Potato Osmosis activity

We talked about semi-permeable membranes and how nutrients, oxygen, and other things pass from the capillaries to the cells, or the alveoli to the capillaries, by diffusion or osmosis. This simple activity demonstrates the effects of osmosis. You cut a potato in half and put half of it in plain water and the other half in salt water. Then wait a few hours. (Put the cut side facing down; this picture shows it after we'd already turned it back over.) 

(White dish) Because there is a lower concentration of water in the salt water (the salt is "taking up" some of the space), the water in the potato cells moves out via osmosis. That makes the cells "floppy" or flaccid, since they have lost some of their water. You can feel that the potato is squishy and limp.

(Clear dish) The potato in the plain water stays firm and crisp. Because the concentration of water on the outside of the potato (in the dish) is the same as the concentration on the inside (in the cells), no osmosis occurs, and the cells stay firm and maintain their shape.

For another explanation of this, see here.