What can freely cross the cell membranes of the blood-brain barrier?

The correct choice highlights that certain substances can freely cross the cell membranes of the blood-brain barrier due to their molecular characteristics and the nature of the barrier itself. The blood-brain barrier is a selective permeability barrier that protects the brain from potentially harmful substances while allowing essential nutrients to pass through.

Substances that can easily diffuse across this barrier tend to be small, nonpolar, or lipid-soluble. Among options that commonly cross the blood-brain barrier, glucose and oxygen are indeed vital. Glucose is transported by a specific transporter, while oxygen, being a gas, can diffuse freely. Freely crossing does not imply requiring a specific transport mechanism or being tightly regulated.

Vitamins, while essential for health, often do not cross the blood-brain barrier effectively, as many require specific transport mechanisms or simply don’t have the lipid-soluble properties necessary for diffusion. Water does cross the barrier but is regulated through specialized aquaporin channels rather than freely.

Therefore, understanding that the blood-brain barrier allows for the diffusion of certain substances, the correct answer emphasizes a type of molecule that can pass through without restriction, reflecting the unique properties of the blood-brain barrier itself.