I used this lab BEFORE I taught any vocabulary such as osmosis, equilibrium, hypertonic, hypotonic, or isotonic. I wanted students to visually see what happens to cells in fresh water vs. salt water before I threw any vocabulary at them. Students were really excited to see the cells change within a matter of 60 seconds. Here are a few tips when doing this lab for the first time:
1. You cannot use the dry layers of the onion skin. You need to use the very top of the purple fleshy layer. It can be a little bit tricky to get a specimen that is thin enough, so I decided to do it myself and hand each kid a piece. I took metal tweezers, gently pushed them under the purple layer, and slid the tweezers out so a small flap of onion skin was loose. I peeled it off, handed it to each kid, and they set up their own wet mounts. No dye needed!
3. Tell students to wait at least 2 minutes before drawing the salt water image, because sometimes it takes a little time for the cytoplasm to shrivel up. Below are images of the onion cells in fresh water (left) and salt water (right) on 100x magnification. We had a discussion on whether or not the cell wall shriveled as well. Students automatically said yes, because the cell wall is almost transparent and harder to see. Once I told them to switch to high power (400x) they were able to see the cell wall more clearly and realize that the cell walls were still intact, while the membrane and cytoplasm shriveled.
I hope your students enjoy the lab as much as mine did! Other than having my classroom smell like onion for a day, it was a total win! Want more ideas? Check out this blog post that has 10 resources for teaching cell membranes!