The Dialysis Bag Diffusion Experiment
A student fills a dialysis bag (a semipermeable membrane with pores that allow water, glucose, and iodine to pass but block starch) with a mixture of 2% starch and 1% glucose solution, then seals it and places it in a beaker of distilled water containing iodine. After 30 minutes, the water outside the bag turns blue-black. Tests show glucose is present in the beaker water but starch is not. The student claims this proves the membrane is completely impermeable to starch and that no glucose remained inside the bag. The teacher points out the student did not test the fluid inside the bag and asks the student to revise the conclusion and explain what drove each movement.
1. Explain which substances crossed the membrane and what transport process moved each one, then correct the student's claim about starch permeability.
Read the worked answer
Iodine and glucose crossed the membrane; starch did not. Iodine is small enough to pass through the pores and diffuses down its concentration gradient into the bag, where it reacts with starch to form the blue-black complex. But the blue color appeared in the beaker water, which means starch molecules also reached the outside - wait, that cannot be right if starch is blocked. The correct reading: iodine diffused inward first, and the color change in the beaker indicates iodine was present there and reacted with starch that had leaked out. Re-examining the evidence, the safer conclusion is that the membrane is selectively permeable: iodine and glucose pass freely, while starch is largely retained. The student's claim of complete impermeability is too strong because some starch may have escaped, and the claim that no glucose remained inside is unsupported because the inside was never tested. Glucose moves by facilitated diffusion through the pores; water moves by osmosis toward the higher solute concentration inside the bag.
2. Predict what would happen to the bag's mass if it were placed in a 10% glucose solution instead, and justify your prediction using water potential.
Read the worked answer
The bag contains 2% starch plus 1% glucose, giving a total solute concentration lower than the 10% glucose outside. Water moves by osmosis from the region of higher water potential (lower solute concentration) to lower water potential (higher solute concentration). Since the outside solution is more concentrated, the bag's contents have the higher water potential, so water will leave the bag and its mass will decrease. The membrane's starch-impermeability means starch cannot leave to equalize, so the osmotic gradient persists and shrinkage continues until water potentials equalize or the bag becomes flaccid. If the bag were instead in distilled water, water would enter and mass would increase. This shows that direction of osmosis depends on relative solute concentration, not on the absolute amount of solute.