AQA GCSE Biology Biology Paper 1 (Higher), June 2023: Question 3

✅ Mark Scheme Answer

Bacteria release / produce toxins (or poisons).

❌ Common Errors

• Writing just "toxins" without stating they are produced or released.
• Claiming that bacteria “damage cells by invading them” — this confuses bacterial toxins with viral reproduction.

💡 Key Knowledge: Two-Part Logical Chain

1. Immune response in the animal:

• Vaccine introduces dead/inactivated pathogen or specific antigens.
• White blood cells (lymphocytes) produce specific antibodies.
• Memory cells remain; if infected, antibodies are produced rapidly and in larger quantities to kill Salmonella.

2. Preventing human food poisoning:

• Bacteria are killed, so fewer/no pathogens enter meat, milk, or eggs.
• Fewer bacteria are eaten by humans, meaning numbers never reach infectious levels and fewer toxins are produced in humans.

🧠 Exam Technique: Level 2 Criteria

This is a Level of Response question:

• Level 2 (3–4 marks): You must provide a clear, logical link connecting the animal’s internal immune response directly to why humans do not get infected.
• Level 1 (1–2 marks): Giving disconnected facts (e.g. describing how vaccines make antibodies, but completely omitting what happens to food products).

✅ Any TWO of the following (1 mark each):

• Disinfect bench / work surfaces / hands.
• Sterilise Petri dish or agar culture medium before use (e.g. autoclaving).
• Pass inoculating loop or forceps through a Bunsen flame.
• Work near a Bunsen flame (creates convection currents) or in a fume hood.
• Tilt the Petri dish lid rather than fully removing it.
• Secure Petri dish lid with adhesive tape (not fully sealed).

❌ Common Misconceptions

• Vague answers like "wash hands" or "clean table" — you must say disinfect or sterilise.
• Writing "sterilise equipment" without naming the specific item or method.
• Stating “seal the dish completely with tape” (this promotes anaerobic pathogen growth and loses marks).

✅ Mark Scheme Breakdown

• Mark 1: 37 °C is human body temperature.
• Mark 2: Salmonella bacteria grow best / faster / replicate rapidly at 37 °C.
• Mark 3: 25 °C reduces or prevents the growth of bacteria / pathogens that are harmful to humans.

🧠 Exam Technique

Always address both temperatures mentioned in the prompt:

• Link 37 °C to optimal enzyme activity and body conditions for pathogenic bacteria.
• Link 25 °C directly to safety precautions in school labs (inhibiting human pathogens).

✅ Mark Scheme Answer

Acts as a control / provides a comparison to prove that any bacterial death is caused by the antibiotic and not simply by paper itself.

❌ Critical Trap

Do NOT write "control variable" ! A control variable is a factor kept constant. The blank disc is an experimental control (a baseline comparison).

✅ Mark Scheme Answer

They killed the most bacteria / produced the largest zone of inhibition / stopped the most bacteria from growing.

❌ Incomplete Answers

• Do not just write "largest clear area" without explaining that bacteria were killed or inhibited.
• Do not single out just B (e.g. “B was the biggest”); the question asks why either A or B could be chosen.

✅ Mark Scheme Answer

Measure the diameter / radius of each clear zone OR calculate / measure the area of each clear zone.

📐 Math in Biology: Area of a Zone

To calculate the zone of inhibition accurately:

1. Measure diameter (d) across two perpendicular axes using a millimetre ruler.
2. Calculate radius: r = d / 2 .
3. Use the formula: Area = πr² .

✅ Mark Scheme Answer

The bacteria developed resistance / became resistant to antibiotic B.

❌ Absolute Taboo in Biology

NEVER say bacteria became "immune" . Bacteria do NOT have immune systems or antibodies. Only host organisms become immune; bacteria evolve resistance.

✅ Step-by-Step Marking Points

1. Mark 1: Water enters the bacterial cell...
2. Mark 2: ...by osmosis (down a water concentration gradient).
3. Mark 3: The damaged / weakened cell wall cannot withstand the internal water / turgor pressure.

💡 The Role of the Bacterial Cell Wall

Normally, bacteria take up water by osmosis, but the rigid peptidoglycan cell wall pushes back, providing structural support and preventing lysis.

When antibiotics inhibit cell wall synthesis or damage its structure, internal pressure increases until the cell membrane ruptures (bursts).