Required practical 1 · Biology · Biology Paper 1, both tiers

Microscopy

Use a light microscope to observe, draw and label plant and animal cells, with a magnification scale.

The exam trap. Magnification is not resolution: a bigger image does not show more detail. Convert mm to µm (× 1000) before dividing.

Variables

Independent (you change): The specimen observed (onion epidermis or cheek cells)
Dependent (you measure): The cell structures seen and their measured image size
Control (you keep the same): The same microscope and light setting for each specimen
The same stain for each specimen of one type
Starting at the lowest-power objective each time

Peel a thin layer of epidermis from the inside of an onion scale and lay it flat on a slide.
Why Light must pass through the specimen, so it has to be one cell thick.
Add a drop of iodine solution to the onion (or methylene blue to cheek cells).
Why Stains make the colourless structures, such as the nucleus, visible.
Lower a cover slip at an angle with a mounted needle.
Why Lowering at an angle pushes air out, so bubbles do not hide the cells.
Clip the slide on the stage and start with the lowest-power objective. Focus with the coarse focus, then the fine focus.
Why Low power has the widest field of view, so the cells are easiest to find.
Switch to a higher-power objective and refocus with the fine focus only.
Why The coarse focus at high power can crash the lens into the slide.
Draw a few cells with a sharp pencil: clear continuous lines, no shading, labels with ruled lines, and the magnification.
Why An accurate, labelled drawing with a scale is what the exam credits.

Hazard	How to reduce the risk
Iodine solution is an irritant and stains skin.	Wear eye protection and wash off any spills.
A scalpel is sharp.	Cut on a tile, away from your body.
Glass slides and cover slips break easily.	Handle by the edges and report breakages.

Specimen	Structures seen	Image length in mm	Magnification	Real length in µm
Onion epidermis	cell wall, nucleus, cytoplasm, membrane	60	×400	150
Cheek cell	nucleus, cytoplasm, membrane	24	×400	60

Magnification = image size ÷ real size. Put both lengths in the same unit first (1 mm = 1000 µm).
Onion cells show a cell wall but no chloroplasts, because the bulb grows underground; cheek cells have no wall.
A light microscope cannot resolve ribosomes: you need the higher resolution of an electron microscope.

Confusing magnification with resolution: making the image bigger does not show more detail.
Instead Resolution is the ability to distinguish two close points; only a better microscope improves it.
Forgetting to convert units in the magnification calculation, giving an answer 1000 times too small.
Instead Write both lengths in µm (or both in mm) before dividing.
Shading drawings and drawing structures you did not see.
Instead Draw only what is visible, with single clean lines and ruled label lines.