AQA GCSE Biology Biology Paper 1 (Higher), June 2025: Question 8

✅ Correct Answer

Bacterial cells are prokaryotic and yeast cells are eukaryotic.

💡 Key Knowledge

• Prokaryotes (e.g., bacteria): Lack a true nucleus; their genetic material is a single loop of free DNA and plasmids. They also lack membrane-bound organelles.
• Eukaryotes (e.g., animals, plants, fungi): Yeast is a single-celled fungus, meaning it possesses a distinct nucleus and membrane-bound organelles.

❌ Common Misconceptions

Students often mistake yeast for a bacterium because both are microscopic single-celled organisms. Remember: yeast is a fungus, which places it firmly in the eukaryotic kingdom.

✅ Correct Answers (Any Two)

• Nucleus (allow: nuclear membrane / chromosomes)
• Vacuole (large central permanent vacuole visible in diagram)
• Mitochondria (visible oval shapes with folds)

🧠 Exam Technique: Use the Diagram

The question specifies "Use Figure 10". Look closely at the yeast cell drawing:

• The large round shaded circle is the nucleus.
• The large empty clear circular space in the center is the vacuole.
• The small sausage-shaped structures with inner lines are mitochondria.

❌ Common Errors

• Naming Chloroplasts: Yeast cells are fungi, not plants—they never have chloroplasts.
• Giving structures bacteria also have: Stating cell wall, cytoplasm, or cell membrane scores zero because bacteria also have them.
• Stating "ribosomes": Both have ribosomes (though bacterial ribosomes are smaller, simply writing "ribosomes" is not creditworthy).

✅ Model Comparative Response

• Initial similarity: All three antibiotics show no significant effect for the first 4 to 5 hours, with bacteria multiplying at similar rates.
• Antibiotic C is the most effective:
  • Starts reducing bacteria earliest (peaks at ~4.5 hours).
  • Reduces the population the fastest (steepest negative gradient).
  • Has the lowest peak bacterial count.
  • Is the only antibiotic that kills all bacteria (count drops to 0 at ~12 hours).
• Antibiotic B is intermediate:
  • More effective than A, but less effective than C.
  • Starts reducing later than C (at ~8 hours) but before A.
  • Reduces the population to a very low number by 14 hours, but not to zero.
• Antibiotic A is the least effective:
  • Takes the longest time to act (bacteria grow until ~8.5 hours).
  • Reduces bacteria the slowest.
  • Leaves the highest number of surviving bacteria at 16 hours.

🧠 Top-Grade Strategy

To secure a Level 2 (4/4 marks), follow this 3-step checklist:

1. Mention all three: Rank them clearly (C is most effective, B is intermediate, A is least effective).
2. Identify key graph phases:
   • Initial lag / delay (0–4 hours)
   • Time when numbers start falling (onset of bactericidal effect)
   • Final outcome at 16 hours (C = 0, A = high).
3. Quote time milestones: e.g., "Antibiotic C reaches zero bacteria at around 12 hours".

✅ Correct Method (Mark Scheme)

1. Dilute each sample and count the number of bacteria using a microscope [1 mark].
2. Calculate the mean number of bacteria per sample and scale/multiply up to 1 cm³ [1 mark].

📐 Calculations & Scaling Factors

Two valid mathematical routes to scale up to 1 cm³:

❌ Common Errors & Traps

• Missing the dilution step: The question states the cells were "too close together to be counted accurately". You must explain how to fix this: by diluting the sample (serial dilution).
• Multiplying by 10 instead of 50: 10 is the number of samples, not the volume conversion factor! 0.02 cm³ goes into 1 cm³ exactly 50 times.
• Forgetting to find the mean: Taking repeat samples is only useful if you calculate an average to improve reliability.