Final answer:
The claim that only vitamins can move freely across the blood-brain barrier is false. The BBB allows passage of certain substances including glucose, amino acids, water, gases, and ions, whereas plasma proteins generally do not cross into the tissue fluid via facilitated diffusion.
Step-by-step explanation:
The statement that only vitamins can freely move across the cell membrane of the blood-brain barrier (BBB) is False. The BBB is a highly selective permeability barrier that prevents most compounds from entering the central nervous system (CNS), but it does allow certain substances to pass through. Not just vitamins, but also essential nutrients such as glucose and amino acids can cross the BBB. Water, gases, and ions are also known to cross, albeit through various specialized mechanisms. Conversely, larger molecules and most immune cells are restricted from freely passing the BBB, which provides protection to the brain from toxins and pathogens but also complicates the delivery of therapeutic drugs.
Plasma proteins typically do not cross the capillary cell membrane into the tissue fluid via facilitated diffusion due to their size and the selective nature of capillary walls. Instead, they are retained within the blood vessels to maintain osmotic balance and perform other functions. Some small molecules that are hydrophobic, like lipid-soluble vitamins and gases such as oxygen and carbon dioxide, can pass directly through the plasma membrane by simple diffusion thanks to their solubility in the hydrophobic core of the membrane.