AQA GCSE Combined Science: Trilogy Biology Paper 1 (Higher), 2019: Question 4

✅ Correct answer

Protist

💡 Key knowledge

• Malaria is caused by a protist called Plasmodium.
• It is not caused by a bacterium, fungus, or virus.

❌ Common errors

• Ticking bacterium because malaria is “spread by mosquitoes”.
• Saying “virus” just because it is an infectious disease.

✅ Correct answers

• Insecticides – to kill mosquitoes / the vector.
• Mosquito nets – to avoid being bitten.
• Insect repellents – to make bites less likely.
• Vaccination – so people are immune to malaria.
• Anti-malaria tablets – to kill the pathogen / protist.
• Drain swampy ground / remove standing water / put oil on water – to reduce breeding grounds for mosquitoes.
• Release GM or sterile mosquitoes – to reduce reproduction.

💡 Key knowledge

• Malaria is spread by mosquitoes, so many prevention methods target the mosquito.
• The reason must match the method, for example:
  • Net → stops bites
  • Insecticide → kills mosquitoes
  • Repellent → mosquitoes less likely to bite
  • Tablets → kill the pathogen

🧠 Exam technique

• Choose two different methods.
• Write each method clearly and then a reason on the next line.
• To get the mark for the reason, it must be linked to that exact method.
• If you write “kills insects” for insecticides, that is usually fine.

❌ Common errors

• Saying only “kill insects” without naming mosquitoes — accepted sometimes, but be specific.
• Writing a method with no reason.
• Giving a reason that does not match the method.
• Saying “acts as a physical barrier” for nets — this may be accepted, but “avoid being bitten” is clearer.

✅ Correct answers

• A bacterial cell does not have a nucleus.
• A bacterial cell has plasmids.
• A bacterial cell is smaller.
• A bacterial cell has circular DNA / a single strand of DNA.
• A bacterial cell does not have mitochondria or other membrane-bound organelles.

💡 Key knowledge

• Bacteria are prokaryotic cells.
• Eukaryotic cells have a nucleus.
• Plasmids are small rings of DNA found in bacteria.

🧠 Exam technique

• Write two separate differences.
• Short clear comparison statements score best.
• Examples:
  • “Bacterial cells have no nucleus; eukaryotic cells do.”
  • “Bacterial cells have plasmids; eukaryotic cells do not.”

❌ Common errors

• Saying bacteria “have no chloroplasts” — not accepted here.
• Writing something too vague like “they are different”.
• Only giving one difference twice in different words.

✅ Correct answer

To allow air / oxygen into the bottle so the bacteria can respire aerobically.

💡 Key knowledge

• Bacteria need oxygen for aerobic respiration.
• Respiration releases energy for growth and division.

❌ Common errors

• Talking about “ventilation” without linking to oxygen.
• Saying the holes are “for bacteria to get out”.
• Missing the idea of respiration.

✅ Correct answers by stage

• Stage A: no change in population size because there is no / limited cell division or reproduction.
• Stage B: rapid increase because cells divide rapidly as nutrients/food are plentiful.
• Stage C: population size stays the same because the rate of cell division equals the rate of cell death.
• Stage D: population decreases because nutrients run out or wastes/toxins build up, so cells die.

💡 Key knowledge

• This is a typical bacterial growth curve.
• Look for the stage shape:
  • Flat = no growth
  • Steep rise = rapid growth
  • Flat at top = balanced birth and death
  • Falling = more death than division

🧠 Exam technique

• Use the graph shape in your answer.
• For top marks, mention the biological cause: nutrients, food, division, death, waste build-up.
• “It grows” is too vague on its own.

❌ Common errors

• Saying “the bacteria are getting bigger” instead of dividing.
• Writing “population increases because of photosynthesis” — bacteria here are not plants.
• For Stage C, saying the population is “static because bacteria stop living” — the key idea is birth rate = death rate.

📐 Calculations – step by step

Use the cubes shown:

• Cube X side length: 0.0002 cm
• Cube Y side length: 11 cm

1. Find surface area and volume for Cube X
   SA = 6 × (0.0002 × 0.0002) = 2.4 × 10⁻⁷ cm²
   Volume = 0.0002 × 0.0002 × 0.0002 = 8 × 10⁻¹² cm³
2. Find the SA:V ratio for Cube X
   2.4 × 10⁻⁷ : 8 × 10⁻¹² = 30,000:1
3. Find surface area and volume for Cube Y
   SA = 6 × 11 × 11 = 726 cm²
   Volume = 11 × 11 × 11 = 1331 cm³
4. Find the SA:V ratio for Cube Y
   726:1331 = 0.55:1

✅ Correct explanation

• The multicellular organism has a lower surface area to volume ratio.
• So diffusion distance is longer.
• Each unit of surface area has to supply a greater volume of cells.
• Therefore diffusion is too slow to supply enough nutrients / remove wastes quickly enough.

💡 Key knowledge

• Small objects have a large surface area compared with volume.
• Large organisms have more volume but not enough surface area to meet demand by diffusion alone.
• Cells deep inside a large organism are far from the surface, so substances must travel further.

🧠 Exam technique

• Show your working clearly.
• Use the correct units:
  • Surface area in cm²
  • Volume in cm³
• When giving a ratio, simplify it correctly.
• Link the maths to biology: smaller SA:V = less efficient diffusion.

❌ Common errors

• Using the wrong units or forgetting units completely.
• Calculating only volume or only surface area.
• Writing the ratio backwards or not simplifying it.
• Saying “multicellular organisms are bigger” without explaining why that affects diffusion.
• Not mentioning that diffusion rate per unit volume is slower in a multicellular organism.

🧠 Top responses did these things

• Used the exact biology term required, such as protist and plasmids.
• Matched each malaria prevention method with a clear reason.
• For the graph, linked each stage to division, nutrients, and death.
• For the calculation, gave clear working and then used the answer to explain why diffusion fails in large organisms.

❌ Where students lost marks

• Using vague statements like “it stops them” without saying what “it” is.
• Not linking prevention methods to the mosquito or pathogen.
• Mixing up bacterial and eukaryotic features.
• Forgetting that bacteria divide by binary fission.
• Not using the surface area to volume ratio to support the diffusion explanation.