Cells: the living units — Biology, 11–13 years
All living things are made from cells, from a tiny yeast cell to the many cells in your body. Seeing what cells do gives us a way to explain life without treating it as one mysterious whole.
What a cell does
A cell is a tiny living space surrounded by a thin boundary. Inside, chemical reactions release energy, build materials and remove waste, while the boundary controls what enters and leaves. Some living things are one cell; your body is made of many cells working together.
Why scientists needed cells
Before microscopes, a leaf or a person looked like one continuous piece, so it was hard to explain how life could grow, heal or fail. Better microscopes revealed repeating tiny structures. In the 1600s, Robert Hooke named them “cells” after seeing spaces in cork; later observations showed that living cells are active, not empty boxes.
Looking at onion cells
1. Peel a very thin layer from an onion and place it on a slide. 2. Add a drop of iodine stain, which makes structures easier to see, then lower the coverslip. 3. Start with the lowest microscope power and focus. You should see brick-like cells with boundaries and darker nuclei, not a single solid sheet.
A cell is not a tiny organ
It is tempting to think that a cell is just a smaller version of a whole animal, with a tiny heart, stomach and brain. The comparison feels sensible because both cells and bodies have jobs and need energy. But most cells use different internal parts, and a cell’s nucleus is not a miniature brain; it stores genetic instructions.
Cells outside school
Doctors use knowledge of cells to recognise infections, observe cancer cells and choose treatments. A blood test can count different kinds of cells, while a culture can help identify bacteria. These applications work because disease can change how cells look, move or carry out their normal jobs.
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