The cell
The cell is the basic unit of living things. Its membrane separates the interior from the surroundings and allows exchange. Some cells live as independent organisms; others form tissues. Eukaryotic cells have a membrane-bound nucleus; prokaryotes organise their DNA without that nucleus.
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Exercises from this rung
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In a photo at 500× magnification, a cell appears 20 mm across. How big is it really, in micrometres?
Answer 40
Why Magnification = size in the image ÷ real size, with both measurements in the same unit: 20 mm is 20000 µm, and 20000 ÷ 40 = 500. The classic mistake is not converting to the same unit, which makes the answer wrong by a thousand. A micrometre is a thousandth of a millimetre.
easierA yeast can live as a single cell. What does this show?
- Cells exist only inside animals
- Every living thing needs several organs
- One cell can be a whole organism
Why A cell can exchange substances, obtain energy and reproduce. In yeast, this one unit carries out the organism's functions; in an animal, many cells specialise and cooperate.
in betweenA spherical cell doubles its radius. What happens to its surface-area-to-volume ratio?
- The ratio halves
- The ratio stays the same
- The ratio doubles
Why Surface area scales with radius squared, so it increases fourfold. Volume scales with radius cubed, so it increases eightfold. Surface area per unit volume becomes 4/8, or half as large, limiting exchange in large cells.
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