Onion Osmosis

In our first microscope lab, Casia was given unknown samples of hair from family members and had to use the microscope to compare them to known samples and attempt to identify them. She learned the parts of the microscope, how to place a slide on the stage and how to adjust the magnification and focus. In the end, she wasn't able to correctly identify more than one sample, but she had fun and learned to use her microscope.

In this lab, Casia got a chance to do an experiment under the microscope. I was looking for an interesting way to demonstrate osmosis and ran across this: Onion Osmosis Lab. 

I started by peeling a thin layer of onion and placing it on the slide. I then stained the onion with tincture of iodine (CAUTION- this makes a big mess if you spill it, which I learned the hard way in this lab...) and placed a cover over the sample. Casia then found a nice section of the onion that showed an entire cell. It was very exciting for her because after having studied cell structures, she was able to identify many of it's parts. We are fortunate enough to have a microscope with a camera attached, but I still made Casia draw a picture of what she saw. Below, I've included the pictures from her microscope.

Initial onion cells under the microscope at 10x magnification.

In the next step Casia placed a small piece of paper towel on one side of the slide cover. On the opposite side of the slide cover I dropped salt water, using a medicine dropper. I'd never used this technique before, but it was really neat to see the paper towel absorb the iodine and the salt water be drawn in to replace it.  Unfortunately, we didn't see a change in the cells even after a few minutes. I thought it might have been because I didn't have enough fluid under the slide, so I removed the slide cover and dropped more salt water directly onto the onion. This is what we saw: the cell membranes shrunk in response to losing water from osmosis. 

Onion cell after adding salt water.

Overall, I think it was a pretty easy lab to execute (minus the iodine mishap) and recommend it. Casia has gotten very interested in cell biology, so I'm sure we'll have more exciting microscope labs in the near future.