TESSA Secondary
Resource 3
Teacher resource for planning or adapting to use with pupils
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.
TESSA Secondary
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.