MCAT Biochemistry Question 106: Answer and Explanation

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Question: 106

1. A student is trying to determine the type of membrane transport occurring in a cell. She finds that the molecule to be transported is very large and polar, and when transported across the membrane, no energy is required. Which of the following is the most likely mechanism of transport?

  • A. Active transport
  • B. Simple diffusion
  • C. Facilitated diffusion
  • D. Exocytosis

Correct Answer: C

Explanation:

We are asked to identify the type of transport that would allow a large, polar molecule to cross the membrane without any energy expenditure. This scenario describes facilitated diffusion, which uses a transport protein (or channel) to facilitate the movement of large, polar molecules across the nonpolar, hydrophobic membrane. Facilitated diffusion, like simple diffusion, does not require energy, which explains why no ATP was consumed during this transport process.

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