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 (Extended): 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 but does not fall to zero: photosynthesis carries on at a steady rate because another factor is now limiting.

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

  Wrong (word equation asked): 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 (Extended):

  When asked: "State where maltase acts"

  Too vague: "In the small intestine"

  Correct: "On the membranes of the epithelial cells lining the small intestine"

  Be specific - maltase sits on the membranes of the epithelium, not inside the cells.

• Purpose of digestion:

  When asked: "Explain why digestion is necessary"

  Incomplete: "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 fits the active site, which has a complementary shape; an enzyme-substrate complex forms; products are released" (enzyme-substrate complex is an Extended term)

• 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 a chemical reaction and is not changed by the reaction"

  Two halves, one mark each: "increases" (not "changes"), and "not changed / not used up".

• "Killed" instead of denatured:

  When asked: "Explain why this human enzyme stops working at 70 °C"

  Wrong: "The enzyme is killed", or saying enzymes are denatured at low temperature

  Correct: "The enzyme is denatured: the active site changes shape, so the substrate no longer fits"

• Diagram interpretation:

  When shown: A lock-and-key diagram

  Tip: The enzyme is the structure that is the same before and after. The substrate is changed into products (split apart, or joined together).

What Goes Wrong:

• Half a species definition:

  When asked: "Define the term species"

  Weak: "Similar organisms that can breed together" (no fertile offspring)

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

• Dichotomous keys:

  When asked: To construct a dichotomous key for four animals

  Wrong: Steps with more than two choices, or features you cannot see ("Is it fast?")

  Correct: Each step splits the group into two using one visible feature ("Has wings" / "Has no wings"), and each organism ends at its own name

• Arthropod groups:

  When asked: To place an arthropod in its group from a drawing

  Count legs and body parts: insects 3 pairs of legs, 3 body parts; arachnids 4 pairs, 2 body parts; crustaceans more than 4 pairs, 2 pairs of antennae; myriapods many segments, each with legs

  A spider is an arachnid, not an insect.

• 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: "Water molecules have more kinetic energy, so more water evaporates from mesophyll cell surfaces and diffuses out through the stomata faster"

• Wind and humidity:

  When asked: "Explain why transpiration is faster on a windy day than a still, humid day"

  Wrong: "The wind blows water out of the leaf"

  Correct: "Wind removes water vapour around the leaf, keeping a steep concentration gradient, so vapour diffuses out faster; humid air makes the gradient shallower" (humidity is Extended)

• 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: why a shoot is cut and fitted into a potometer 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 the DNA in a bacterial cell as a "nucleus" (bacteria have no nucleus: it is circular DNA)

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

• Size relationships:

  When asked: To put a cell, a gene, a nucleus and a chromosome in order of size

  Wrong: Putting an allele smaller than a gene, or a chromosome bigger than the nucleus

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

  Remember: an allele is a version of a gene, not part of one. A chromosome is one long DNA molecule carrying many genes, inside the nucleus.

• Labelling technique:

  When asked: "Label the diagram"

  Wrong: A freehand or curved line, or one that stops short of the structure

  Correct: A ruled line that touches the structure (a plain line, not an arrow)