Eggs Again

I posted about this lab last year, and I've since revised the lab sheets. This year I used some food coloring for the different corn syrup solutions to help keep straight which dilution was which, then collected the eggs in a jar to represent a collection of cells and demonstrate the idea that the cells in our bodies are bathed in fluid into & out of which materials flow. I should have added some tubing to represent the capillaries, but I thought of it after the fact and adding the tubes would likely have burst the fragile eggs, creating a horrible mess.

I like to do this lab even though it replicates some of the concepts in the Diffusion Through a Membrane required lab. The eggs are of course a good size to work with and I'm always partial to labs where we can measure the results and graph the data. They also respond pretty consistently and dramatically to the different soultions, making them virtually foolproof if the kids measure correctly (and the eggs don't break - always plan extras). The occasional broken egg adds to the "ick" factor. The basic strategy is this:

Students weigh the eggs before soaking in vinegar. This is the baseline mass that we use for comparison after it soaks in the other liquids, so we are investigating how the egg deviates from that original mass (we ignore the mass of the dissolved shell since it is pretty trivial compared to the overall mass of the egg and the changes we get from osmosis.) We can estimate the original water content (%) in the egg by interpolating from the graph the point at which the change in mass would have been zero - the point where the line they graphed intersects the y x axis. The point where the line intersects the y x*axis represents the point at which the concentration of water inside the cell would be equal to the concentration of water outside the cell. Needless to say the kids get a little confused with the terminology and using a graph in this way. There are a couple of tricky mathematical components to the lab, but all age/grade appropriate - it's just the kids are used to doing math in math class and apparently not comfortable applying the skills to science class. Aside from the graphing, a few kids were uncomfortable subtracting a large initial mass from a smaller final mass (final mass - initial mass) to get a negative change. We also did not simply use the change in mass, since the eggs were all somewhat different sizes in the beginning, so we calculated the % change to control for that variable (thanks to Mathew Davies, my G-K12 fellow last year, who helped me revise the lab).

You need at least 4 days for this lab,longer if you let students soak all the eggs in the various solutions. It is possible to check the eggs at the beginning of the period and have some time left to do other things afterward. You may need to refresh the vinegar after the first day. The eggs don't seem to suffer from leaving over the weekend if you have to start in the middle/end of the week. Have lots of paper towels. I've estimated the water content of corn syrup solutions, but the numbers may be off - something about the nutrition label doesn't make sense, and I got conflicting numbers from the USDA as to the sugar/water content of corn syrup, so check those numbers for yourself if you're concerned about the accuarcy of the details. It's bothersome to me but I can live with the rough estimates.

*I meant the y=0 point, which is actually the x axis. Sorry 'bout that...