Cell Membrane Model Demonstration Using Dialysis Tubing

Summary

Mr. W demonstrates selective permeability using dialysis tubing. A selectively permeable boundary allows some substances to cross while preventing others from passing.

Demonstration and procedure

1. Tie one end of a piece of dialysis tubing to form a bag.
2. Fill it with a solution containing starch and fructose, then tie the other end closed.
3. Place the bag in a cup of water and add iodine solution to the cup.
4. Save some of the cup’s solution in a glass test tube as a comparison sample. The glass prevents substances from crossing.
5. Let the setup sit, then observe the bag and cup and test the cup’s solution for fructose.

Observations and conclusions

• The solution inside the bag turns purple-black, showing that iodine entered the bag and reacted with the starch.
• The solution in the cup tests positive for fructose, showing that fructose left the bag and crossed into the cup.
• The cup’s solution does not turn purple-black, indicating that starch did not leave the bag.
• The iodine solution in the cup becomes lighter because iodine entered the bag and reacted with starch; the reacted iodine is no longer freely present in the cup.

The results show that the dialysis tubing is permeable to iodine and fructose, but not to starch.

Molecular explanation

Diffusion moves substances down their concentration gradients:

• Starch and fructose begin at higher concentrations inside the bag, while iodine begins at a higher concentration outside it.
• The small fructose and iodine molecules can pass through the tubing’s pores.
• Starch is a large polysaccharide and is too large to pass through. Fructose is a much smaller monosaccharide, and iodine is smaller still.

Mr. W compares the tubing to a chain-link fence: a large starch molecule could not fit through, whereas smaller fructose and iodine molecules could.

Broader lesson

The demonstration models how a selectively permeable boundary works. Cell membranes have a different structure from dialysis tubing, but they also regulate which substances can cross. This selective permeability is a fundamental feature of living systems.

Speaker

• Mr. W — narrator and presenter.