6 IGCSE Biology Mistakes Students Keep Making

What Goes Wrong:

• Forgetting CO₂: When asked for raw materials, students often only mention water or say "sunlight" (which is a requirement, not a raw material)
• Limiting factors graphs: Students say "light is the limiting factor" for the entire graph, but different parts of the curve have different limiting factors

  In a rate of photosynthesis vs light intensity graph:

  • Steep rising part: Light is limiting (more light = faster rate)

  • Plateau/flat part: Light is no longer limiting - now CO₂ or temperature is limiting

  The rate stops increasing because another factor has become the bottleneck.

• Symbol vs word equations: Writing chemical symbols when the question asks for words

  Wrong: CO₂ + H₂O → C₆H₁₂O₆ + O₂

  Correct: Carbon dioxide + Water → Glucose + Oxygen

• Not explaining "why": Describing what happens without giving a reason

  Weak: "As light intensity increases, the rate of photosynthesis increases"

  Better: "...because light provides the energy needed for the reaction"

What Goes Wrong:

• Egestion vs Excretion confusion:

  When asked: "State the function of the anus in the digestive system"

  Wrong: "Excretion of waste"

  Correct: "Egestion of undigested food / faeces"

  Excretion = metabolic waste (urea, CO₂). Egestion = undigested food.

• HCl misconception:

  When asked: "State the role of hydrochloric acid in the stomach"

  Wrong: "HCl digests / breaks down food"

  Correct: "Kills bacteria and provides optimum pH for pepsin to work"

  Enzymes digest food - HCl just creates the right conditions.

• Maltase location:

  When asked: "State where maltase acts"

  Wrong: "In the small intestine"

  Correct: "In the epithelial cells lining the small intestine"

  Be specific - maltase is in the cell membrane of epithelial cells.

• Purpose of digestion:

  When asked: "Explain why digestion is necessary"

  Wrong: "To break down food chemically"

  Correct: "To break large insoluble molecules into small soluble molecules so they can be absorbed"

  Always include the purpose - "so they can be absorbed into the blood".

What Goes Wrong:

• Vague descriptions:

  When asked: "Describe the action of an enzyme"

  Wrong: "Enzymes break down food"

  Correct: "The substrate binds to the active site, an enzyme-substrate complex forms, products are released"

• Catalyst definition:

  When asked: "Define the term catalyst"

  Wrong: "A substance that changes the rate of reaction"

  Correct: "A substance that increases the rate of reaction and is not used up"

  Must say "increases" not "changes", and mention it's unchanged.

• Incomplete mechanism:

  When asked: "Using the diagram, describe how enzymes work"

  Missing: Forgetting to say the enzyme is free to work again

  Include: "The enzyme is unchanged and can be reused"

• Diagram interpretation:

  When shown: A lock-and-key diagram

  Tip: The enzyme is the structure that stays the same. The substrate breaks into products.

What Goes Wrong:

• Genus vs Species:

  When asked: "Define the term species"

  Wrong: Giving the definition of genus instead

  Correct: "A group of organisms that can interbreed to produce fertile offspring"

• Dichotomous keys:

  When asked: "State what type of key is shown in Fig. 2.1"

  Wrong: Not recognising it's a dichotomous key

  Tip: If it has yes/no or either/or questions that branch, it's a dichotomous key

• Classification order:

  When asked: "Arrange these classification groups in order, starting with the largest"

  Remember: Kingdom → Phylum → Class → Order → Family → Genus → Species

  "King Philip Came Over For Good Soup"

• Observable features:

  When asked: "State one visible feature of organism X" (with photo)

  Wrong: "It is an arthropod" (that's a classification group)

  Correct: "It has wings" or "It has 6 legs" (visible features)

What Goes Wrong:

• Describe vs Explain:

  When asked: "Explain why the plant lost mass at higher temperatures"

  Wrong: "The mass decreased as temperature increased" (that's describing)

  Correct: "Higher temperature increases kinetic energy of water molecules, so more evaporate from leaves"

• Temperature effects:

  When asked: "Describe the effect of temperature on the rate of transpiration"

  Wrong: "The temperature goes up and down" (describing the temperature itself)

  Correct: "Higher temperature increases transpiration rate because water molecules have more energy to evaporate"

• Evaporation location:

  When asked: "State where water evaporates in the leaf"

  Wrong: "Through the stomata" (that's where vapour exits)

  Correct: "From the surface of mesophyll cells into air spaces"

• Cutting underwater:

  When asked: "Explain why the stem is cut under water"

  Correct: "To prevent air bubbles entering the xylem and blocking water flow" or "To maintain a continuous water column"

What Goes Wrong:

• Organelle identification:

  When shown: A cell diagram and asked to identify organelles

  Wrong: Labelling the large clear area as "nucleus" (it's the vacuole)

  Wrong: Labelling sausage-shaped structures as "nucleus" (they're mitochondria)

  Tip: Nucleus = large, dark, central. Vacuole = large, clear. Mitochondria = small, sausage-shaped.

• Size relationships:

  When asked: "Arrange these in order of size: chromosome, allele, nucleus, gene"

  Wrong: Random order or thinking chromosomes are bigger than the nucleus

  Correct: Allele → Gene → Chromosome → Nucleus (smallest to largest)

  Remember: Alleles are on genes, genes are on chromosomes, chromosomes are in the nucleus.

• Labelling technique:

  When asked: "Label the diagram"

  Wrong: Using arrows (→) or curved lines

  Correct: Use straight lines that touch the structure, no arrowheads