MCAT Biochemistry Question 147: Answer and Explanation

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Question: 147

12. Which of the following best explains why cytosolic NADH can yield potentially less ATP than mitochondrial NADH?

  • A. Cytosolic NADH always loses energy when transferring electrons.
  • B. Once NADH enters the matrix from the cytosol, it becomes FADH2.
  • C. Electron transfer from cytosol to matrix can take more than one pathway.
  • D. There is an energy cost for bringing cytosolic NADH into the matrix.

Correct Answer: C

Explanation:

The wording of these answer choices is critical. The electrons from cytosolic NADH can enter the mitochondrion through one of two shuttle mechanisms: the glycerol 3-phosphate shuttle, which ultimately moves these electrons to mitochondrial FAD, and the malate–aspartate shuttle, which ultimately moves these electrons to mitochondrial NAD+. If the electrons are transferred using the malate–aspartate shuttle, then no energy is lost, making choices (A) and (D) incorrect. NADH cannot enter the matrix directly, making choice (B) incorrect. It is the fact that electrons can use more than one pathway—one of which loses energy that could be used for ATP synthesis—that accounts for the potentially decreased yield of ATP from cytosolic NADH.

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