MCAT Biochemistry Question 107: Answer and Explanation

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Question: 107

2. A researcher treats a solution containing animal cells with ouabain, a poisonous substance that interferes with the sodium–potassium ATPase embedded in the cell membrane, and the cell lyses as a result. Which of the following statements best describes ouabain's effects?

  • A. Treatment with ouabain results in high levels of extracellular calcium.
  • B. Treatment with ouabain results in high levels of extracellular potassium and sodium.
  • C. Treatment with ouabain increases intracellular concentrations of sodium.
  • D. Treatment with ouabain decreases intracellular concentrations of sodium.

Correct Answer: C

Explanation:

This question requires an understanding of osmosis and the action of the sodium–potassium pump. When a cell is placed in a hypertonic solution (a solution having a higher solute concentration than the cell), fluid will diffuse out of the cell and result in cell shrinkage. When a cell is placed in hypotonic solution (a solution having a lower solute concentration than the cell), fluid will diffuse from the solution into the cell, causing the cell to expand and possibly lyse. The sodium–potassium pump moves three sodium ions out of the cell for every two potassium ions it lets into the cell. Therefore, inhibition of the sodium–potassium pump by ouabain will cause a net increase in the sodium concentration inside the cell and water will diffuse in, causing the cell to swell and then lyse.

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