Cell Structure (B1.1)
Cell biology underpins every other biology topic. Expect cell components, magnification calculations and specialised-cell questions in every series.
Eukaryotic vs prokaryotic
| Feature | Eukaryotic | Prokaryotic (bacteria) |
|---|---|---|
| Nucleus | ✓ membrane-bound | ✗ — DNA free in cytoplasm as circular chromosome |
| Size | 10–100 μm | 1–10 μm |
| Examples | Animal, plant, fungi | All bacteria |
Animal cell components
- Nucleus — contains DNA; controls cell activity
- Cytoplasm — jelly-like matrix; site of most metabolic reactions
- Cell membrane — selectively permeable; controls entry and exit
- Mitochondria — site of aerobic respiration; ATP production
- Ribosomes — site of protein synthesis
Plant cell — three extra structures
- Cell wall (cellulose) — rigid support and shape
- Permanent vacuole (cell sap) — maintains turgor pressure
- Chloroplasts — contain chlorophyll; site of photosynthesis
Bacterial cell
DNA is a single circular chromosome in the nucleoid (not a nucleus). Also: peptidoglycan cell wall, cell membrane, smaller ribosomes, often plasmids, sometimes flagella and pili.
Specialised cells
| Cell | Key adaptation | Function |
|---|---|---|
| Sperm | Flagellum, many mitochondria, acrosome | Fertilise egg |
| Red blood cell | Biconcave, no nucleus, packed haemoglobin | Carry O₂ |
| Root hair cell | Long thin projection | Absorb water and mineral ions |
| Neurone | Long axon, myelin sheath | Transmit electrical impulses |
| Ciliated epithelial | Cilia on surface | Sweep mucus along airway |
Differentiation and stem cells
Cells become specialised by differentiation — switching on specific genes. Stem cells retain the ability to divide and differentiate. Embryonic stem cells are pluripotent (any cell type); adult stem cells are multipotent (restricted range). Therapeutic uses: treating Parkinson's, diabetes, spinal injury. Ethical concern: destroying a potential human life.
Microscopy
Magnification formula:
magnification = image size ÷ actual size
Always use the same units (1 mm = 1 000 μm). Convert before dividing.
| Microscope | Max mag | Resolution | Can see |
|---|---|---|---|
| Light | ×2 000 | ~200 nm | Cells, nuclei, chloroplasts |
| Electron | ×2 000 000 | ~0.1 nm | Ribosomes, organelle ultrastructure |
Required practical: water drop → onion epidermis → iodine stain → coverslip at angle → low power first.
Common exam errors
- Saying bacteria have no DNA — they do, just no nucleus.
- Using "magnification" when "resolution" is meant.
- Forgetting unit conversion in magnification calculations.
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