How do lipid-soluble molecules cross the blood-brain barrier?

How do lipid-soluble molecules cross the blood-brain barrier?

• A. They cross by simple diffusion.
• B. They cross via active transport.
• C. They cannot cross BBB.
• D. They cross via gap junctions.

Answer: (A)

Lipid-soluble molecules cross the blood-brain barrier primarily by simple diffusion across the endothelial cell membranes. The barrier is made up of tightly joined endothelial cells that limit paracellular passage, so substances move transcellularly rather than through gaps. If a molecule is lipid-soluble, it can dissolve in the cell membrane’s lipid bilayer and diffuse down its concentration gradient into brain tissue without needing energy or specific transport proteins. This is why gases like O2 and CO2, as well as many lipid-soluble drugs and steroid hormones, readily reach the brain. Active transport would be needed for polar or charged substances that can’t diffuse, not for lipid-soluble ones. Gap junctions connect neighboring cells but do not provide a route for blood-to-brain translocation of solutes, so they aren’t the mechanism here.

Lipid-soluble molecules cross the blood-brain barrier primarily by simple diffusion across the endothelial cell membranes. The barrier is made up of tightly joined endothelial cells that limit paracellular passage, so substances move transcellularly rather than through gaps. If a molecule is lipid-soluble, it can dissolve in the cell membrane’s lipid bilayer and diffuse down its concentration gradient into brain tissue without needing energy or specific transport proteins. This is why gases like O2 and CO2, as well as many lipid-soluble drugs and steroid hormones, readily reach the brain. Active transport would be needed for polar or charged substances that can’t diffuse, not for lipid-soluble ones. Gap junctions connect neighboring cells but do not provide a route for blood-to-brain translocation of solutes, so they aren’t the mechanism here.