AQA GCSE Combined Science: Trilogy Biology Paper 1 (Higher), 2021: Question 4

✅ Correct answers

• Gonorrhoea → Bacterium
• Malaria → Protist
• Measles → Virus

1 mark for each correct line. Do not draw more than one line from any disease box.

💡 Key knowledge

• Gonorrhoea is a bacterial sexually transmitted infection.
• Malaria is caused by a protist parasite.
• Measles is caused by a virus.
• GCSE questions often test simple pathogen recall: bacterium, virus, protist, fungus.

🧠 Exam technique

• Read the disease name carefully before choosing the pathogen.
• If drawing lines on paper, make sure each disease has one line only.
• Use the exact pathogen type from the list provided.

❌ Common errors

• Mixing up malaria with a virus or bacterium.
• Thinking all diseases are caused by the same type of pathogen.
• Drawing more than one line from a box, which loses marks.

✅ Correct answer

• Releasing sterile male mosquitoes reduces breeding / reproduction.
• This means fewer mosquitoes are produced.
• So there are fewer mosquitoes to bite people or fewer mosquitoes to pass on the malaria protist.

2 marks total: one for reducing reproduction, one for explaining the knock-on effect on spread.

💡 Key knowledge

• Male sterile mosquitoes are infertile, so they do not produce offspring.
• Malaria is spread by mosquitoes acting as vectors.
• Fewer vectors means the disease spreads less.

🧠 Exam technique

• Use a cause → effect → outcome chain.
• For 2-mark explain questions, give both the biological idea and the final effect on malaria.
• Words like fewer, less likely, reduced spread are useful.

❌ Common errors

• Only saying “mosquitoes die” — the mark scheme is about reproduction and transmission.
• Writing that sterile mosquitoes “kill the malaria pathogen” — they do not.
• Forgetting to link fewer mosquitoes to fewer bites / less spread.

✅ Correct answer

Electron microscope

1 mark for the correct piece of equipment.

💡 Key knowledge

• Viruses are too small to be seen with a light microscope.
• Electron microscopes have much higher magnification and resolution.
• GCSE wording: do not accept “light microscope”.

🧠 Exam technique

• If the object is a virus, think electron microscope immediately.
• “Microscope” alone may be too vague in some mark schemes, so use the full term.

📐 Calculations: step-by-step

1. Read the sizes from the figure:
   Fungal spore = 16 μm
   Virus = 2.5 × 10⁻⁷ m
2. Convert 16 μm into metres:
   16 μm = 1.6 × 10⁻⁵ m
3. Divide the fungal spore size by the virus size:
   1.6 × 10⁻⁵ ÷ 2.5 × 10⁻⁷
4. Calculate:
   = 64

Number of times longer = 64

3 marks: correct conversion, correct substitution, correct final answer.

💡 Key knowledge

• 1 μm = 1 × 10⁻⁶ m
• Comparing sizes usually requires both values in the same unit.
• Scientific notation helps avoid errors with lots of zeros.

🧠 Exam technique

• Always check whether the question wants a ratio or a difference.
• Here it asks “how many times longer” so you must divide, not subtract.
• Keep units tidy and write the final answer clearly.

❌ Common errors

• Subtracting the lengths instead of dividing.
• Forgetting to convert μm to m.
• Writing 6.4 or 640 by moving the decimal the wrong way.
• Using the wrong virus length from the diagram.

✅ Correct answer

• Discolouration in leaves or less chlorophyll in leaves.
• This means reduced photosynthesis.
• So less glucose is made, meaning less amino acids / proteins / cellulose are produced, or less glucose is available for respiration so less energy is released for growth.

3 marks: one for reduced chlorophyll/discolouration, one for reduced photosynthesis, one for reduced glucose leading to less growth.

💡 Key knowledge

• Viruses damage plant cells and can cause mottled or discoloured leaves.
• Chlorophyll is needed to absorb light for photosynthesis.
• Less photosynthesis means less glucose for making biomass and for respiration.

🧠 Exam technique

• Build the explanation in order: virus effect → photosynthesis → glucose → growth.
• Use “so” to show cause and effect, which helps earn linked marks.
• One idea of “less” is enough per mark, but the chain must make sense.

❌ Common errors

• Saying only “the plant has less energy” without explaining where that comes from.
• Talking about the mosaic pattern instead of discolouration.
• Missing the link to glucose and biomass production.
• Writing vague statements like “the plant is sick so it grows slowly”.

Pathogen types

• Gonorrhoea → bacterium
• Malaria → protist
• Measles → virus

Key ideas

• Sterile mosquitoes reduce breeding, so transmission falls.
• Viruses need an electron microscope.
• Compare sizes by converting to the same unit and dividing.
• Plant viruses can reduce chlorophyll and photosynthesis.