Cell structure and microscopy
Biology · GCSE · The School
Three kinds of cell
An ANIMAL cell has a nucleus, cytoplasm, cell membrane, mitochondria and ribosomes. A PLANT cell has all of those plus a cellulose cell wall, a permanent vacuole and usually chloroplasts. A BACTERIAL cell is prokaryotic: no nucleus at all, its DNA loose in the cytoplasm as a single loop, often with small rings called plasmids. That last difference is the one worth most marks.
The magnification equation
Magnification equals image size divided by actual size, and the two sizes must be in the SAME unit before you divide. Most lost marks here are unit conversions, not the equation. One millimetre is 1,000 micrometres, and one micrometre is 1,000 nanometres. Write the conversion down as a separate line rather than doing it in your head.
Why resolution is the real limit
Resolution is the smallest distance between two points that can still be seen as two points. A light microscope resolves to about 200 nm and magnifies usefully to around 1,500 times; an electron microscope resolves to about 0.1 nm. That is why sub-cellular structures were unknown until electron microscopy, and it is a favourite exam question about how technology changes what science can claim.
A cell appears 50 mm across in a photograph taken at 2,000 times magnification. ACTUAL SIZE = image ÷ magnification = 50 ÷ 2,000 = 0.025 mm. Now convert: 0.025 mm × 1,000 = 25 μm, which is a believable size for a plant cell, so the answer passes a sanity check. NOW BACKWARDS, which is the harder version: a cell is 8 μm wide and you want the image at 1,500 times. Convert first — 8 μm is 0.008 mm — then multiply: 0.008 × 1,500 = 12 mm. Notice both directions needed the units matched BEFORE the arithmetic, and that is where nearly every mark is lost.