Osmosis is the scientific term used to describe the movement of water through a membrane. The direction of movement will always be toward the area with the smallest amount of water. Essentially, the water flows to where it is needed most. Help kids understand osmosis by allowing them to pour beans mixed with salt into a colander. Discuss why the salt’s smaller size allowed it to pass through the holes in the colander while the larger beans did not. This activity demonstrates the semi-permeability cell membranes, which selectively allows some materials to pass through based on size.
An egg is a good material to use in an osmosis experiments because it clearly resembles an animal cell and has a visible inner membrane. To reveal the membrane underneath the egg’s shell, place a raw egg in a transparent container, cover the egg with white vinegar and seal with a lid. Students will immediately observe bubbles that appear from the presence of carbon dioxide gas that occurs when the vinegar reacts with the limestone of the shell. After 72 hours, the shell will have dissolved and leaving the rubbery membrane behind. Because the water from the vinegar passed through the membrane into the egg, it will be recognizably bigger.
The same egg can be used again to show how the process of osmosis allows water to flow out of a cell’s membrane when it is surrounded by an area that contains less water. Place the same egg, or one soaked in vinegar for three days, into a transparent jar and fill with three inches of corn syrup. After three more days, the egg will be small with not much left inside the membrane. Students can easily see how the process of osmosis allowed the water from inside the egg to pass through the membrane into the corn syrup, leaving only the material that would not pass through the egg's rubbery lining.
Many substances with a cell membrane can be used to show how cells take in or lose water. Consider weighing an object before, and then after, soaking it in different liquids to see what kinds of fluids are more easily absorbed. As students begin to understand how water passes in out of a cell, use more complex experiments to prepare them for other scientific concepts related to osmosis and cellular activity. Slice a potato in half and demonstrate how the spud will absorb fresh water while it won’t take in saltwater. Make sure children have an active part in every experiment and give them time to discuss what they see and think about the results.