This lab uses dialysis tubing to model the semi-permeability of cell membranes. You'll need to have dilute iodine solution and starch solution (I just use corn starch and water) prepared ahead of time. Students create a "bag" out of the dialysis tubing, fill it with starch, and then place the bag in iodine. Over a few minutes, the smaller iodine atoms diffuse into the bag, turning the starch purple/black. The lab goes on to ask students to answer a series of analysis questions and to write a conclusion.
This lab is also great because it allows you to connect the students back to earlier content: Iodine is an element so it is very small, starch is a macromolecule so it is very large and therefore cannot fit through the tiny holes in the dialysis tubing.
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