Eukaryotic and prokaryotic cells, the function of sub-cellular structures (nucleus, mitochondria, ribosomes, chloroplasts, cell wall, vacuole, plasmids), light and electron microscopy, the magnification equation, and the use of standard form and SI units for cell sizes.

What this topic is asking

OCR wants you to describe the structures in eukaryotic and prokaryotic cells, state the function of each sub-cellular structure, explain the difference between light and electron microscopy, use the magnification equation, and handle cell sizes written in standard form with SI units.

Eukaryotic and prokaryotic cells

Animal cells contain a nucleus (controls the cell's activities and holds the DNA), cytoplasm (where most chemical reactions happen), a cell membrane (controls what moves in and out), mitochondria (the site of aerobic respiration, which releases energy) and ribosomes (where protein synthesis happens). Plant and algal cells have all of these plus a cellulose cell wall for strength and support, chloroplasts containing chlorophyll that absorb light for photosynthesis, and a permanent vacuole filled with cell sap that helps keep the cell firm (turgid). A bacterial cell is far smaller (about $1$ micrometre, written $1 \times 10^{-6}$ m) than a typical animal or plant cell ($10$ to $100$ micrometres). Instead of a nucleus, its single loop of DNA floats in the cytoplasm, often with one or more plasmids that can carry extra genes such as antibiotic resistance.

Microscopy and resolution

Cells were first seen with the light microscope, which passes light through a thin specimen and uses glass lenses to magnify it up to about 2000 times. Its limit is resolution, the ability to distinguish two points that are close together: a light microscope cannot resolve the smallest organelles. The electron microscope uses a beam of electrons instead of light and has both much higher magnification and much higher resolving power, 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 cannot usually be alive, so light microscopes are still used to view living cells.

Working with cell sizes

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