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Biology · Diffusion · Membrane model
Visking tubing as a model membrane
A bag of Visking tubing containing starch and glucose solution is suspended in a beaker of distilled water. Visking tubing has microscopic pores: small molecules pass through, large ones cannot. After 30 minutes, test both the bag contents and the surrounding water for each sugar.
t = 0 min / 30 min
Summary
| Starch? | Glucose? | |
|---|---|---|
| Bag contents | + (large) | + (small) |
| Beaker water | − | + |
Starch has not crossed the membrane (molecules are too large). Glucose has diffused into the surrounding water (small enough to pass).
Exam-style questions
1. After 30 minutes, iodine added to the water OUTSIDE the bag stays orange-brown. What does this show?
2. After 30 minutes, Benedict's reagent added to the water OUTSIDE the bag gives a brick-red precipitate. What does this show?
3. Visking tubing is a model of which cell structure?
4. Why must the OUTSIDE of the bag be washed with distilled water BEFORE submerging it in the beaker?