Biology, Answering the Big Questions of Life/Osmosis

--Red Onion Osmosis Lab -- Make a wet mount of the onion leaf. Use a small piece of paper towel to pull a salt solution through the mount by placing a drop of salt water on one side of the coverglass, and the paper towel on the other.

Plasmolysis in red onion cells

Using red onion really helps in this lab because the cells are already dyed. The problem is that you cannot use the thin membrane between the onion layers to perform this experiment. You must peel the top layer off of the onion off to do this lab. For this reason, It may be prudent for the instructor to use a razor blade and prepare the samples in advance.

Smaller samples will make better pictures. Closing the diaphram on the condensor will prevent the sample from appearing washed out under the microscope.

Students should focus on only the colored layer of cells. For best results focus on the cells on the edge of the sample as the salt will affect them first.

We prepared a 20% by weight salt solution and observed results within 5 minutes.

Alternatives

This laboratory can be done with other plant cells, but cells with many chloroplasts, such as Elodea are the best.

Onion skin slides can be made and dyed with iodine which makes the cells slightly yellow. (Note) Iodine fixes (kills) the cells. To make onion skin slides, peel a piece of onion. The skin is a thin area on the bottom of an onion leaf that can be peeled off and laid between a slide and a coverglass in a wet mount.

A demonstration can be done with blood cells, but remember that in this era fear of AIDS means that all blood samples must be treated with extreme caution. See your administrator before doing any laboratory using blood.