IGCSE Biology Paper 4 Key Notes

Small Intestine & Villi

  • Villi: increase \text{surface area} for absorption of digested food into blood.
  • Structure of a villus:
    • Finger-like projection, one-cell-thick epithelium containing microvilli.
    • Dense capillary network for quick transport of monosaccharides, amino acids, water-soluble vitamins.
    • Lacteal (lymph vessel) for absorption of \text{lipids} (fatty acids + glycerol).
    • Goblet cells secrete mucus; muscle fibres enable slow movement/mixing.

Blood & Transport

  • Nutrient-carrying component: plasma.
  • Assimilation: incorporation of absorbed molecules into the body’s cells/tissue for growth & repair.
  • Blood proteins examples: fibrinogen, antibodies (immunoglobulins), albumin, haemoglobin (any two).

Cholera Infection

  • Vibrio cholerae releases toxin that opens \text{Cl}^- channels in intestinal epithelium.
  • Massive efflux of \text{Cl}^- and \text{HCO}_3^- into lumen; water follows osmotically.
  • Results: severe diarrhoea, dehydration, loss of ions → disrupts digestion & absorption.

Hormones & Blood Glucose

  • Hormone: chemical messenger made by endocrine glands, transported in blood plasma, alters activity of target organs/cells, effective in small quantities.
  • Insulin: lowers blood glucose by stimulating uptake into liver/muscle, conversion to glycogen, and increased respiration rate.

Glucose Tolerance Graph (Fig. 2.1)

  • Person A (diabetic): higher peak, slower decline, prolonged hyperglycaemia.
  • Person B: smaller peak, returns to baseline ≈90 min due to normal insulin response.
  • After 90 min in B: insulin secretion → glycogenesis and cellular uptake reduce glucose towards fasting level.
  • % increase A (60 → 90 min): \dfrac{(\text{value}{90}-\text{value}{60})}{\text{value}_{60}} \times 100 ≈ answer expected 40–50 % (calc not included here).
  • Type 1 diabetes treatment: regular insulin injections / pump, controlled diet & monitoring of blood glucose.

Blood Cells (Fig. 3.1)

  • Red blood cell: biconcave, no nucleus; transports O_2 via haemoglobin.
  • Phagocyte (neutrophil): lobed nucleus, granular cytoplasm; engulf/digest pathogens.
  • Lymphocyte: large nucleus, thin cytoplasm; produces antibodies & coordinates immune response.

Blood Clotting

  • Sequence: damaged vessel → platelets → thrombin converts fibrinogen → fibrin → mesh traps cells.
  • Roles: prevents blood loss; blocks pathogen entry; initiates tissue repair.

Sex-linked Inheritance (Haemophilia)

  • X-linked recessive: males affected if X^hY; females if X^hX^h.
  • Pedigree genotypes: P X^hY, Q X^HX^h (carrier), R X^HY.
  • Probability SX^HX^h × TX^HY ⇒ 25 % haemophilic sons.
  • Sex-linked: allele located on sex chromosome; phenotype shows different pattern in males vs females.

Respiration Equation

  • Aerobic (balanced): C6H{12}O6 + 6O2 \rightarrow 6CO2 + 6H2O + \text{energy}.

Biofuel Crop Choice

  • Country 12–24 °C, 1000 mm rain → wheat or corn fit temp; wheat higher rainfall match = best.
  • Wheat gives reasonable energy yield 53–84\,\text{GJ ha}^{-1} and optimum 24 °C / 800–1200 mm.

Transpiration & Humidity

  • Lower external \text{water vapour} concentration ↑ gradient between leaf air spaces & air.
  • ↑ diffusion of water vapour through stomata; more water pulled up xylem (cohesion-tension).

Vegetative Propagation (Cuttings)

  • Genetically identical, preserve desired traits.
  • Faster, reliable, can reproduce seedless/sterile varieties.
  • Uniform crop maturity/quality; bypass seed dormancy.

Reptiles vs Plants

  • Reptile features: dry scaly skin; lay shelled eggs on land; ectothermic.
  • Plant-only cell structures: cell wall, chloroplasts, large central vacuole (any two).

Enzymes & Digestion

  • Starch digestion sites: \text{mouth (saliva)}, \text{small intestine lumen} (pancreatic amylase).
  • Enzyme specificity: active site’s 3-D shape complementary to substrate; forms enzyme-substrate complex → efficient hydrolysis; wrong shape prevents binding.

Conservation Strategies

  • Protected areas / reserves; captive breeding & reintroduction; seed banks & tissue culture; laws against hunting/trade; public education; habitat restoration; international agreements (CITES);
    sustainable ecotourism.

Ecology: Food Web Roles

  • Producer example: algae / phytoplankton.
  • Secondary consumer: pipefish / blenny / spotted sandpiper etc.
  • Feeds at two trophic levels: stone crab (eats oyster & algae) or similar.
  • Pyramid: many producers capture solar energy; energy lost (respiration, waste) ≈90\% each level → fewer top consumers.

Nitrogen Cycle (Fig. 6.2)

  • A: nitrification; organism: nitrifying bacteria (e.g., Nitrosomonas/Nitrobacter).
  • Nitrate uptake: enters root hairs by active transport against concentration gradient; requires ATP from respiration; moves in through xylem to leaves.
  • D (protein synthesis): occurs on ribosomes within plant cells.
  • B: decomposition / ammonification (proteins → ammonium ions).