Osmosis in Action

 

The Shrinking-Expanding Onion Cell

A remarkably easy and interesting experiment is to watch the shape of onion cells change when exposed to a high salt solution. You are watching “osmosis in action”.

Have ready: a cup of a high salt solution (just dump table salt into a container and add a small amount of water to dissolve and make a concentrated solution; don’t need to be too precise) and a cup of tap water. Place a then peel of a red onion on a microscope slide (see preparation of red onion).

Start a time lapse movie (1 frame/sec; 60X mag); after a few seconds of recording, put one or two drops (with a pipette or a medicine dropper) of high salt solution on the section of the onion slice under view in the microscope. Watch the cells “shrivel”. But they are not dead. Place several drops of regular tap water to dilute the salt concentration. The shriveled onion cells will gradually expand to their original shape (or close to it). This shrinking and expanding is largely due to a process called osmosis and illustrates that cells have membrane that restrict the passage of salt but allow the passage of water. When you add a drop of concentrated salt solution, water flows out of the cell (which has a lower salt concentration than the outside) across it membrane, and it shrinks. When you dilute the salt/sugar, water flows across the cell membrane into the cell and it expands. More detailed explanations can be found in a basic biology textbook or How Stuff Works (http://science.howstuffworks.com/question29.htm). Kids might guess that there is a “membrane” surrounding the cell to hold it together, but this illustrates the properties of this membrane in action. Human cells also need proper salt conditions; if the salt concentration of a blood transfusion is not correct (does not match the salt concentration in your blood), then your red blood cells can change shape and potentially pop. Onion cells are a little sturdier, in part because they also have a “cell wall” outside of their membrane which provides mechanical rigidity for plant cells.

Osmosis in onion cell (6.6 mb movie)