CB1.1 — Cells, structure and microscopy (Edexcel 1SC0)
Edexcel 1SC0 opens biology with cell biology because every downstream topic — from osmosis to DNA to ecosystems — depends on understanding what cells are and how we observe them.
Eukaryotic vs prokaryotic
Eukaryotic cells have a membrane-bound nucleus and complex organelles. Animals, plants, fungi and protists are eukaryotes. Prokaryotic cells (bacteria) have no nucleus — DNA sits as a single circular chromosome in the cytoplasm, with plasmids as optional extra rings of DNA.
| Feature | Animal cell | Plant cell | Bacterial cell |
|---|---|---|---|
| Nucleus | Yes | Yes | No (nucleoid) |
| Cell wall | No | Yes (cellulose) | Yes (peptidoglycan) |
| Chloroplasts | No | Yes | No |
| Vacuole | Small/absent | Large permanent | No |
| Plasmids | No | No | Often |
Sub-cellular structures and functions
- Nucleus — contains DNA; controls cell activities
- Cell membrane — controls entry/exit of substances
- Cytoplasm — site of most chemical reactions
- Mitochondria — aerobic respiration; ATP production
- Ribosomes — protein synthesis
- Chloroplasts (plants) — photosynthesis; contain chlorophyll
- Cell wall (plants/bacteria) — provides support and shape
Specialised cells
| Cell | Key adaptation | Function |
|---|---|---|
| Red blood cell | Biconcave; no nucleus; haemoglobin | Carry oxygen |
| Sperm | Long tail; many mitochondria; acrosome | Fertilise egg |
| Root hair | Long extension; large surface area | Absorb water/ions |
| Ciliated epithelial | Cilia on surface | Move mucus up airway |
| Nerve cell | Long axon; branched dendrites | Transmit impulses |
Microscopy
Light microscope: max ×2,000; resolution ~200 nm; can see cells and nuclei. Electron microscope: max ×2,000,000; resolution ~0.1 nm; can see ribosomes, mitochondria structure.
Magnification equation: $$\text{magnification} = \frac{\text{image size}}{\text{actual size}}$$
If image = 40 mm, actual = 0.05 mm → magnification = 40 ÷ 0.05 = ×800.
Required practical: staining onion epidermis cells with iodine; making a temporary slide with coverslip lowered at an angle to avoid air bubbles.
Common Edexcel exam errors
- Saying bacteria have no DNA — they do, just no nucleus.
- Confusing chloroplasts (photosynthesis) with mitochondria (respiration).
- Not converting units before calculating magnification.
AI-generated · claude-opus-4-7 · v3-edexcel-combined-science