MCAT Biology Question — Passive Diffusion
- May 27, 2018
- MCAT Question of the Day
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 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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