Selective Permeability and Osmosis in Dialysis

Steps:

• Identify membrane properties: allows sugar and water diffusion, blocks starch and protein.
• Determine movement: non-permeable starch and protein create hypertonic inside, drawing water in via osmosis.
• Account for sugar: diffuses based on gradient, but doesn't reverse osmotic effect from large molecules.
• Predict outcome: bag volume increases after 30 minutes due to net water influx.

Why B is correct:

• B states the bag swells, matching osmosis definition where water moves to higher solute concentration across semi-permeable membrane.

Why the others are wrong:

• A: No volume change ignores osmotic pressure from impermeable solutes.
• C: Bag shrinks assumes hypotonic inside, but setup implies hypertonic.
• D: Wrong substance movement violates permeability rules for starch/protein.

Final answer: B