Diffusion Through a Membrane Lab Set-up

I spent several hours today setting up the fairly elaborate lab required by NY State for the regents exam, "Diffusion Through a Membrane." It's a nice enough lab, but way too much set-up for teachers in my situation, in rooms with no running water that are used by multiple teachers and classes throughout the day. The basic idea is to produce cell models with dialysis tubing, cut into segments, filled with a mixture of a starch solution and a glucose solution, and tied at both ends a little like a sausage link. This "cell" is placed in a hypotonic environment (distilled water) into which a few drops of Lugol's iodine solution is placed.

While allowing the cells to soak in water, students will perform a series of tests, using known solutions of starch and glucose, plus Benedict's solution and the iodine mentioned above as indicators. They will observe that the light blue Benedict's solution will turn orange/red when heated (water bath method) if glucose is present, and the Lugol's solution will turn black in the presence of starch, without heating. There are of course controls to show that it is indeed the glucose & starch that are responsible for the color changes.

Meanwhile, the little cell will have turned a dark purplish-black color inside, showing that iodine has diffused across the membrane into the cell. A test of the distilled water outside the cell will reveal that glucose has diffused through the membrane out of the cell. The color of the distilled water outside the membrane will remain amber from the iodine, indicating that starch has not diffused out of the cell through the membrane.

• Glucose - diffuses out of the cell, no indicator in the water, so no color change. Only detected when water is added to indicator and heated.


• Iodine - diffuses into the cell, turns color inside the cell to indicate starch.


• Starch - does not diffuse out of cell, so water outside cell with Lugol's does not change color. Inside cell turns blackish.

And that's just Part 1! In part 2 We get to hook up the microscopes and watch onion cells shrivel or plump up when placed in saltwater or distilled water environment.

Follow-up Post