MCAT Biochemistry Question 142: Answer and Explanation

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Question: 142

7. In which of the following reactions is the reactant oxidized?

  • A. FAD → FADH2
  • B. NAD+ → NADH
  • C. NADPH → NADP+
  • D. ADP → ATP

Correct Answer: C

Explanation:

To answer this question, we must remember that reduction is a gain of electrons, while oxidation is a loss of electrons. In the case of the energy-storing molecules of cellular respiration, the high-potential electrons generally come from hydride ions (H–). Because the question is asking us to determine in which reaction the reactant gets oxidized, our task is to select the equation in which the reactant loses hydride ions. From the given choices, the only one that matches our prediction is choice (C). Another way to look at this question is to notice that NADP+ has a +1 charge, which represents an increase from the zero charge of NADPH, implying than an electron was lost in the conversion from NADPH to NADP+.

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