Microscopy: onion epidermal cells
Prepare a wet mount of onion epidermis, stain it with iodine, and draw what you actually see at ×40 and ×400 — cell walls, nuclei and all.
WHAT YOU'LL LEARN
- ✓Making a wet mount without air bubbles
- ✓Focusing from low to high power without crunching the slide
- ✓Calculating magnification and drawing biological diagrams that examiners accept
ON YOUR BENCH
- □Light microscope (×4, ×10, ×40 objectives)
- □Microscope slides and cover slips
- □Fresh onion, forceps, scalpel
- □Iodine solution and dropper
- □Blotting paper, mounted needle
The protocol, step by step
The same guide is printed and waiting at your bench.
- 01
Peel the epidermis
Snap an onion scale backwards and peel off the thin transparent skin from the inner surface with forceps. You need a single layer — if it's white and opaque, it's too thick.
- 02
Mount it flat
Lay the peel flat on a drop of iodine on the slide, teasing out folds with the mounted needle. Folded tissue gives you dark unreadable stripes.
- 03
Lower the cover slip
Rest one edge of the cover slip against the drop, support it with the needle, and lower it slowly at an angle. This pushes air out ahead of it — dropping it flat traps bubbles that look confusingly like cells.
- 04
Focus low power first
Start on ×4 with the stage racked up, then focus by moving the stage down, away from the slide. Centre a good region, then switch to ×10 and refocus with the fine knob only.
- 05
High power and draw
Switch to ×40 — fine focus only. Draw 3–4 adjacent cells with sharp pencil lines, no shading, and label the cell wall, cytoplasm and nucleus. Note the total magnification (eyepiece × objective).
What you should see
- →Brick-like rectangular cells in neat rows, with the cell wall stained pale yellow and the nucleus a darker orange dot near the wall.
- →No chloroplasts — this is an inner epidermis that never sees light, and a common exam trick question.
- →At ×400 the total magnification with a ×10 eyepiece is ×400; a typical onion cell is about 0.25 mm long.
⚠ BEFORE YOU START
- •Carry the microscope with both hands — one under the base.
- •Cover slips are thin glass and snap easily; sweep any fragments with the brush provided, not fingers.
- •Iodine stains — keep it off the microscope stage.
Try it, then run it for real.
Practise the whole thing on the virtual bench, then book real lab time and do it with your own hands.