osmosis red onionFor the chapter on cell membranes we did two labs last year.  The first experiment involved looking at red onion cells under the microscope while adding salt water to the slide.  The salt water increases the concentration of solute outside the cell so water leaves the cell causing it to shrink.  This is very obvious with the red onion cells because the red/purple area shrinks away from the cell walls as you can see in the photo.  The cells at the bottom of the picture have not be in contact with the salt water yet so they still look ok.  close of red onion cellAs we watched through the microscope we saw the membranes shrink and the red/purple color become more concentrated in the center of each cell – this is called plasmolysis and you can find the lab I used at www.explorebiology.com.

The second lab we did involved chicken eggs.  A few days before class, we soaked the eggs in vinegar to dissolve the shell.   During class the  students found the mass of the eggs and made a prediction in their lab books about what they thought would happen to the eggs.  The eggs were then either soaked in water or corn syrup.  eggs osmosisEvery 30 minutes they pulled the eggs out of the liquid, patted them dry and recorded their mass again.  Everyone was very surprised to see that the egg soaking in corn syrup actually shrunk and the egg soaking in water swelled up.  Students found the percent change in mass of each egg and made graphs of their results.  The egg in the corn syrup, hypertonic solution,  shrunk (egg on the left in the photo) because the water left the cell (yes, a chicken egg is just one big cell!) to try to dilute the sugary surroundings, while the egg soaked in water, a hypotonic solution, absorbed more water trying to dilute its interior (egg on the right in the photo). You can see in the picture if was a pretty obvious change.  The middle school class did these labs too, but we only measured the eggs twice, at the beginning and the end of class so it was more of a demonstration.   We actually let the eggs soak for a few days after class,  just to see how much more they would change. shrunk egg The egg soaking in water didn’t really change much after the first few hours but the egg in corn syrup basically turned into just the yolk (see photo).

biology inquiriesThis and some of the other labs we did can be found in Biology Inquires: Standards-Based Labs, Assessments, and Discussion Lessons by Martin Shields.  

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