Top 10 IGCSE Biology MCQ Mistakes (2022-2025)
The most common Multiple Choice mistakes from Cambridge Examiner Reports. These errors appear year after year - learn to avoid them.
Why MCQ mistakes matter: Multiple choice papers (11-13 for Core, 21-23 for Extended) are worth 30% of your grade. The same conceptual errors appear every year. This guide shows exactly what examiners see students getting wrong - and how to fix it.
The Top 10 MCQ Mistakes
Active Transport Direction
Appears every year: 2022, 2023, 2024, 2025What the Syllabus Expects:
Define active transport and explain why it requires energy from respiration.
What Goes Wrong:
- • Selecting diagrams showing movement from high to low concentration (that's diffusion!)
- • Not understanding why oxygen is needed for active transport
- • Forgetting that a protein carrier is required in the membrane
Example Question:
Q: Which diagram shows active transport?
WRONG CHOICE
Diagram showing particles moving from HIGH to LOW concentration
CORRECT CHOICE
Diagram showing particles moving from LOW to HIGH concentration
Why: Active transport moves substances AGAINST their concentration gradient (low → high). This requires energy from respiration, which is why oxygen must be present.
Remember:
- ✓ Active = Against gradient = LOW to HIGH = needs ATP
- ✓ Diffusion/Osmosis = WITH gradient = HIGH to LOW = passive (no energy)
- ✓ No oxygen → no respiration → no ATP → no active transport
Diffusion vs Osmosis Confusion
Appears every year: 2022, 2023, 2024, 2025What the Syllabus Expects:
Define diffusion. Describe factors affecting rate: concentration gradient, temperature, surface area.
What Goes Wrong:
- • Thinking diffusion only involves water (that's osmosis!)
- • Believing diffusion needs energy (it's passive)
- • Selecting "decrease concentration gradient" to speed up diffusion (opposite!)
Example Question:
Q: How can the rate of diffusion be increased?
WRONG CHOICES
• Decrease surface area
• Decrease concentration gradient
• Decrease temperature
CORRECT CHOICE
• Increase surface area
• Increase concentration gradient
• Increase temperature
Why: Greater gradient = faster diffusion. Larger surface = more area for particles to cross. Higher temperature = faster particle movement.
Remember:
- ✓ Diffusion = ANY particle moving HIGH → LOW
- ✓ Osmosis = only WATER through a membrane
- ✓ Both are PASSIVE (no energy needed)
Root Hair Cells Have Chloroplasts?
9 mistakes recorded - most common single errorWhat Goes Wrong:
- • Believing root hair cells contain chloroplasts (they're underground - no light!)
- • Thinking sperm cells have a cell wall (animal cells don't have walls)
- • Confusing cell membrane with cell wall for "partially permeable"
Example Question:
Q: Which structures are found in a root hair cell?
WRONG
Cell wall, cell membrane, cytoplasm, nucleus, chloroplasts
CORRECT
Cell wall, cell membrane, cytoplasm, nucleus, mitochondria, vacuole (NO chloroplasts)
Why: Root hair cells are underground where there's no light. Chloroplasts are only in cells that photosynthesize (leaves, green stems).
Remember:
- ✓ Chloroplasts = only in GREEN parts exposed to LIGHT
- ✓ Cell wall = plants, fungi, bacteria (NOT animals)
- ✓ Cell membrane = ALL cells, controls what enters/exits
Photosynthesis Inputs vs Outputs
Appears in 2022, 2023, 2025What Goes Wrong:
- • Saying CO₂ is a product (it's a reactant!)
- • Confusing what enters vs what leaves the leaf
- • Not recognizing oxygen bubbles indicate photosynthesis
- • Forgetting both light AND chlorophyll are needed
Example Question:
Q: Which gas moves OUT of a leaf during photosynthesis?
WRONG
Carbon dioxide
CORRECT
Oxygen
Why: CO₂ + H₂O → Glucose + O₂. Carbon dioxide goes IN, oxygen comes OUT. The bubbles you see from pondweed in light are oxygen.
Remember:
- ✓ IN: Carbon dioxide + Water (+ Light energy)
- ✓ OUT: Glucose + Oxygen
- ✓ Bubbles from water plant = oxygen = photosynthesis happening
Easy Way to Remember:
Trees clean up our mess!
- Us (respiration): O₂ in, CO₂ out (we pollute)
- Trees (photosynthesis): CO₂ in, O₂ out (they clean up)
Plants do the OPPOSITE of what we do - that's why we need them!
Potato in Sugar Solution
Appears in 2022, 2023, 2025What Goes Wrong:
- • Thinking potato gains mass in concentrated sugar (it loses mass!)
- • Believing sucrose moves during osmosis (only water moves)
- • Describing osmosis as movement of "particles" not specifically water
Example Question:
Q: A potato chip is placed in concentrated sugar solution. What happens?
