IL6 - The Onion Cell Lab

Background: Onion tissue provides excellent cells to study under the microscope. The main cell structures are easy to see when viewed with the microscope at medium power. For example, you will observe a large circular nucleus in each cell, which contains the genetic material for the cell. In each nucleus, are round bodies called nucleoli. The nucleolus is an organelle, which synthesizes small bodies called ribosomes. Ribosomes are so small, you cannot see them with the light microscope. Also present in the onion cell, is a well developed cell wall and a cell membrane just beneath it.

Purpose: To study the structure of the onion epidermal cell, with particular emphasis on the nucleus and nucleoli.

Materials: The following materials are required; onion, microscope, glass slide, coverslip, iodine stain (Note: iodine is toxic and will stain - handle with care).


    1. Obtain a section of a scale leaf from an onion bulb.
    2. Remove the single layer of epidermal cells from the inner (concave) side of the scale leaf.
    3. Place the single layer of onion cell epithelium on a glass slide. Make sure that you do not fold it over or wrinkle it.
    4. Place a drop of iodine stain on your onion tissue.
    5. Put the coverslip on the stained tissue and gently tap out any air bubbles.
    6. Observe the cells under 40´ . Now observe the cells under 100´ with the diaphragm wide open. Slowly reduce the light intensity by closing the diaphragm, and observe the image. Which light intensity revealed the greatest cellular detail?_____________
    7. In the space provide below, draw a group of 10 neighbouring cells at 100´ . In one cell, label all the parts you see.
    8. Switch to high power at 400´ . Can you see a whole cell? If you can, draw one cell and label it. If no, go back to 100´ and draw one cell and label it.











Onion cells at 100´













Onion cell at 400´ or 100´


  1. What is the function of the nucleus?
  2. Where is the nucleolus found and what does it produce?
  3. Describe what ribosomes do in the cell?