Cell biology

Every living thing is built from cells, and the differences between them decide what each can do. This reference covers AQA GCSE Biology 8461, topic 4.1 Cell biology.

How the exam treats this topic:

• Paper 1 (4.1–4.4) carries Cell biology. Required practicals: RP1 (light microscope), RP2 (antiseptics/antibiotics on agar, biology only) and RP3 (osmosis in plant tissue).
• Culturing microorganisms is biology only; standard-form answers for bacterial numbers are HT only.
• Calculations the exam demands: magnification (image ÷ real), real size from image and magnification, area by πr², percentage change in mass, surface-area-to-volume ratio, bacterial division counts.

• Eukaryotic 真核 cells (plant and animal): genetic material enclosed in a nucleus.
• Prokaryotic 原核 cells (bacteria) are much smaller; cytoplasm and cell membrane surrounded by a cell wall; genetic material is not in a nucleus — a single DNA loop plus one or more plasmids (small rings of DNA).

Scale: use the prefixes centi (10⁻²), milli (10⁻³), micro (10⁻⁶), nano (10⁻⁹) and order-of-magnitude comparisons in standard form.

Specialised cells relate structure to function (learn one feature ↔ one advantage each):

Differentiation 分化: cells acquire different sub-cellular structures to become specialised. Animal cells differentiate mostly at an early stage; plant cells throughout life. In mature animals, cell division is mainly for repair and replacement.

Microscopy: a light microscope uses lenses; an electron microscope has much higher magnification and resolving power 分辨能力, so far more sub-cellular structures could be seen and understood.

Worked example. A cell image is 4.8 cm wide at magnification ×1200.

• Convert first: $4.8\ \text{cm} = 4.8\times10^{-2}$ m $= 48\,000\ \mu\text{m}$.
  $$\text{real size} = \frac{\text{image size}}{\text{magnification}} = \frac{48\,000\ \mu\text{m}}{1200} = 40\ \mu\text{m}$$

RP1: observe, draw and label plant and animal cells with a magnification scale included (e.g. a scale bar or the printed magnification).

Bacteria multiply by binary fission 二分裂 as often as once every 20 minutes with enough nutrients and a suitable temperature. They grow in nutrient broth or as colonies on agar gel.

Aseptic technique 无菌操作 (each point earns a mark — know the why):

• sterilise Petri dishes and culture media before use — kills unwanted microorganisms;
• flame the inoculating loop before transferring bacteria — kills contaminants on the loop;
• secure the lid with adhesive tape and store upside down — stops airborne contamination and condensation dripping;
• in schools, incubate at a maximum of 25 °C — prevents growth of pathogens harmful to humans.

Worked example (division count). One bacterium divides every 20 minutes for 3 hours.

• 3 hours = 180 min = 9 division times; number $= 2^9 = 512$.

Worked example (zone area). A clear zone of radius 4 mm around an antibiotic disc.

RP2: effect of antiseptics/antibiotics on bacterial growth — measure zones of inhibition with a ruler (several diameters, mean, ÷2), controls with no substance.

• The nucleus contains chromosomes 染色体 made of DNA; each chromosome carries many genes; body cells have chromosomes in pairs (humans: 23 pairs).
• The cell cycle: DNA replicates → growth (more ribosomes and mitochondria) → mitosis 有丝分裂 (one set of chromosomes pulled to each end; nucleus divides) → cytoplasm and membranes divide → two identical cells.

Mitosis matters for growth and development of multicellular organisms, and repair. Recognise contexts where mitosis is occurring (a growing root tip, healing skin, a tumour).

A stem cell 干细胞 is an undifferentiated cell that can give rise to many more of the same type and, by differentiation, to other cell types.

Uses and evaluation (the credited pairs — benefit and risk/objection):

• Medicine: potential treatment for diabetes and paralysis; therapeutic cloning makes an embryo with the patient's own genes, so cells are not rejected.
• Risks/ethics: transfer of viral infection; ethical and religious objections to using embryos.
• Plants: meristem clones — rare species preserved from extinction; disease-resistant crops cloned quickly and economically.

Rate of diffusion increases with: concentration gradient, temperature, surface area of the membrane.

Surface area to volume ratio: single-celled organisms have a large SA:V — enough exchange across the surface alone. Multicellular organisms need exchange surfaces and a transport system. Exchange surfaces are effective by: large surface area, thin membrane (short diffusion path), (animals) good blood supply, (animals, gas exchange) ventilation.

Adapted exchange surfaces to know: small intestine and lungs in mammals, gills in fish, roots and leaves in plants.

Worked example (SA:V). A cube of side 2 mm: SA $= 6\times2^2 = 24$ mm², V $= 8$ mm³, SA:V $= 3:1$. A cube of side 4 mm: SA $= 96$, V $= 64$, SA:V $= 1.5:1$ — doubling the side halves the ratio.

RP3 (osmosis): potato pieces (same surface area, blotted dry, exact masses) in a range of sugar or salt solutions; measure percentage change in mass:

Interpretation: gain in dilute solutions (water enters); no change where the solution matches the cell concentration; loss in concentrated solutions (water leaves). Control variables: volume of solution, temperature, time, surface area.

• Label animal, plant and bacterial cells; match every structure to its function.
• Specialised cells: one structural feature linked to one function each.
• Magnification, real size and image size — convert units before dividing.
• Aseptic technique whys; binary-fission counts; πr² zones (RP1, RP2).
• Cell cycle stages and where mitosis occurs; stem cell sources with benefits and risks.
• Diffusion, osmosis, active transport: direction, energy, examples; the three rate factors.
• SA:V calculations; four exchange-surface features; percentage change in mass (RP3).