The Dialysis Bag: Reading Permeability from Four Observations
A student seals 10 mL of a solution containing 5% glucose and 1% starch inside a length of dialysis tubing whose pores exclude molecules larger than roughly 14,000 daltons, then blots the outside dry. The filled bag weighs 12.4 g. She suspends it in a beaker holding 200 mL of distilled water to which a few drops of amber iodine solution have been added; iodine is a small molecule that turns blue-black wherever starch is present. After 30 minutes she records four observations: the bag weighs 15.1 g; the fluid inside the bag is deep blue-black; the water in the beaker is still amber; and a glucose test strip dipped into the beaker water reads positive. She then sets up a second, identical bag in a beaker of 20% glucose solution instead of distilled water.
1. Using all four observations, state which substances crossed the tubing membrane and which did not. Then rank water, iodine, glucose, and starch from smallest to largest, and explain how each observation supports your ranking.
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What crossed: iodine entered, glucose exited, water entered, and starch stayed inside.
Evidence, one observation at a time:
- The bag's contents turned blue-black, but the beaker water stayed amber. Iodine must have moved inward to reach the starch, and starch must have been unable to move outward, or the beaker would also have turned blue-black.
- The beaker water tested positive for glucose even though no glucose was ever put in the beaker, so glucose crossed out of the bag down its concentration gradient.
- The bag gained 2.7 g. Solute had not been added to it, so the added mass is water that moved inward osmotically, driven by the higher total solute concentration inside.
Size ranking (smallest to largest): water < iodine < glucose < starch.
The ranking follows from the results plus the stated pore limit. Water and iodine crossed freely, glucose crossed but is a larger molecule than water, and starch - a long polymer of hundreds of glucose units, far above the 14,000-dalton cutoff - was retained. The membrane behaves as a sieve: small particles pass, large ones do not, and the tubing's pore size rather than any active transport explains the pattern.
2. Predict whether the second bag (inside: 5% glucose plus 1% starch; beaker: 20% glucose) will gain or lose mass after 30 minutes. Explain the direction of net water movement and the direction of net glucose movement, and note whether the two gradients push material the same way or opposite ways.
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Prediction: the bag loses mass and becomes flaccid or shriveled.
Water: the beaker holds 20% solute against roughly 6% total solute inside the bag. Water moves by osmosis from the region of lower solute concentration (higher water potential) to the region of higher solute concentration, so net water movement is outward, out of the bag.
Glucose: the beaker's glucose concentration is far higher than the bag's, so glucose diffuses inward, down its own gradient.
Do the two gradients agree? Yes. Water leaves and glucose enters; both fluxes move material from the beaker side toward the bag side or the reverse in a way that reduces both concentration differences. This is the key contrast with the first trial, where water entered even though glucose was leaving - because there the inside solution was the more concentrated one overall. The lesson is that water movement is governed by total solute concentration across the membrane, not by the direction any single solute happens to be diffusing. Over a longer time, glucose would equilibrate and the bag would stop shrinking.
3. A second group's bag was nicked with a scissors blade while being tied shut. Fifteen minutes later their beaker water has turned blue-black and their bag's mass has dropped. What can this group legitimately conclude about membrane permeability, and what should they do next?
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Conclusion they can draw: essentially nothing about permeability.
The blue-black color in the beaker means starch reached the water, but a visibly damaged bag is the most economical explanation - starch escaped through a physical tear, not through the membrane's pores. The mass loss is likewise explained by bulk leakage of solution rather than by osmosis. Both results are artifacts of a broken seal.
Using this trial to claim that "starch can cross dialysis tubing" would be a design error, not a discovery. The membrane was never actually tested, because the barrier being studied was breached.
What they should do: discard the run and repeat with a new bag. Before starting, they should fill the bag, blot it, and hold it over an empty beaker for a minute to check for drips. They should also keep the beaker-water starch test as the standard check for membrane integrity in every trial, so that any future leak is caught immediately. An experimental result is only as good as the assumption that the apparatus is doing what it is supposed to do.