The use of light and electron microscopy to study cells, the difference between magnification and resolution, the magnification equation, rearranging it to find real size or image size, and using standard form and SI units (mm, micrometre, nm) for cell sizes.

What this dot point is asking

OCR wants you to describe how light and electron microscopes are used to study cells, explain the difference between magnification and resolution, use and rearrange the magnification equation, and handle cell sizes written in standard form with the correct SI units.

Light and electron microscopy

Cells were first seen with the light microscope, which passes light through a thin specimen and uses glass lenses to magnify the image. To make the structures visible, the specimen is usually stained with a dye that binds to particular parts of the cell. A light microscope magnifies up to about 2000 times and can be used to view living cells, which is why it is still used today.

The electron microscope uses a beam of electrons instead of light. It has both much higher magnification and much higher resolution, so it reveals fine detail such as the internal structure of mitochondria and individual ribosomes. The trade-off is that specimens must be specially prepared and placed in a vacuum, so they cannot usually be alive.

The magnification equation

A common memory aid is a formula triangle with image size on top, and magnification and real size on the bottom. Cover the quantity you want, and the other two show you whether to multiply or divide. Whatever method you use, the marks are split between the rearrangement and the unit conversion, so show both.

Working with cell sizes and standard form

OCR rewards confident use of standard form and SI prefixes. A length such as $0.0025$ m is clearer written as $2.5 \times 10^{-3}$ m. Practise converting between metres, millimetres, micrometres and nanometres, because microscope questions mix the units deliberately to test whether you convert correctly.

A useful order of magnitude to memorise: most animal and plant cells are tens of micrometres across, a bacterium is around a micrometre, and the largest structure you study under a light microscope (such as a whole onion epidermis cell) is a fraction of a millimetre. When you calculate a real size, always check it is sensible against these numbers.

The microscopy practical (PAG B1)

In the required microscopy practical you prepare a slide (for example onion epidermis cells stained with iodine), focus it with the low-power objective first and then a higher power, and produce a labelled biological drawing. You may be asked to use a graticule and a stage micrometer to measure real sizes. Exam questions test the method (clean slide, lower the objective carefully to avoid cracking the slide, focus upwards), the safety points, and the magnification calculation.