AQA GCSE Biology Biology Paper 1 (Foundation), June 2025: Question 4

✅ Model 6-Mark Method (Level 3)

1. 1 Sterilise area: Disinfect the workbench and wash hands. Work close to a lit Bunsen burner so upward convection currents carry airborne microbes away.
2. 2 Pour plate: Flame the neck of the molten agar bottle. Lift the Petri dish lid slightly at an angle, pour the agar in, and allow it to set completely.
3. 3 Sterilise loop: Pass the metal inoculating loop through the Bunsen flame until red hot, then allow it to cool so it doesn't kill the bacteria.
4. 4 Inoculate: Flame the neck of the bacterial culture bottle. Dip the loop in and streak the bacteria evenly over the set agar, opening the lid minimally.
5. 5 Incubate: Secure the lid with adhesive tape (not all the way around), invert the dish (upside down), and incubate at 25 °C.

🧠 How to Secure Level 3 (5–6 Marks)

• The "Holy Trinity" of RP2: Examiners look for three clear stages: (1) Preparing the plate, (2) Inoculating with bacteria, and (3) Specific aseptic actions.
• To reach Level 3, your method must be logically sequenced. You cannot streak bacteria before the agar sets!
• Always state how equipment is sterilised (e.g., passing loop through Bunsen flame, wiping bench with disinfectant).

❌ Common Errors & Misconceptions

• Taping all the way round: Never say "seal the dish completely". Sealing prevents oxygen entry, promoting the growth of dangerous anaerobic pathogens.
• Forgetting to cool the loop: Dipping a red-hot loop directly into bacterial culture kills the bacteria.
• Taking the lid off completely: Leaving the lid flat on the desk exposes the open agar to falling airborne spores.

💡 Mark Scheme Breakdown

• Level 3 (5–6 marks): Valid, logically sequenced method covering agar plate preparation, bacterial transfer, and aseptic techniques.
• Level 2 (3–4 marks): Covers plate preparation and bacterial transfer, but some steps missed or unordered.
• Level 1 (1–2 marks): Fragmented aseptic steps without clear coherence.

✅ Correct Answer

To prevent the growth of pathogens (harmful microorganisms / bacteria / fungi).

🧠 Exam Technique

Always use the word pathogen or "harmful / disease-causing microorganisms". Human pathogens thrive at human body temperature (37 °C). Keeping school cultures at 25 °C greatly reduces the risk of culturing something dangerous.

❌ Examiner Pitfall

Do NOT write "bacteria will be denatured" or "bacteria will die at higher temperatures". Bacteria grow faster at 37 °C; the restriction is purely for safety!

✅ Correct Box to Tick

☑ To show the effect of no antibiotics.

💡 Why is this needed?

The paper disc soaked in sterile water acts as a negative control. It proves that the paper disc itself or the water is not inhibiting bacterial growth—only the antibiotic active ingredients are.

📐 How to Draw on Figure 7

• Around Disc A: Draw a circular clear ring (zone of inhibition) that is noticeably larger in diameter than the shaded ring around disc B.
• Around Disc C: Draw a circular clear ring that is noticeably smaller in diameter than the ring around B (or leave no clear ring at all, touching the disc).
• Note: Shading/hatching is not required by examiners, but the boundary circle must be clear.

💡 Scientific Principle

The larger the clear zone (zone of inhibition) around an antibiotic disc, the more effective the antibiotic is at killing or preventing the reproduction of the bacteria.

• Antibiotic A (Most effective): Largest clear zone.
• Antibiotic B (Intermediate): Medium clear zone (already drawn).
• Antibiotic C (Least effective): Smallest clear zone (or none).

✅ Correct Box to Tick

☑ None of the bacteria would have been killed.

💡 Key Recall Fact

Bacterial resistance means the bacteria are entirely unaffected by that specific antibiotic. They will grow right up to the edge of the disc, resulting in a zone of inhibition of 0 mm.