Top 10 IGCSE Biology MCQ Mistakes
Ten multiple-choice mistakes that cost IGCSE Biology students easy marks, with the fix for each.
Why MCQ mistakes matter: Multiple choice papers (11-13 for Core, 21-23 for Extended) are worth 30% of your grade: 40 questions in 45 minutes, about one minute each. This guide shows ten easy ways to lose marks, and how to avoid each one.
The Top 10 MCQ Mistakes
Active Transport Direction
What the Syllabus Expects:
Describe active transport as movement through a cell membrane from lower to higher concentration (against the gradient), using energy from respiration. Paper 2 also: protein carriers move the particles, e.g. ion uptake by root hairs.
What Goes Wrong:
- • Selecting diagrams showing movement from high to low concentration (that's diffusion!)
- • Not understanding why oxygen is needed for active transport
- • Paper 2: forgetting that protein carriers in the membrane move the particles
Example Question:
Q: A cell already holds more potassium ions than the liquid around it, yet it keeps taking more in. Which process is this?
WRONG CHOICE
Diffusion
CORRECT CHOICE
Active transport (outside LOW → inside HIGH)
Why: Active transport moves substances AGAINST their concentration gradient (low → high). This needs energy from respiration. With little oxygen, cells respire anaerobically and release much less energy, so active transport slows.
Remember:
- ✓ Active = Against gradient = LOW to HIGH = needs energy from respiration
- ✓ Diffusion/Osmosis = WITH gradient = HIGH to LOW = passive (no energy from respiration)
- ✓ Less oxygen → less aerobic respiration → less energy → less active transport (e.g. slower ion uptake by roots)
Diffusion vs Osmosis Confusion
What the Syllabus Expects:
Describe diffusion as net movement from higher to lower concentration, powered by the particles' own kinetic energy. Factors: concentration gradient, temperature, surface area and distance.
What Goes Wrong:
- • Thinking diffusion only involves water (that's osmosis!)
- • Believing diffusion needs energy from respiration (it uses the particles' own kinetic energy)
- • 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. Shorter distance (thin walls) = faster diffusion.
Remember:
- ✓ Diffusion = NET movement of ANY particle HIGH → LOW
- ✓ Osmosis = only WATER, through a partially permeable membrane
- ✓ Both are PASSIVE (no energy from respiration)
Root Hair Cells Have Chloroplasts?
A favourite cell-structure trapWhat 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, ribosomes, 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, partially permeable, controls what enters/exits; the cell wall is fully permeable and gives support
Photosynthesis Inputs vs Outputs
What 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 does a leaf give out overall in bright light?
WRONG
Carbon dioxide
CORRECT
Oxygen
Why: carbon dioxide + water → glucose + oxygen (light and chlorophyll needed). In bright light photosynthesis is faster than respiration, so carbon dioxide goes IN and oxygen comes OUT overall. The bubbles you see from pondweed in light are oxygen.
Remember:
- ✓ IN: Carbon dioxide + Water (+ Light energy)
- ✓ PRODUCTS: Glucose + Oxygen. Oxygen is the only gas made; glucose becomes starch (store) or sucrose (transport)
- ✓ Bubbles from water plant = oxygen = photosynthesis happening
Easy Way to Remember:
Plants respire ALL the time, just like us.
- Bright light: photosynthesis beats respiration, so CO₂ in, O₂ out overall
- Dark: respiration only, so O₂ in, CO₂ out, the same as us
Trap: "plants respire only at night" is wrong. They respire day and night.
Potato in Sugar Solution
What 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 / goes soft (Paper 2 word: flaccid)
Why: The sugar solution is more concentrated than the cell sap. Water moves OUT of the cells by osmosis, from the dilute solution inside to the concentrated one outside, through the partially permeable membrane; the sugar does not cross. In pure water the chip would gain mass instead. (Paper 2: the solution has the lower water potential.)
