Cambridge International A-Level Biology

## Units of length Cells and their contents are measured in three units, each a thousand times smaller than the one before. | Unit | Symbol | Size | |---|---|---| | millimetre | mm | $10^{-3}$ m | | micrometre | µm | $10^{-6}$ m, so 1 mm = 1000 µm | | nanometre | nm | $10^{-9}$ m, so 1 µm = 1000 nm | A typical animal cell is tens of micrometres across, a bacterium is one to five micrometres, a ribosome is around 25 nanometres and a virus is tens to hundreds of nanometres. Choosing the unit that gives a number between about 1 and 1000 keeps calculations manageable, and converting between units is the commonest place to lose a factor of a thousand. ## Magnification and resolution **Magnification** is how many times larger an image is than the real object. **Resolution** is the smallest distance between two points at which they can still be seen as separate. A microscope that magnifies without resolving more detail only makes a blurred image larger, which is why the two ideas are kept apart: magnification can be increased without limit on paper, but resolution is set by the wavelength of the radiation used. A light microscope uses visible light, whose wavelengths (roughly 400 to 700 nm) limit resolution to about 200 nm (0.2 µm). Two structures closer together than that merge into one blob, however high the magnification. Useful magnification therefore stops at about ×1500. Electron microscopes use a beam of electrons, whose wavelength is far shorter, so they resolve structures…