WRONG
Gains mass / swells / becomes turgid
CORRECT
Loses mass / shrinks / becomes flaccid
Why: Concentrated sugar = LOW water potential outside. Water moves OUT of potato cells by osmosis (high → low water potential). Result: cells lose water, potato shrinks.
Remember:
- ✓ Concentrated solution = LOW water potential
- ✓ Water moves from HIGH to LOW water potential
- ✓ Osmosis = ONLY water, through a membrane
Enzyme Action & Optimum pH
Appears in 2022, 2023What Goes Wrong:
- • Thinking all enzymes work best at pH 7 (pepsin works at pH 2!)
- • Confusing substrate and product in diagrams
- • Saying protease is a substrate (it's an enzyme/catalyst)
Example Question:
Q: At what pH does pepsin work best?
WRONG
pH 7 (neutral)
CORRECT
pH 2 (acidic - stomach conditions)
Why: Different enzymes have different optimum pH. Pepsin works in stomach acid (pH 2). Amylase works in saliva (pH 7). Lipase works in small intestine (pH 8).
Remember:
- ✓ Substrate binds to enzyme → product released
- ✓ Enzyme = catalyst (speeds up reaction, unchanged)
- ✓ Different enzymes = different optimum pH
Water Movement in Leaves
Appears in 2022, 2023What Goes Wrong:
- • Saying water leaves stomata by evaporation (it's diffusion of water vapour)
- • Not knowing evaporation occurs at spongy mesophyll cell surfaces
- • Confusing how humidity affects concentration gradient
Example Question:
Q: How does water escape from stomata?
WRONG
Evaporation through stomata
CORRECT
Diffusion of water vapour through stomata
Why: Evaporation happens INSIDE the leaf (liquid → vapour at cell surfaces). The vapour then DIFFUSES out through stomata (high concentration inside → low outside).
Remember:
- ✓ Evaporation = liquid to gas (happens at cell surfaces)
- ✓ Diffusion = gas moving out through stomata
- ✓ High humidity = smaller gradient = slower transpiration
Expired Air Percentages
Tested repeatedly in 2022What Goes Wrong:
- • Not knowing approximate percentages of O₂ and CO₂
- • Confusing inspired vs expired air composition
You MUST memorize:
| Gas | Inspired Air | Expired Air |
|---|---|---|
| Oxygen | ~21% | ~16% |
| Carbon dioxide | ~0.04% | ~4% |
| Nitrogen | ~78% | ~78% |
Remember:
- ✓ Oxygen DECREASES (21% → 16%) - used in respiration
- ✓ CO₂ INCREASES (0.04% → 4%) - produced by respiration
- ✓ Nitrogen stays the SAME (not used in body)
Genus vs Species
Appears in 2022, 2023, 2025What Goes Wrong:
- • Selecting species name when asked for genus (e.g., "sapiens" instead of "Homo")
- • Not knowing mules are infertile (so horse and donkey = different species)
- • Unable to identify organisms as protoctists
Example Question:
Q: In the name Aquila chrysaetos, what is the genus?
WRONG
chrysaetos
CORRECT
Aquila
Why: Binomial naming: GENUS first (capital letter), species second (lowercase). Aquila = genus, chrysaetos = species.
Remember:
- ✓ Species = can interbreed to produce FERTILE offspring
- ✓ Genus = first name, Capital letter, italicized
- ✓ Mules are hybrids = infertile = parents are different species
Where Urea is Made
Appears in 2022, 2024What Goes Wrong:
- • Saying urea is made in the kidneys (it's made in the LIVER!)
- • Not knowing kidneys also excrete excess ions
- • Confusing excretion with egestion
Example Question:
Q: Where is urea produced?
WRONG
Kidneys
CORRECT
Liver (by deamination of excess amino acids)
Why: The LIVER breaks down excess amino acids (deamination) to produce urea. The KIDNEYS then filter urea from the blood and excrete it in urine.
Remember:
- ✓ LIVER = makes urea (deamination)
- ✓ KIDNEYS = filter and remove urea + excess water + excess ions
- ✓ Excretion = metabolic waste. Egestion = undigested food
Quick Reference: The 10 MCQ Killers
- 1. Active transport: LOW → HIGH (needs energy)
- 2. Diffusion: Any particle, no energy needed
- 3. Root hair cells: NO chloroplasts
- 4. Photosynthesis: CO₂ IN, O₂ OUT
- 5. Potato in sugar: LOSES mass (water out)
- 6. Enzymes: Different optimum pH
- 7. Transpiration: Evaporation inside, diffusion out
- 8. Expired air: 16% O₂, 4% CO₂
- 9. Binomial names: Genus first (capital)
- 10. Urea: Made in LIVER, not kidneys
Source: Cambridge IGCSE Biology 0610 Examiner Reports (2022-2025)
Analysis of 181 MCQ mistakes across 10 exam sessions.