AQA GCSE Biology Biology Paper 1 (Foundation), November 2021: Question 8

✅ Acceptable Answers (Any Two)

• Sterilise equipment / surfaces before use (e.g. wipe work surface with disinfectant; pass inoculating loop through a roaring Bunsen flame).
• Use sterilised agar (autoclaved).
• Secure the Petri dish lid with tape (adhesive tape placed in spots, not sealed completely air-tight).
• Only lift the lid a small amount or at an angle when inoculating or adding discs.

❌ Common Errors & What Not to Say

• "Clean the table" – Too vague! Examiners strictly ignore "clean"; you must say sterilise or disinfect.
• "Wash hands" – Hand washing is basic hygiene, not an accepted laboratory aseptic technique for this mark scheme.
• "Tape the lid all the way around" – Incorrect science: this prevents oxygen entering, encouraging the growth of dangerous anaerobic pathogens!

💡 Key Knowledge: Why Aseptic Technique Matters

Aseptic techniques ensure that only the intended bacterial strain grows, preventing contamination from airborne microbes or tools, and preventing pathogens from escaping into the environment.

🧠 Exam Technique

Always describe what is sterilised and how. Say: "Flame the metal loop until red hot" or "Disinfect the bench before starting".

✅ Correct Answer

Antibiotic B

Reason: It kills the fewest bacteria / it produces the smallest clear zone (area where no bacteria were growing).

🧠 Exam Technique: The "Both Parts" Trap

This is a 1-mark question that requires both the correct letter and the correct biological reason. If you only name B without stating that it has the smallest clear zone or kills the least bacteria, you score 0 marks.

💡 Key Knowledge

Antibiotics diffuse through the agar gel. The larger the clear zone (zone of inhibition) around the filter disc, the more effective that antibiotic is at killing or preventing the growth of that bacterium.

❌ Common Errors

Selecting antibiotic C (confusing least effective with most effective) or just stating "it's the smallest" without specifying the clear zone or bacteria killed.

❌ Common Errors

• Forgetting to divide diameter by 2: Squaring the diameter ( 2.2² ) instead of radius gives an answer 4× too large.
• Unit mismatch: Giving the number calculated from cm but writing mm² , or omitting units altogether.
• Subtracting the paper disc: The mark scheme explicitly ignores subtracting the disc area—keep it simple and calculate the whole circular area!

🧠 Exam Technique: Error Carried Forward (ECF)

Even if you misread your ruler and get an incorrect radius, you can still gain 4 out of 5 marks if you write down the formula, substitute correctly, complete the math, and write the matching area unit!

✅ Acceptable Suggestions (Any One)

• Repeat the experiment and calculate a mean (or repeat to identify and eliminate anomalies).
• Use a negative control disc (e.g. a sterile paper disc soaked in sterile water / disc with no antibiotic) to prove the paper alone doesn't kill bacteria.
• Test on different types of bacteria to see if the antibiotics are broad-spectrum.

❌ Insufficient Answers

• "Repeat it" – On its own, writing just "repeat" gets 0 marks. You must state why: "repeat and calculate a mean" or "repeat to spot anomalies".
• "Set up a control" – Too vague. You must describe the control: "use a disc soaked in sterile water".