Which molecules require a carrier system to cross the blood-brain barrier?

The correct choice is the one that identifies metabolic by-products and glucose as molecules that require a carrier system to cross the blood-brain barrier. This barrier is a selective permeability barrier that protects the brain from potentially harmful substances while allowing essential nutrients to pass through.

Glucose, a vital source of energy for the brain, cannot simply diffuse through the blood-brain barrier due to its size and polar nature. Instead, it is transported into the brain by specific transporter proteins, such as the GLUT1 transporter, which facilitate its movement across the barrier.

Metabolic by-products, depending on their nature, often also need specific transport mechanisms to be effectively transported out of the brain tissue or into circulation. This allows for the regulation of metabolic processes and the maintenance of homeostasis within the brain.

In contrast, molecules such as oxygen and carbon dioxide can freely diffuse across the blood-brain barrier due to their small size and lipid solubility. Similarly, alcohol and lipids can pass through the barrier without the need for specific carriers, as they can easily navigate through the lipid bilayers of cell membranes. Neurotransmitters and hormones often do not cross the blood-brain barrier directly; instead, they interact with specific receptors on the brain's endothelial cells, thus influencing