Body systems
Digestive system
Digestion breaks down large, insoluble food molecules into small, soluble molecules that can be absorbed into the blood.
Mechanical digestion: physical breakdown (chewing, stomach churning) — increases surface area. Chemical digestion: enzymes hydrolyse (break) bonds in large molecules.
| Enzyme | Substrate → Product | Location made | Optimum pH |
|---|---|---|---|
| Amylase | Starch → maltose (then glucose) | Salivary glands, pancreas | ~7–8 |
| Protease (pepsin/trypsin) | Protein → amino acids | Stomach, pancreas | 2 (stomach); 7–8 (small intestine) |
| Lipase | Fats → fatty acids + glycerol | Pancreas | ~7–8 |
Bile (made in liver, stored in gall bladder): not an enzyme — emulsifies fats (breaks large fat droplets into smaller ones, increasing surface area for lipase). Also neutralises stomach acid in the duodenum.
Absorption of soluble products occurs in the small intestine via villi (singular: villus):
- Villi have microvilli (brush border) to increase surface area enormously.
- Rich blood supply (capillaries) for rapid transport.
- Thin walls — short diffusion distance.
- Lacteal (lymph vessel) absorbs fatty acids + glycerol.
Breathing and gas exchange
The respiratory system moves air into and out of lungs for gas exchange (O₂ in; CO₂ out).
Alveoli are adapted for efficient gas exchange:
- Large surface area (millions of alveoli)
- Thin walls (one cell thick — short diffusion distance)
- Moist lining (gases dissolve)
- Rich capillary network (maintains steep concentration gradient)
Mechanism of breathing (CCEA):
- Inhalation: diaphragm contracts (flattens); intercostal muscles contract; rib cage moves up and out; thorax volume increases; pressure falls below atmospheric; air rushes in.
- Exhalation: diaphragm relaxes (domes up); intercostal muscles relax; rib cage falls; volume decreases; pressure rises; air pushed out.
Circulatory system
The double circulatory system:
- Pulmonary circuit: right ventricle → lungs → left atrium (deoxygenated → oxygenated blood).
- Systemic circuit: left ventricle → body → right atrium (oxygenated → deoxygenated blood).
Blood vessels:
| Vessel | Wall | Lumen | Direction |
|---|---|---|---|
| Artery | Thick, elastic, muscular | Narrow | Away from heart |
| Vein | Thin | Wide, has valves | Towards heart |
| Capillary | One cell thick | Very narrow | Connects arteries to veins |
Components of blood:
- Red blood cells: no nucleus; contain haemoglobin; carry O₂. Small and biconcave — large surface area.
- White blood cells: defend against pathogens (phagocytes engulf; lymphocytes produce antibodies).
- Platelets: cell fragments; trigger clotting.
- Plasma: liquid; carries dissolved substances (glucose, CO₂, urea, hormones, antibodies).
Photosynthesis
Word equation: Carbon dioxide + Water → Glucose + Oxygen (using light energy + chlorophyll).
Symbol equation: 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂
Factors that limit the rate of photosynthesis (limiting factors):
- Light intensity — more light → faster rate (up to a saturation point).
- CO₂ concentration — more CO₂ → faster rate.
- Temperature — too low slows enzyme reactions; too high denatures enzymes (≈45 °C).
Glucose produced by photosynthesis is used for: respiration (energy), cellulose (cell walls), starch (storage), proteins (with nitrates from soil), fats.
⚠Common mistakes
- Confusing respiration and breathing — breathing is physical (lungs); respiration is chemical (cells).
- Saying "veins carry deoxygenated blood" as an absolute rule — the pulmonary vein carries oxygenated blood to the heart.
- Forgetting that bile emulsifies (not digests) fats — no enzyme activity.
- Light and temperature both limit photosynthesis at different points on a rate graph — read the graph carefully.
AI-generated · claude-opus-4-7 · v3-ccea-biology