B2 Scaling up — OCR Gateway Biology (J257/01)
Cell differentiation
After fertilisation, cells divide by mitosis and then differentiate — they develop specialised structures to carry out specific functions. This occurs because, although every cell in the body contains the same DNA, different genes are switched on or off in different cell types.
Examples of specialised cells:
| Cell type | Adaptations | Function |
|---|---|---|
| Red blood cell (erythrocyte) | Biconcave disc (large SA); no nucleus (more room for Hb); flexible membrane | Carry oxygen via haemoglobin |
| Sperm cell | Long tail (flagellum); many mitochondria; acrosome (enzymes to penetrate egg) | Fertilise egg |
| Nerve cell (neurone) | Long axon; myelin sheath (insulation); synaptic terminals | Transmit electrical signals |
| Root hair cell | Long extension (↑ surface area); thin wall; many mitochondria | Absorb water + minerals |
| Palisade mesophyll cell | Densely packed chloroplasts; long shape to intercept light | Photosynthesis |
Stem cells
Stem cells are undifferentiated cells that can divide and differentiate into many cell types.
Embryonic stem cells:
- Found in the inner cell mass of a blastocyst (3–5 day embryo).
- Totipotent/pluripotent — can become almost any cell type.
- Ethical controversy: the embryo is destroyed to harvest them.
Adult stem cells:
- Found in specific tissues (bone marrow, skin, gut lining).
- Multipotent — can only produce a limited range of cell types.
- Fewer ethical concerns; used in bone marrow transplants (treat leukaemia).
Therapeutic cloning:
- Transfer patient's nucleus into enucleated egg → stimulate division → harvest stem cells.
- Cells are genetically identical to patient → no immune rejection.
OCR examiner focus: Be ready to evaluate the ethical arguments (treat as an extended-response opportunity with "for" and "against" points).
Surface area : volume ratio (SA:V)
As an organism grows, volume increases faster than surface area. A large SA:V ratio allows sufficient exchange of materials; as organisms grow larger, SA:V decreases and exchange surfaces become insufficient — hence specialised exchange organs evolve.
Calculating SA:V for a cube:
- Cube with side length n: SA = 6n², Volume = n³, SA:V = 6n²/n³ = 6/n.
- As n increases, SA:V decreases.
Example: n=1 cm → SA:V = 6:1; n=2 cm → SA:V = 3:1; n=4 cm → SA:V = 1.5:1.
Implications:
- Small cells / single-celled organisms: SA:V is large → can exchange by diffusion alone.
- Mammals: SA:V is tiny → need specialised exchange surfaces (lungs, villi, gills).
Exchange surfaces — adaptations
Efficient exchange surfaces share four features:
- Large surface area — folding, villi, microvilli.
- Thin — short diffusion distance.
- Maintained concentration gradient — blood flow / ventilation keeps gradient steep.
- Moist — gases dissolve to cross membrane.
Alveoli (lungs):
- Tiny air sacs (~300 million) → enormous SA (~70 m² total).
- One-cell-thick walls + capillary walls → short diffusion path.
- Rich blood supply (constantly removing O₂ and delivering CO₂) → steep gradient maintained.
- Moist lining.
Small intestine villi:
- Finger-like projections (villi) + microvilli (brush border) → huge SA for absorption.
- Thin epithelium; dense capillary network (glucose + amino acids → blood); lacteals (fat → lymph).
Fish gills:
- Gill filaments with lamellae → huge SA.
- Countercurrent system: water flows over gills in opposite direction to blood — maintains steep O₂ gradient along entire length → near-total extraction of O₂ from water.
Transport in plants — xylem and phloem
Plants have two vascular tissues:
Xylem:
- Transports water and dissolved mineral ions from roots → leaves.
- Vessels made of dead, lignified cells — hollow tubes with no cross walls.
- Water moves up by transpiration pull (cohesion-tension mechanism) + root pressure.
- Lignin waterproofs and strengthens.
Phloem:
- Transports dissolved sugars (mainly sucrose) and amino acids from leaves (source) to growing regions/storage organs (sink) — this is translocation.
- Made of sieve tube elements (with perforated sieve plates) + companion cells (which provide ATP for active loading of sucrose).
Transpiration (PAG B2.1 — measuring with a potometer):
- Water evaporates from leaf surface (mainly stomata) → water is pulled up xylem.
- Factors increasing transpiration rate: higher temperature, lower humidity, higher light intensity (stomata open wider), increased air movement.
- A potometer measures water uptake as a proxy for transpiration rate — the air bubble moves as water is drawn into the cut stem.
Transport in animals — the heart and blood vessels
Double circulatory system in mammals:
- Pulmonary circuit: right ventricle → lungs → left atrium (oxygenation).
- Systemic circuit: left ventricle → body → right atrium (oxygen delivery).
Blood vessels:
| Vessel | Wall structure | Pressure | Direction |
|---|---|---|---|
| Artery | Thick, elastic, muscular | High | Away from heart |
| Vein | Thin, less muscular; valves | Low | Towards heart |
| Capillary | One cell thick (endothelium) | Lowest | Tissue exchange |
Valves in the heart:
- Atrioventricular (AV) valves (tricuspid and mitral): prevent backflow from ventricles → atria.
- Semilunar valves (pulmonary and aortic): prevent backflow from arteries → ventricles.
Common OCR examiner traps
- Transpiration is water loss from leaves, not the whole plant. Don't say "water loss from roots."
- Xylem is dead; phloem is living. The companion cells of phloem require ATP.
- Osmosis is passive — water enters root hair cells by osmosis (no ATP), but mineral ions require active transport.
- Double vs single circulation: fish have single; mammals have double. Advantage: higher pressure to body.
- SA:V formula for cubes — always divide SA by volume and simplify.
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