Remember:
- ✓ Water moves from a DILUTE solution to a more CONCENTRATED one
- ✓ Paper 2: say HIGH to LOW water potential (concentrated = LOW)
- ✓ Osmosis = ONLY water, through a partially permeable membrane
Enzyme Action & Optimum pH
What Goes Wrong:
- • Thinking all enzymes work best at pH 7 (the stomach protease, pepsin, works best near pH 2!)
- • Confusing substrate and product in diagrams
- • Saying protease is a substrate (it's an enzyme/catalyst)
Example Question:
Q: At about what pH does the protease made in the stomach work best?
WRONG
pH 7 (neutral)
CORRECT
pH 2 (acidic - stomach conditions)
Why: Different enzymes have different optimum pH. The stomach protease (Paper 2 name: pepsin) works in acid, about pH 2, because hydrochloric acid is made there. Amylase in the mouth works near pH 7; enzymes in the small intestine work at slightly alkaline pH, about 8. Away from the optimum, activity falls; far from it the enzyme is denatured.
Remember:
- ✓ Substrate fits the active site (complementary shape) → products released
- ✓ Enzyme = catalyst (speeds up reaction, unchanged)
- ✓ Different enzymes = different optimum pH
Water Movement in Leaves
What Goes Wrong:
- • Saying water leaves stomata by evaporation (it's diffusion of water vapour)
- • Not knowing evaporation occurs at mesophyll cell surfaces, into the air spaces
- • Getting factors backwards: hotter or windier = FASTER (Paper 2: more humid = slower)
Example Question:
Q: Where does liquid water turn into water vapour in a leaf?
WRONG
In the stomata, as the water leaves
CORRECT
At the surfaces of the mesophyll cells, into the air spaces
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
- ✓ Hotter or windier = faster; Paper 2: humid air = smaller gradient = slower
Expired Air Percentages
What Goes Wrong:
- • Not knowing approximate percentages of O₂ and CO₂
- • Confusing inspired vs expired air, or forgetting water vapour (expired air is moister, and still holds about 16% oxygen)
Learn the pattern (approximate values):
| Gas | Inspired Air | Expired Air |
|---|---|---|
| Oxygen | ~21% | ~16% |
| Carbon dioxide | ~0.04% | ~4% |
| Water vapour | varies | more (saturated) |
Remember:
- ✓ Oxygen DECREASES (21% → 16%) - used in respiration
- ✓ CO₂ INCREASES (0.04% → 4%) - produced by respiration
- ✓ Water vapour INCREASES - evaporates from the moist alveolus lining (nitrogen stays about 78%)
Genus vs Species
What 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)
- • Paper 2: unable to place organisms in the protoctist kingdom
Example Question:
Q: The grey wolf is Canis lupus. Which word is its genus?
WRONG
lupus
CORRECT
Canis
Why: Binomial naming: GENUS first (capital letter), species second (lowercase). Canis = genus, lupus = 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
Liver part is Paper 2 onlyWhat Goes Wrong:
- • Paper 2: saying urea is made in the kidneys (it's made in the LIVER!)
- • Not knowing kidneys also excrete excess ions
- • Confusing excretion with egestion, or forgetting the lungs excrete carbon dioxide
Example Question:
Q: Where is urea produced?
WRONG
Kidneys
CORRECT
Liver (by deamination of excess amino acids)
Why: The LIVER removes the nitrogen-containing part of excess amino acids (deamination) to make 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 = removing metabolic waste (CO₂, urea) and excess substances. Egestion = removing undigested food as faeces
Quick Reference: The 10 MCQ Killers
- 1. Active transport: LOW → HIGH (needs energy)
- 2. Diffusion: Any particle, no energy from respiration
- 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: less O₂ (~16%), more CO₂ (~4%), more water vapour
- 9. Binomial names: Genus first (capital)
- 10. Urea: Made in LIVER, not kidneys
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