Cells: Semipermeable Membranes

You don't have to get very far into your study of cells before your students are presented with the term "semi-permeable," used to describe cell membranes.  Just as easily as you can define it for your students, you can show them!

Before class mix together 1/2 cup of sand (or salt) and 1/2 cup of marbles (or dried beans, pebbles, or other objects larger than the holes in a colander).  Place the mixture in a beaker or glass jar.

When you get to semi-permeable membranes during class, show the students your mixture.  Then pour the mixture through a colander (make sure you have a bowl or pan underneath!).

The beans stay in the colander while the salt/sand passes right through.  Just like a cell membrane, particles that are small enough to pass through the holes do so and particles larger than the holes stay put.