Microscopy
GCSE Biology (8461) · Required practical 1 — method, variables, the marks examiners report students losing.
Use a light microscope to observe, draw and label plant and animal cells, and calculate the magnification of the image.
Apparatus
- Light (optical) microscope with low- and high-power objective lenses
- Microscope slide and coverslip
- Onion epidermis (plant cells) and/or prepared cheek cells (animal cells)
- Iodine solution or methylene blue stain
- Mounted needle, forceps and a dropping pipette
- Pencil and ruler for a labelled biological drawing
Method
- 1Peel a thin, single layer of epidermis from an onion and place it flat on a slide in a drop of water.
- 2Add a drop of iodine stain to make the nucleus and cell walls show up clearly.
- 3Use a mounted needle to lower the coverslip slowly at an angle, so no air bubbles are trapped.
- 4Clip the slide onto the stage and select the lowest-power objective lens first.
- 5Use the coarse focus to bring the cells roughly into view, then the fine focus to sharpen the image; switch to a higher-power lens to see more detail.
- 6Make a clear, labelled pencil drawing of a few cells and record the magnification used.
Results & processing
- Total magnification = eyepiece lens magnification x objective lens magnification (e.g. x10 eyepiece x x40 objective = x400).
- Magnification = size of image / size of real object — rearrange to find real size = image size / magnification.
- The drawing should show cell wall, cytoplasm, nucleus and (in plant cells) a vacuole, with a magnification or scale stated.
Where students lose marks
Trapping air bubbles under the coverslip.
Fix: Lower the coverslip slowly at an angle with a mounted needle so air is pushed out.
Starting on the highest-power lens.
Fix: Always start on the lowest power to find the cells, then increase magnification.
Confusing magnification with resolution.
Fix: Magnification is how many times bigger the image is; resolution is the ability to see two points as separate — an electron microscope has higher resolution.
Improve the method
- Use a stain to increase the contrast between structures.
- Focus on low power first to protect the slide and lens, then move up.
- Use an eyepiece graticule (calibrated against a stage micrometer) for accurate size measurements.
Try it — exam-style
A cell has an actual width of 0.06 mm. Its image under the microscope is 24 mm wide. Calculate the magnification.
A student uses a x10 eyepiece lens and a x40 objective lens. State the total magnification and explain why iodine is added to the onion cells.
Questions are written in the style of past AQA papers — never copied from them.
Drill it properly
Stuck on microscopy?
Microscopy marks are won on slide prep and the magnification maths — I drill both, and your first lesson is free.