Inside a cell: plant and animal cells
Cell theory, how small cells are, the parts of plant and animal cells, and bacteria. With an onion-skin microscope experiment.
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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.[1]
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.[1]
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.[2][3]
Animal and plant cells have a nucleus that stores the DNA, and mitochondria that release energy from food in a process called respiration.[3]
Plant cells also have a cell wall made of cellulose, chloroplasts for photosynthesis and a large fluid-filled vacuole that helps control the cell's water.[3]
Bacteria are living things made of just one cell. Their cells are much smaller and have no nucleus: the DNA sits in a region called the nucleoid.[2]
Try it: Onion skin under the microscope
You need
- Half an onion (cut by an adult)
- Microscope slides and coverslips
- A school light microscope
- Tweezers
- A dropper
- Water and a few drops of blue food colouring
- Paper towels
Safety
Only an adult uses the knife. Slides and coverslips are glass: handle them by the edges and tell an adult about any breakage. Never use direct sunlight as the microscope's light source. Food colouring stains: wear an apron.
What to do
- An adult cuts the onion and separates one of the thick layers.
- With tweezers, peel off the very thin, see-through skin from the inside of the layer.
- Lay it flat on a slide with one drop of water. Lower a coverslip slowly from one side to avoid air bubbles.
- 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.
- Look at both slides with the lowest power first, then a higher power. Draw what you see. Which slide shows the cells more clearly?
Play: Plant, animal or both?
Is each cell part found only in plant cells, mainly in animal cells, or in both? Pick the box.
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Quiz
Other ages: Ages 6–7 · Ages 15–16