 4 H⁺ → 2 H₂O
B. NADPH → NADP⁺ + e^– + H⁺
C. NADP⁺ + e^– + H⁺ → NADPH
D. Ubiquinone (Q) → Ubiquinol (QH₂)

Correct Answer: A

Explanation:

Reduction is a gain of electrons, which eliminates choice (B) because it is an oxidation reaction. NADPH, choice (C), is a product of the pentose phosphate pathway. Ubiquinone, choice (D), transfers electrons during the course of the electron transport chain, but is not the final electron acceptor. This title belongs to oxygen.

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