This activity was sent to me by Heather Burgess.

Purpose

To provide a model of a semi-permeable membrane and to emphasize the limits of pore size on the movement of molecules by simple diffusion. This activity is also related to osmosis.

Background

This activity follows a laboratory experience where students have observed osmosis in de-shelled eggs placed in different concentrated solutions. The students have been introduced to the concepts of the fluid-mosiac model of a cell membrane, the phosolipid bilayer composition of membranes, concentration gradient, osmosis, and diffusion.

Preparation

I buy several small and large-sized nerf balls to represent small and large molecules. I take two large lawn garbage bags and cut them into sheets and then tape the two sheets together to form one wide sheet. I then use a jack-knife and cut holes into the plastic. The holes have to be large enough for the smaller nerf balls to pass through, but not large enough for the large nerf balls to pass.

Activity

I usually do this as a demonstration, but it could work just as well as an activity with the entire class involved the students could make the membranes or even design them themselves from different materials. I tell the students that the nerf balls are two different sized molecules, the small one being water and the large one being some biomolecule, like glucose. I then have two students hold the membrane up and have several other students throw the nerf balls at the membrane. Ideally, only the small nerf balls will make it across by going through the holes (you may want to have students with good aim throw the balls although the fact that it does take a while for balls to get across does lead to a discussion on the randomness/slowness of simple diffusion and osmosis in cells.)We then develop and emphasize the concept of osmosis from the students’ observations from this simulation.