Cell structure — AQA GCSE Biology

Revision notes

9 short notes, in the order of the specification. Each one in short:

1. Eukaryotes and prokaryotes

   Eukaryotic cells, such as plant and animal cells, have a cell membrane, cytoplasm and genetic material enclosed in a nucleus. Prokaryotic cells, such as bacteria, are much smaller. Their genetic material is not enclosed in a nucleus: it is a single DNA loop, and there may be one or more small rings of DNA called plasmids.

2. Animal and plant cells

   Most animal cells have a nucleus, cytoplasm, a cell membrane, mitochondria and ribosomes. Plant cells have these too, and often also have chloroplasts, which absorb light for photosynthesis, and a permanent vacuole filled with cell sap. Plant and algal cells also have a cell wall made of cellulose, which strengthens the cell.

3. Required practical 1: using a light microscope

   Practical

   Required practical 1 uses a light microscope to observe, draw and label plant and animal cells. Mount thin tissue on a slide with water or a stain and a cover slip, focus on the lowest-power objective with the coarse focusing knob, then sharpen with the fine knob. Total magnification is the eyepiece lens magnification multiplied by the objective lens magnification.

4. Cell specialisation

   A specialised cell has structures that suit it to a particular function. A sperm cell has a tail and many mitochondria to release energy for swimming. A nerve cell is long, with branched connections, to carry impulses. A root hair cell has a long hair-like projection, giving a large surface area to absorb water and mineral ions.

5. Cell differentiation

   Differentiation is the process by which a cell acquires different sub-cellular structures so that it can carry out a certain function, becoming a specialised cell. Most types of animal cell differentiate at an early stage, but many types of plant cell keep the ability to differentiate throughout life. In mature animals, cell division is mainly for repair and replacement.

6. Microscopy

   An electron microscope uses electrons instead of light, so it has much higher magnification and resolving power than a light microscope. This lets biologists see much finer detail, such as ribosomes and the internal structure of mitochondria and chloroplasts. Magnification is calculated with magnification = size of image ÷ size of real object, using the same units for both.

7. Culturing microorganisms

   Triple only

   Bacteria multiply by binary fission, a simple cell division, as often as once every 20 minutes if they have enough nutrients and a suitable temperature. They are grown in nutrient broth or on agar gel plates using aseptic technique: sterilise the dishes, media and inoculating loop, tape the lid, store the dish upside down and incubate at 25 °C.

8. Culture calculations

   Triple only

   To find the number of bacteria after a certain time, divide the total time by the mean division time to get the number of divisions. Then multiply the starting number by 2 to the power of the number of divisions. The cross-sectional area of a colony or clear zone is πr², where the radius is half the diameter.

9. Required practical 2: antiseptics and antibiotics

   PracticalTriple only

   Required practical 2 tests the effect of antiseptics or antibiotics on bacterial growth. Paper discs soaked in each substance are placed on a bacterial lawn on an agar plate, with a sterile water control, and incubated at 25 °C. The larger the clear zone of inhibition around a disc, the more effective the substance is against that bacterium.