How to prepare an onion cell slide

This activity is one of the first uses of an optical microscope that most school children encounter in a school biology lab. Onions are used because they have large cells that are easily visible under a student microscope. This experiment is normally performed by children between the ages of 11 and 14.

An onion is made of many concentric layers. Each layer is separated by a thin skin or membrane. In this experiment you will make a slide and look at the cells of the membrane under a microscope.

Method

1. Take a small piece of onion and using forceps (tweezers) peel off the membrane from the underside (the rough side).
2. Lay the membrane flat on the surface of a clean glass slide, and then add one drop of dye (iodine or methylene blue). Be very careful; these dyes will stain your skin and clothes.
3. Using a pin, lower a thin glass cover slip onto the slide. Make sure there are no air bubbles.
4. Put the slide onto the stage of the microscope. Make sure the objective lens is set on low power, and the microscope light is turned on.
5. Look through the eyepiece lens and turn the focusing wheel until you can see the cells. They should look like lizard skin.
6. Swap the objective lens for a high powered one so that you can see the cells at a greater magnification. You should be able to make out the nucleus.

• Onion
• School Biology experiments and demonstrations
• Microscope
• How-to
• Slide
• Biological cell