SAT Subject Biology Practice Question 299: Answer and Explanation

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Question: 299

9. Human cells maintain concentration gradients across their plasma membranes, such that there is a high sodium concentration outside the cell and a high potassium concentration inside the cell. Suppose that within the cell membrane are sodium "leak" channels. These channels would allow sodium to

A. move out of the cell by simple diffusion
B. move into the cell by simple diffusion
C. move out of the cell by facilitated diffusion
D. move into the cell by facilitated diffusion
E. move into the cell by active transport

Correct Answer: D

Explanation:

D Break this question down one piece at a time. First, the question states that there is a high sodium concentration outside the cell. This means sodium wants to move into the cell (where concentration is lower). Choices A and C can be eliminated because they state that sodium would move out of the cell. Active transport requires ATP, and because there is no ATP involved, choice E can be eliminated. Last, because sodium moves across the membrane with the help of a channel, it is moving by facilitated diffusion (choice B is eliminated).

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