Dialysis Bags, Osmosis, and an Enzyme That Cannot Get In
A student fills four dialysis bags (molecular weight cutoff 12,000 Da) with 10 mL of solution, ties them, weighs each, and suspends each in a separate beaker containing 100 mL of test solution at room temperature. After 60 minutes each bag is blotted and reweighed, and the beaker fluid is tested with Benedict's reagent for glucose and with iodine for starch.
- Bag 1: 10% glucose inside; beaker of distilled water.
- Bag 2: 10% starch inside; beaker of distilled water.
- Bag 3: 10% starch inside; beaker of 1% amylase in water (amylase is about 54,000 Da).
- Bag 4: 10% glucose inside; beaker of distilled water plus amylase.
Mass changes at 60 min: bag 1 +3.8 g, bag 2 +1.1 g, bag 3 +1.0 g, bag 4 +3.9 g.
Beaker tests: bag 1 Benedict's positive, iodine negative; bag 2 Benedict's negative, iodine negative; bag 3 Benedict's negative, iodine negative; bag 4 Benedict's positive, iodine negative.
1. Bags 1 and 2 held the same mass concentration of solute but gained different amounts of water. Explain why.
Read the worked answer
Osmosis responds to the number of dissolved particles, not to grams on the label. 10% glucose is roughly 100 g/L ÷ 180 g/mol ≈ 0.56 mol/L of particles. Starch is a polymer of thousands of glucose units, so 10% starch is a very small molar concentration, on the order of millimoles, and each dissolved molecule holds far less water than the equivalent mass of glucose.
The water potential difference across bag 2's membrane was therefore much smaller, so less water entered - hence +1.1 g versus +3.8 g. Neither solute crossed the 12,000 Da membrane, so both mass gains are osmotic rather than diffusive, which makes the comparison clean. The general point: equal percent concentration is not equal osmotic strength.
2. Bag 3's beaker contained amylase, yet no glucose appeared in the beaker and no starch appeared outside the bag. Explain what happened and what the iodine result adds.
Read the worked answer
Both molecules were too large for the 12,000 Da cutoff. Amylase is roughly 54,000 Da and starch is far larger, so amylase could not enter the bag and starch could not leave it. Enzyme and substrate never met, so no hydrolysis occurred and no glucose was produced.
The iodine test on the beaker fluid is the useful control: it stayed negative, showing the membrane was intact and starch was still inside. That rules out a leak as the explanation. The correct conclusion concerns membrane permeability, not enzyme function - a student who writes "amylase does not digest starch" has over-read a negative result.
3. Predict the results for bag 3 if the membrane cutoff were raised to 100,000 Da.
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Amylase (~54,000 Da) would now pass into the bag and reach the starch, hydrolysing it to maltose and glucose. Glucose (180 Da) would then diffuse down its gradient out into the beaker, so the beaker fluid should give a positive Benedict's test within the hour. Iodine would still be negative, because starch remains too large to escape.
Water movement would change over time. Chopping one large starch molecule into many small sugars sharply raises the particle count inside, pulling water in transiently before glucose leaks out and the gradient falls. Mass change alone therefore becomes ambiguous, and the chemical tests, not the balance, are what identify digestion.