Cell structure, eukaryotes, prokaryotes and microscopy
Cells are the smallest units that count as alive. The first big idea in GCSE biology is that all cells share some features but split into two families with very different organisation.
Eukaryotic vs prokaryotic — the headline difference
Eukaryotic cells (animals, plants, fungi, protists) keep their genetic material inside a nucleus and contain membrane-bound organelles. Examples: human cheek cells, palm-tree cells, yeast.
Prokaryotic cells (bacteria) are smaller (typically 1–10 μm vs 10–100 μm), have no nucleus — their DNA is a single loop in the cytoplasm — and they may carry small extra rings of DNA called plasmids. They have no mitochondria and no chloroplasts.
What every cell has
- Cell membrane — a thin layer that controls what enters and leaves.
- Cytoplasm — jelly-like fluid where reactions happen.
- Genetic material (DNA) — instructions for making proteins.
- Ribosomes — where proteins are made.
Animal cell extras
- Nucleus — controls the cell, holds DNA on chromosomes.
- Mitochondria — site of aerobic respiration (release energy).
Plant cell extras (on top of all the animal-cell features)
- Cell wall of cellulose — gives strength and shape.
- Permanent vacuole filled with cell sap — keeps the cell turgid.
- Chloroplasts — contain chlorophyll, site of photosynthesis.
Specialised cells
Most cells in a multicellular organism are differentiated — adapted for one job. Common GCSE examples:
- Sperm cell — long tail, many mitochondria, acrosome with enzymes.
- Nerve cell — long axon, branched dendrites, myelin sheath insulates.
- Muscle cell — protein fibres that contract; lots of mitochondria.
- Root hair cell — long projection increases surface area for water/mineral uptake.
- Xylem and phloem cells — adapted for transport in plants.
Microscopy and magnification
Light microscopes magnify ×1000–2000 and resolve to ~200 nm — fine for cells, organelles only just visible. Electron microscopes magnify ×500,000+ and resolve to ~0.2 nm — they reveal organelle ultrastructure.
The key equation for any image (often examined):
magnification = image size ÷ real (actual) size
Always convert units first — 1 mm = 1000 μm = 1,000,000 nm. A common exam slip is forgetting to convert; e.g. 30 mm image at ×1500 → 30/1500 = 0.02 mm = 20 μm, not 0.02.
Culturing microorganisms (required practical)
Bacteria are grown on agar plates sterilised at 121 °C. Inoculation uses a sterile loop, then plates are sealed (not fully — to stop anaerobes) and incubated at 25 °C in school labs (lower than body temperature to reduce risk of growing human pathogens). The zone of inhibition around an antibiotic disc is calculated as area of a circle: A = πr².
⚠Common mistakes— Common mistakes / exam traps
- "Bacteria have no DNA" — wrong; they have a single loop plus plasmids.
- "Plants don't have mitochondria" — they do; plants respire as well as photosynthesise.
- Mixing up magnification units — always convert to the same unit before dividing.
- Forgetting that not every plant cell has chloroplasts — root cells don't.
Links
This connects to B1.2 (cell division and stem cells), B1.3 (transport in cells — surface area to volume), and B2 (organisation into tissues and organs).
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