What it shows
The thin skin of an onion layer is a sheet of box-like plant cells. A stained slide under the microscope shows the cell wall and the nucleus, and a simple trick lets you estimate how big each cell is.
The science
Plants are made of cells. Each plant cell has a firm cell wall of cellulose outside the thin cell membrane, a nucleus that controls the cell, cytoplasm and usually a large vacuole. The cells of the onion skin are close together with no gaps, like bricks in a wall. Iodine stains the nucleus brown or yellow, so it becomes easier to see. Onion bulb cells have no chloroplasts, because they lie under the soil and are not exposed to light.
Materials
Cost: under Rs 50. Many items come from home or from waste; spare parts such as LEDs, motors and wire are in the store.
- An onion
- A school compound microscope
- Glass slides and cover slips
- Tweezers, a dropper and a blotting paper
- Dilute iodine solution (a few drops of tincture of iodine in a cup of water) or a drop of red ink in water
- A transparent ruler marked in millimetres
- A pencil and a notebook
Safety
For this project: An adult must cut the onion, prepare the dilute iodine and supervise. Iodine stains skin and clothes and must not be swallowed; keep it away from eyes. Glass slides break easily. Wash hands after the practical.
Never use mains electricity: power models with cells, batteries or a low-voltage adapter. Sharp and hot tools, flames and lab chemicals need an adult or your teacher beside you. Keep magnets, button cells and small parts away from young children, and wash your hands after making.
All the safety rulesStep by step
8 steps. Do them in order, and test each part before you go on to the next. Write what you notice in a notebook as you go.
Ask an adult to cut an onion into quarters and take out one fleshy scale leaf.
Bend the leaf a little and use tweezers to peel off a thin, clear skin from the inner side, about 1 cm square.
Place the skin flat in a drop of water on a slide and avoid folds.
Add one drop of dilute iodine, wait one minute, then lower a cover slip on one side slowly to avoid air bubbles.
Blot off any extra liquid at the edges with blotting paper.
Look first through the low power lens, then through the high power, and find the rectangular cells, the cell walls and one round nucleus in each.
Count the number of cells that fit across the field of view and measure the field’s width under low power by laying a transparent ruler on the stage and reading it.
Divide the field width by the number of cells across to estimate the length of one cell, and draw two or three cells with labels.
What you should see
You see rows of rectangular cells with clear walls, each with one stained nucleus, and your estimate puts a cell at roughly 0.2 to 0.5 mm long. The labelled drawing shows the cell wall, cytoplasm and nucleus.
Viva questions
A teacher or a judge will ask how it works. Try to answer each question before you open it, then compare.
1What is the outer boundary of a plant cell?
The cell wall, with the cell membrane just inside it.
2What does the nucleus do?
It controls the activities of the cell and holds the genetic material.
3Why does the onion skin have no chloroplasts?
The bulb grows underground without light, so it does not make food by photosynthesis.
4Why add iodine?
It stains the nucleus so that it is easy to see.
Take it further
Compare onion cells with a thin leaf peel from a plant such as Rhoeo and note the difference, such as green chloroplasts.
Change one thing, measure what happens, and add a table or chart of your readings: that is what turns a model into your own investigation.
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