Look closely · Biology
Visking tubing
What it is for
A partially permeable bag: small molecules pass, large ones do not.
Mistake to avoid Not rinsing the outside of the bag after filling it: solution left on the outside gives a positive test that never came through the wall.
Drawn in ink: the site's lab kit has no model of it yet, so the one at school may look a little different.
How it is asked
- Use it as a model cell: the tubing is a partially permeable membrane, like a cell membrane.
- Explain changes in mass or liquid level by osmosis: water moves from a dilute solution to a more concentrated one.
- Show which molecules pass: test the water outside with iodine solution for starch, and heat a sample with Benedict’s solution for reducing sugars.
Exam-style question
A bag of Visking tubing is filled with starch and glucose solution, rinsed, and hung in distilled water. After 30 minutes the water outside is tested.
- The water outside gives a positive Benedict’s test but a negative iodine test. Explain these results.
- The bag is heavier after 30 minutes. Explain why.
- Why is the outside of the bag rinsed before it goes into the water?
Show what a good answer says
- Glucose molecules are small enough to pass through the tubing, so glucose diffuses out, from its higher concentration inside to the lower one outside. Starch molecules are too large, so they stay inside.
- There is a net movement of water into the bag by osmosis, from the distilled water outside to the more concentrated solution inside, through the partially permeable tubing.
- To wash off solution spilt on the outside, which would give a positive test without passing through the tubing.
Teaching points for checking your own answer, not official Cambridge marks.
