Cell structure

Every living thing is made of cells, and every cell is one of two types. The difference that matters is where the genetic material is kept.

In a eukaryotic cell — animal, plant or fungal — the DNA is enclosed inside a nucleus. In a prokaryotic cell — a bacterium — there is no nucleus. The genetic material is a single loop of DNA sitting free in the cytoplasm, and there may also be one or more small rings of DNA called plasmids. A prokaryotic cell is much smaller, and it has a cell wall surrounding its cell membrane.

You are also expected to handle the scale of cells, and to make order of magnitude calculations — comparisons rounded to the nearest power of ten. A typical animal cell is about 10–30 μm across; a bacterium is about 1–5 μm, so an animal cell is one order of magnitude — ten times — wider.

Sizes this small are written in standard form, and you are expected to use the prefixes centi, milli, micro and nano: 1 m = 100 centimetres (cm), 1 cm = 10 millimetres (mm), 1 mm = 1000 micrometres (μm) and 1 μm = 1000 nanometres (nm).

Learn each sub-cellular structure together with its job. Questions here nearly always ask what a structure does, or why a particular cell has a lot of one of them.

Most animal cells have a nucleus, cytoplasm, a cell membrane, mitochondria and ribosomes. Plant and algal cells have all of those, and in addition often have chloroplasts and a permanent vacuole filled with cell sap. Plant and algal cells also have a cell wall made of cellulose, which strengthens the cell.

The specification also asks for the function of plasmids in bacterial cells. A plasmid is a small ring of DNA separate from the main loop, carrying extra genes — genes for antibiotic resistance, for example — which bacteria can pass to one another.

You are not always given exact measurements, and some questions ask you to judge a size or an area rather than calculate one. Estimate when no measurement is available, when the numbers are awkward and a rough answer settles the comparison anyway, or when all you need is the order of magnitude. Round to one significant figure, do the arithmetic, and say that the answer is an estimate.

Watch the difference between length and area: if one structure is a fifth as wide as another, it covers about a twenty-fifth of the area, because area scales with the square of the width.

A specialised cell has a structure that suits one particular job in a tissue, an organ, an organ system or the whole organism. Marks come from linking the two: name the feature, then say what it allows the cell to do.

As an organism develops, its cells differentiate to form different types of cell. As a cell differentiates it acquires different sub-cellular structures that let it carry out a particular function — it has become a specialised cell.

Most types of animal cell differentiate at an early stage, and in a mature animal cell division is mainly restricted to repair and replacement. Many types of plant cell keep the ability to differentiate throughout life, which is why a cutting taken from a plant can grow into a whole new plant.

Two things separate one microscope from another: magnification and resolution. An electron microscope has much higher magnification and much higher resolving power than a light microscope, so it can be used to study cells in far finer detail. That is how biologists came to see and understand many more sub-cellular structures — a light microscope simply cannot resolve them.

Microscopy techniques have developed over time, and what biologists could see developed with them. Early light microscopes in the seventeenth century were good enough to show that tissue is built from cells at all; better lenses and stains through the nineteenth century sharpened that picture. But a light microscope is limited by the wavelength of light — it cannot resolve anything closer together than about 0.2 μm — so sub-cellular detail stayed out of reach. The electron microscope, developed in the 1930s, uses a beam of electrons instead, and its far higher magnification and resolving power increased understanding of sub-cellular structures enormously: ribosomes and the internal structure of mitochondria were seen for the first time. You are not asked for dates — only for why the newer technique shows more, which comes down to magnification and resolution.

Bacteria multiply by simple cell division, called binary fission, as often as once every 20 minutes if they have enough nutrients and a suitable temperature. They can be grown in a nutrient broth solution, or as colonies on an agar gel plate.

To investigate the action of disinfectants and antibiotics the culture must be uncontaminated, so it is prepared using aseptic technique.

You need to be able to explain each precaution, not just list it. Petri dishes and culture media are sterilised before use, to kill any unwanted microorganisms already on them. Inoculating loops are sterilised by passing them through a flame before microorganisms are transferred. The lid is held on with adhesive tape rather than sealed all the way round, so contaminants cannot get in but harmful anaerobic bacteria cannot grow either. The dish is stored upside down so that condensation cannot drip onto the agar. And in school laboratories cultures are incubated at 25 °C, because warmer temperatures encourage the growth of pathogens that are harmful to humans.