Resource 3: Working with onion cells

Preparing a slide of an onion cell and measuring a cell

You will need:

Microscope

Scissors

Microscope slide

Dropper pipette

Cover slip

Clear plastic ruler

Dilute iodine solution.

Preparing the onion slide

What to do:

1. Slice an onion in two, lengthwise.
2. Remove one of the thick leaf-like structures from inside.
3. Pull away a piece of the thin papery lining of its inner surface.
4. Using scissors, cut a small square of this lining, about 5 mm x 5 mm.
5. Place this square on the centre of a slide.
6. Add a drop of dilute iodine solution – make sure the solution spreads below as well as above the square of onion skin. The iodine acts as a stain to make the structures in the cell easier to see.
7. Carefully lower a cover slip over the onion skin. Try to avoid trapping air bubbles.
8. Place the slide on the microscope stage. Examine first using the low power. Focus carefully.
9. Choose an area of the slide where the cells can be clearly seen. Switch to high power and refocus.
10. Look for the structures shown in the photographs in Resource 1.

Measuring the onion cell

What to do:

1. Place the ruler on the microscope stage under the low power objective lens.
2. Move the ruler so the edge with the scale can be focused in the centre of the field of view of the microscope, as in Diagram 1 below.

Diagram 1: The field of view of a microscope.

3. Use the scale to measure the field of view of your microscope.
4. The diameter of the field of view in Diagram 1 is approximately 5 mm.
5. You can use the measurement of the field of view in your microscope to estimate the size of objects viewed with the same objective lens.
6. The cell viewed in Diagram 2 would be about 2 mm long if viewed with the microscope with the field of view shown above.

Diagram 2: Cell.

7. Estimate the length and width of your onion cell using this method.

Using a microscope

The main parts of a light microscope are shown below

Diagram 3: Main parts of a light microscope.