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MCAT Biology Question — Passive Diffusion

The plasma membrane maintains the concentration gradients between the intracellular and extracellular spaces. In so doing, some molecules can diffuse directly through the membrane. Which of the following molecule(s) can do so?


I. Water
II. Testosterone
III. Glucose
IV. Na+


A. I only
B. I and II only
C. I, II, and III only
D. I, II, III, and IV

Click for Explanation

This question asks the examinee to identify the types of molecules that are able to enter the cell through passive diffusion. To answer this question, you must understand the various transport mechanisms for molecules into and out of the cell. Passive diffusion, facilitated diffusion, active transport, and secondary active transport are the four types of transport processes.

This question asks about passive diffusion, which involves the movement of molecules down a concentration gradient directly through the plasma membrane (without transmembrane proteins). The passive diffusion of molecules depends on two things: 1) size and 2) polarity.

The size of a molecule is important because molecules that are too large to fit between the phospholipids that comprise the cell membrane must use integral proteins. Polarity is also an important consideration because the nonpolar interior of the plasma membrane. This hydrophobic environment prevents the passage of all charged ions.

Of the choices above, water is small enough to pass through the membrane. Testosterone is a steroid hormone, which freely passes through the plasma membrane. Glucose is too large to pass through the membrane and sodium ions are charged and thus unable to pass through the membrane. Therefore, I and II are correct making B the answer.

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