AQA GCSE Biology · Cell Biology
Microscopy required practical
Use a light microscope to observe, draw and label plant and animal cells, with a magnification or scale bar.
You prepare a thin slide, focus up from the lowest power, and work out the real size of what you see from its image size and the magnification.
Equipment
- Light microscope
- Microscope slides and cover slips
- Onion, tweezers and a scalpel
- Iodine solution (for onion) or methylene blue (for cheek cells)
- Pipette and paper towel
- Ruler, sharp pencil and plain paper
Variables
- Independent you change
- The type of cell or the magnification used
- Dependent you measure
- What you can see: the cell’s size and structures
- Control you keep the same
- Thin layer of tissue, one cell thick
- Same stain for the same cell type
- Focus from the lowest power upwards
Method
- 1
Put a drop of water on a clean slide. Peel a thin layer of onion epidermis with tweezers and lay it flat on the drop.
- 2
Add a drop of iodine solution to stain the cells, so the nucleus and cell wall show up.
- 3
Lower a cover slip at an angle with a mounted needle so no air bubbles are trapped.
- 4
Put the slide on the stage and select the lowest-power objective lens.
- 5
Use the coarse focusing knob to move the stage up close to the lens, then focus by moving it away; sharpen with the fine focus.
- 6
Switch to a higher-power objective, refocus with the fine knob, then draw what you see with clear, unshaded lines and label it.
Worked example
Cells measured on a drawing made at ×400 total magnification (×10 eyepiece × ×40 objective).
| Cell | Image length (mm) | Magnification | Real length (µm) |
|---|---|---|---|
| Onion epidermis cell | 36 | ×400 | 90 |
| Human cheek cell | 24 | ×400 | 60 |
| Onion cell nucleus | 8 | ×400 | 20 |
Working
Total magnification = eyepiece × objective = 10 × 40 = ×400
Real size = image size ÷ magnification
Onion cell: 36 mm ÷ 400 = 0.09 mm = 90 µm (1 mm = 1000 µm)
Conclusion
The onion cell is about 90 µm long, and has a cell wall and a large vacuole; the cheek cell is smaller and rounder with no cell wall. Converting mm to µm by multiplying by 1000 makes cell sizes easy to compare.
Risks
- Scalpel is sharpCut downwards onto a white tile, away from your fingers.
- Iodine stains skin and irritates eyesWear eye protection and wipe up spills.
- Mirror can reflect direct sunlight into the eyeNever point the mirror at the sun; use the built-in lamp.
Common mistakes
- Focusing downwards on high power. The lens can crash into the slide and crack it; always move the lens away from the slide while you focus.
- Shading or sketching cell drawings. Use single, clear pencil lines with label lines that don’t cross.
- Forgetting to convert units. Image size is usually in mm but cells are given in µm, so multiply by 1000.
Better results
- Use a thin layer, one cell thick, so light passes through and cells don’t overlap.
- Add a stain so clear structures like the nucleus are visible.
- Measure several cells and work out a mean size.
Launching November 2026
7 exam questions on this practical.
Practise the method, the variables, the maths and what the results mean, then revise the Cell Biology flashcards. It opens to students soon: join the waitlist and we’ll email you once, when it does.