Which mechanism allows lipid-soluble molecules to cross the blood-brain barrier?

Which mechanism allows lipid-soluble molecules to cross the blood-brain barrier?

• A. Lipid-soluble molecules via simple diffusion.
• B. Water via aquaporins.
• C. Ions via active transport.
• D. Proteins via passive diffusion.

Answer: (A)

The key idea is that passage into the brain depends on a molecule’s ability to dissolve in the lipid membranes of the brain’s capillary endothelium. Lipid-soluble (nonpolar) molecules cross the blood–brain barrier by simple diffusion, moving directly through the endothelial cell membranes down their concentration gradient without needing energy or transport proteins. The tight junctions between those cells largely prevent paracellular diffusion, so diffusion through the membrane is the main route for these lipophilic substances. Water mainly uses aquaporins, ions require energy-dependent transport or specific channels, and proteins are too large and polar to diffuse passively.

The key idea is that passage into the brain depends on a molecule’s ability to dissolve in the lipid membranes of the brain’s capillary endothelium. Lipid-soluble (nonpolar) molecules cross the blood–brain barrier by simple diffusion, moving directly through the endothelial cell membranes down their concentration gradient without needing energy or transport proteins. The tight junctions between those cells largely prevent paracellular diffusion, so diffusion through the membrane is the main route for these lipophilic substances. Water mainly uses aquaporins, ions require energy-dependent transport or specific channels, and proteins are too large and polar to diffuse passively.