B1 Cell-level systems — OCR Gateway Biology (J257/01)
Eukaryotic vs Prokaryotic cells
All living cells are either eukaryotic or prokaryotic. Eukaryotic cells (animals, plants, fungi, protists) have a nucleus containing DNA enclosed in a nuclear membrane, plus membrane-bound organelles. Prokaryotic cells (bacteria) have no nucleus — DNA floats freely in the cytoplasm as a single circular chromosome, often supplemented by small rings of DNA called plasmids.
| Feature | Eukaryotic (animal) | Eukaryotic (plant) | Prokaryotic (bacterium) |
|---|---|---|---|
| Nucleus | ✓ | ✓ | ✗ (free DNA) |
| Cell wall | ✗ | ✓ (cellulose) | ✓ (peptidoglycan) |
| Chloroplasts | ✗ | ✓ | ✗ |
| Mitochondria | ✓ | ✓ | ✗ |
| Plasmids | ✗ | ✗ | Often present |
| Ribosomes | Large (80S) | Large (80S) | Small (70S) |
| Typical size | 10–100 µm | 10–100 µm | 1–10 µm |
Key organelles — animal cell:
- Nucleus — contains chromosomal DNA; controls cell activities.
- Mitochondria — site of aerobic respiration; produce ATP.
- Ribosomes — site of protein synthesis (translation).
- Cell membrane — phospholipid bilayer; controls entry/exit of substances.
- Cytoplasm — jelly-like fluid where metabolic reactions occur.
Additional plant-cell features:
- Cell wall (cellulose) — gives structural support; freely permeable.
- Chloroplasts — site of photosynthesis; contain chlorophyll.
- Vacuole — large, permanent; maintains turgor pressure using cell sap.
Microscopy — PAG B1.1
OCR Gateway assesses microscopy in dedicated PAG questions (J257/01, Section A). Key skills:
Calculating magnification:
Magnification = Image size ÷ Actual size
Always check units are consistent (convert to µm or mm before dividing).
Calculating actual size:
Actual size = Image size ÷ Magnification
Example (common exam question): A cell appears 30 mm long under a ×600 microscope. What is the actual size? Actual size = 30 mm ÷ 600 = 0.05 mm = 50 µm.
Light vs electron microscope:
| Light microscope | Electron microscope | |
|---|---|---|
| Maximum magnification | ~×1500 | ×500,000+ |
| Resolution | ~200 nm | ~0.1 nm |
| Living specimens | Yes | No (must be fixed/stained) |
| Colour images | Yes | No (false colour added) |
| Subcellular structures visible | Nucleus, chloroplasts, vacuole | Mitochondria cristae, ribosomes, ER |
Preparing a slide (PAG B1.1):
- Place material on slide; add a drop of water.
- Lower coverslip at 45° to exclude air bubbles.
- Apply iodine stain (plant cells) or methylene blue (animal cells) to improve contrast.
- Observe under low power first; focus with coarse adjustment; switch to high power and use fine adjustment.
Mitosis
Mitosis produces two genetically identical daughter cells. It is used for growth, repair, and asexual reproduction.
Stages (PMAT mnemonic):
- Prophase — chromosomes condense and become visible; nuclear envelope breaks down; spindle fibres form.
- Metaphase — chromosomes line up at the cell equator (metaphase plate); spindle fibres attach to centromeres.
- Anaphase — spindle fibres contract; sister chromatids separate and are pulled to opposite poles.
- Telophase — nuclear envelopes re-form around each set of chromosomes; chromosomes decondense.
- Cytokinesis — cytoplasm divides; two genetically identical daughter cells produced.
Human body cells are diploid (2n = 46). Each daughter cell receives the same 46 chromosomes.
Cancer is uncontrolled mitosis caused by mutations in genes that regulate the cell cycle (tumour suppressor genes and proto-oncogenes). OCR Gateway links this to B6 (disease).
Transport across membranes
The cell membrane is a selectively permeable phospholipid bilayer. Three transport mechanisms:
Diffusion
- Movement of molecules from high → low concentration (down the concentration gradient).
- Passive (no energy / ATP required).
- Factors increasing rate: larger concentration gradient, higher temperature, smaller molecules, thinner membrane, larger surface area.
- Examples: oxygen into respiring cells; CO₂ out of cells; urea into kidney tubule.
Osmosis
- Specific case of diffusion — movement of water molecules from dilute solution (high water potential) → concentrated solution (low water potential) through a selectively permeable membrane.
- If a cell is placed in:
- Hypotonic (dilute) solution → water enters → cell swells (animal cell may burst = lysis; plant cell becomes turgid).
- Hypertonic (concentrated) solution → water leaves → animal cell shrinks (crenation); plant cell becomes plasmolysed.
PAG B1.2 — Investigating osmosis in plant tissue: Cut cylinders of potato; measure mass before/after placing in sucrose solutions of varying concentration (0–1.0 mol/dm³). Plot % change in mass vs concentration. The point where mass = 0% change gives the concentration of cell sap (≈ 0.3 mol/dm³ for potato).
Active transport
- Movement of molecules against the concentration gradient (low → high concentration).
- Requires ATP (produced by respiration) and specific carrier proteins.
- Examples: absorption of mineral ions (nitrates) by root hair cells; glucose absorption in the small intestine when concentration in blood > gut lumen.
Common OCR examiner traps
- Osmosis is only water — never say "glucose moves by osmosis."
- Active transport needs carrier proteins AND ATP — candidates often omit one.
- Magnification calculation units — always check both measurements use the same unit.
- Mitosis produces 2 cells; meiosis produces 4 — don't confuse them.
- Prokaryotes have no nucleus but DO have ribosomes (small 70S type).
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