How to show Students Diffusion and Osmosis

Two very abstract concepts from the topic of Molecular Transport in Cells.


I got the fourth grade matter for the first time last year and man, it was hard to grasp for me at first. You can imagine how hard it was to let the students understand these concepts.

This year, lessons in fourth grade are simpler because now I fully understand the two concepts. This makes life as a teacher so much easier.

Also, our lovely technical education assistant found some lovely demonstration experiments to actually show the students how osmosis and diffusion works.

First of all, we showed a diffusion demonstration by using dialysis tubing that was shaped into an artificial cell with a starch solution in it. The artificial cell was then dropped into a iodine solution. Iodine is an indicator for starch. This means, its original colour will change when it binds to a certain molecule (in this example starch). When iodine binds to starch its colour will change from yellow to purple.

It is based on a video from the '80s a colleague found, but here is a lovely example made by a teacher who perfectly describes how it works:

It is a brilliant demonstration. Diffusion is actually a very simple concept. Molecules move from a higher concentration to a lower concentration, and in this demo you can see that the amount of bound iodine -starch molecules spread from the sides of the cell to the centre of the cell. It also shows that semi-permeable membranes lets some molecules through (in this case IKI which colours the starch in the artificial cell) and others cannot go through.

Now, for osmosis it is a bit different. Osmosis is a bit harder to grasp as it is the movement of water from a solution with a low concentration to a solution with a higher concentration. If you have two of these solutions seperated by a cellular membrane, the water molecules will move freely through it and will try to even out the concentration of both parts by moving the water into the part with the highest concentration. This will cause the water level in the part with the highest concentration to rise (or gain in volume).

Here is again a demo by Mr. Anderson showing how osmosis works.

I did both experiments twice today, and I had so much fun! It makes it so much simpler for students to understand both concepts.
Here are my pictures:
First, the osmosis experiment:

At left, the starting position, you submerge the whole thing under water. The glass bulb has an opening at the bottom where dialysis tube is placed with an elastic. At right, after about ten to fifteen minutes, the water level has risen about a centimetre above my starting line (in black).

At left the start of the diffusion experiment. One artificial cell filled with starch solution and a glass with iodine solution (or IKI). At right the cell is starting to turn purple!

Here we have a nice purple artificial cell. The iodine reacts with starch and this makes iodine a great indicator for starch.
After that, we did something fun. Ascorbic acid reacts with iodine, better than starch, so we put it in the iodine solution.

Bubbles.....

And here we have again a clear solution!
You really need to be a bit geeky to be a teacher and really love these experiments!