Dialysis membrane permeability based on molecular size

Steps:

• Dialysis membranes are semi-permeable, allowing diffusion of small molecules (<500 Da) while blocking larger ones.
• Urea (60 Da) and sodium ions (~23 Da) are small waste products or electrolytes that pass through to remove toxins and balance fluids.
• Glucose (180 Da) is also small and can diffuse, though its concentration is managed in dialysate.
• Insulin (5,800 Da), a peptide hormone, exceeds the membrane's pore size limit, preventing diffusion.

Why B is correct:

• Insulin is a large protein (molecular weight ~5,800 Da), which dialysis membranes exclude to retain essential blood proteins per the principle of size-selective filtration.

Why the others are wrong:

• A. Glucose: Small carbohydrate (180 Da) diffuses through pores designed for metabolites.
• C. Sodium: Tiny ion (23 Da) freely diffuses to maintain electrolyte balance.
• D. Urea: Small waste molecule (60 Da) diffuses out as the primary target of dialysis.

Final answer: B