Looking at Onion Cells Under the Microscope

Looking at an onion sample under the high power objective of a compound light microscope would be an interesting microscope activity that would help the student or child examine specialized plant cells under magnification.

For this activity, the student or child would need an onion, knife, a pair of tweezers, eye dropper, water, microscope slide, slide cover, tissue and iodine, which can be bought from drugstores. In slicing the onion for a sample to look under the microscope, children are advised to ask for an adult’s supervision or help. Take a small portion of the onion (about a fourth of an inch) and remove the thin skin from inside the onion ring by using your pair of tweezers. Now, mount your sample on a blank microscope slide and place a drop of water on the slide with your eye dropper. Gently place your slide cover over the wet mount, flattening the onion sample against the slide. Images seen through the compound light microscope would be blurry if there are contours in the sample, so it would be important to have your slide samples as flattened down as possible for a high-resolution image.

Now that you have prepared a wet mount of your onion sample, examine it under both the high and low power objective of your student microscope.

Try to look at your sample more closely by staining it with tincture of iodine. You can do this by placing a drop of iodine near the slide cover and touching a piece of tissue on the side of the slide cover that is across the drop of iodine. The tissue would absorb the iodine across and under the slide cover, staining your onion sample. Add more stain until it finally shows in the tissue. Make sure that you place more tissues under you slide before doing the actual staining, and wipe off excess iodine from the microscope slide or you might stain not only your clothes, but the compound light microscope as well.

Compare how your onion sample looks now that it had been stained as opposed to when you looked at it under an ordinary wet mount.

Notice the rows of box-shaped structures that you have under the microscope lens? Those are the cells of the onion sample that you are examining. This is mostly similar as to what you would see if you examine a piece of cork under the student microscope, except that the onion cells that you are looking at right now are still alive. You might even see small bubbles inside the cells, which are the nucleus of said cells. By staining the onion sample with iodine, the nucleus, the cytoplasm and the cell walls would stand out more and it would be easier to tell the parts of the cell apart.

Looking at onion under the high-power magnification of a compound microscope would help the child or student gain a basic understanding of living cells and how they are structured.