How does molecular size influence distribution across biological barriers?

How does molecular size influence distribution across biological barriers?

• A. Larger molecules diffuse more quickly
• B. Smaller molecules diffuse more slowly
• C. Larger molecules diffuse more slowly and may be restricted by permeability barriers
• D. Size has no effect on distribution

Answer: (C)

Molecular size governs how easily substances cross biological barriers. As a molecule gets larger, its diffusion coefficient through fluids and across membranes generally decreases, so large molecules move more slowly. Biological barriers like cell membranes, capillary walls, and the blood–brain barrier are selective and tend to allow small, often lipophilic molecules to pass readily while presenting steric hindrance to bulky ones. Because of this, larger molecules face slower diffusion and may be effectively restricted unless they use specific transport mechanisms or channels. Small molecules, in contrast, can diffuse more easily and distribute more widely, assuming other factors like lipophilicity and charge don’t unduly limit passage.

Molecular size governs how easily substances cross biological barriers. As a molecule gets larger, its diffusion coefficient through fluids and across membranes generally decreases, so large molecules move more slowly. Biological barriers like cell membranes, capillary walls, and the blood–brain barrier are selective and tend to allow small, often lipophilic molecules to pass readily while presenting steric hindrance to bulky ones. Because of this, larger molecules face slower diffusion and may be effectively restricted unless they use specific transport mechanisms or channels. Small molecules, in contrast, can diffuse more easily and distribute more widely, assuming other factors like lipophilicity and charge don’t unduly limit passage.