Final answer:
The substance least able to cross the blood-brain barrier is the white blood cell, as it is too large and typically cannot penetrate the barrier. Other substances like water, oxygen, and glucose can pass through, while sodium ions are also restricted due to their charge. Conditions like inflammation may allow white blood cells to cross, but this is not the norm.
Explanation:
Crossing the Blood-Brain Barrier
The blood-brain barrier (BBB) is a selective permeability barrier that protects the brain from harmful substances while allowing necessary nutrients to pass through. Among the options presented:
- Sodium ions: These are charged particles and generally have difficulty crossing the BBB.
- Water: This molecule can cross the BBB fairly easily through aquaporin channels.
- Oxygen: A small nonpolar molecule, oxygen can diffuse readily across the BBB.
- White blood cells: These are large immune cells that typically cannot cross the BBB unless there is inflammation or a pathological condition.
- Glucose: Despite being a crucial energy source, glucose requires specific transporters to cross the BBB.
Thus, of these substances, the white blood cell is the least able to cross the blood-brain barrier under normal physiological conditions.
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