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Cell theory says: all living things are made of one or more cells, the cell is the basic unit of life, and new cells come only from existing cells. The word 'cell' was first used by Robert Hooke in 1665, for the little boxes he saw in cork.

Cells are measured in micrometres (µm): one micrometre is a thousandth of a millimetre. A human red blood cell is about 8 µm across, so about 250 fit across a pinhead.

All cells share four parts: a membrane that controls what goes in and out, a jelly-like cytoplasm, DNA with the instructions, and ribosomes that build proteins.

Experiment: Onion skin under the microscope

1. An adult cuts the onion and separates one of the thick layers.
2. With tweezers, peel off the very thin, see-through skin from the inside of the layer.
3. Lay it flat on a slide with one drop of water. Lower a coverslip slowly from one side to avoid air bubbles.
4. Make a second slide in the same way but use water with blue food colouring. Everything else stays the same: that keeps it a fair test.
5. Look at both slides with the lowest power first, then a higher power. Draw what you see. Which slide shows the cells more clearly?

	Plain water	Blue food colouring
Shape of the cells	Which slide will show the cells best? Why?
Can I see the walls?	Which slide will show the cells best? Why?
Can I see a nucleus?	Which slide will show the cells best? Why?

Questions

1. Write the three ideas of cell theory.

2. Draw an onion cell as you saw it under the microscope. Label the cell wall and the nucleus.

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3. Name two parts that plant cells have but animal cells do not, and say what each does.

4. Why did we keep everything the same on both slides except the colouring?

Sources for every fact on this sheet: echoesof.science/school/cells/sources/

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