AQA GCSE Biology Biology Paper 1 (Higher), November 2021: Question 2

9 marks · Standard Demand difficulty · Short Answer

Describe aseptic techniques, identify the least effective antibiotic from a petri dish, calculate the area of the inhibition zone, and suggest an improvement to the practical.

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Question

Question text

02 A student investigated the effectiveness of three different antibiotics.

Figure 3 shows how the student set up an agar plate.

Figure 3

The student used aseptic techniques to make sure that only one type of bacterium

was growing on the agar.

02.1 Describe two aseptic techniques the student should have used.

[2 marks]

The student placed the agar plate in an incubator at 25 °C for 48 hours.

Figure 4 shows the agar plate after 48 hours.

Figure 4

02.2 Which antibiotic is the least effective?

Give a reason for your answer.

[1 mark]

Least effective antibiotic

Reason

02.3 Calculate the area where no bacteria were growing for antibiotic C.

Use 𝜋𝜋 = 3.14

Give the unit.

[5 marks]

Area = Unit

02.4 Suggest one way the student could improve the investigation.

[1 mark]

Mark scheme

Show the mark scheme

AO /

Question Answers Extra information Mark

Spec. Ref.

02.1 ignore ‘clean’ unqualified AO1

any two from: ignore wash hands 2 4.1.1.6

• sterilise equipment / surfaces allow description of how to RPA2

(before use) sterilise equipment

• (use) sterilised agar allow description of how to

sterilise agar

• secure lid of the Petri dish

with (adhesive) tape

• only lift lid of Petri dish a little

(when setting up plate)

or lift lid of Petri dish at an

angle (when setting up plate)

02.2 B 1 AO3

4.1.1.6

and RPA2

it kills the fewest bacteria

it has the smallest area where allow it has the smallest clear /

no bacteria were growing white area

02.3 an incorrect answer for one step AO2

does not prevent allocation of 4.1.1.6

marks for subsequent steps RPA 2

ignore calculation and

subtraction of filter paper disc

area from total area

(correct measurement) 1

r = 1.1 (cm) allow d = 2.2 (cm)

or or

r = 11 (mm) d = 22 (mm)

allow a tolerance of ±1 mm

(recall of the equation) 1

𝜋𝜋r2

(calculation/substitution)

3.14 x 1.12 allow correct calculation / 1

or substitution using an incorrect

3.14 x 112 measurement

= 3.799(4) (from 3.14 x 1.12) allow 3.8 1

= 379.9(4) (from 3.14 x 112) allow 380

correct unit 9

(3.7994) cm2 do not accept unit with no 1

or attempt at working / answer

(379.94) mm2

02.4 any one from: 1 AO3

• repeat and calculate a mean 4.1.1.6

• repeat and eliminate RPA2

anomalies

• use a control disc allow description of control disc

e.g. disc with water / nothing

ignore set up a control

• use different types of bacteria

Total 9

How to answer it

Investigating Antibiotics and Zones of Inhibition

What this question tests

Required Practical 2 (RPA 2): Culturing Microorganisms & Testing Antimicrobials

Aseptic procedures: Specific techniques used to prevent contamination by unwanted environmental pathogens.
Interpreting agar plates: Recognising clear zones of inhibition and relating diameter/area to antimicrobial potency.
Mathematical skills: Measuring printed diagrams using a ruler, recalling and substituting into the area of a circle formula (Area = πr²), and stating correct scientific units (cm² or mm²).
Experimental design: Identifying meaningful improvements such as repeats for reliability and valid control treatments.

Question 02.1

Aseptic Techniques

Describe two aseptic techniques the student should have used. [2 marks]

✅ Correct Answers (Choose Any Two)

Sterilise equipment / work surfaces before use (e.g. wiping the bench with disinfectant, passing an inoculating loop through a blue Bunsen flame).
Use pre-sterilised nutrient agar / sterilise agar in an autoclave.
Tape the lid of the Petri dish securely (with adhesive tape).
Minimise opening the lid: Only lift the Petri dish lid a small amount or open it at an angle when inoculating.

💡 Key Knowledge

Why do we use aseptic technique?

To prevent contamination by unwanted airborne bacteria or fungi.
Contamination can ruin the experiment (competing with target bacteria) and poses a safety risk if harmless cultures are contaminated with harmful human pathogens.
Note: The dish is incubated at 25 °C in schools to reduce the risk of growing harmful human pathogens (which thrive at body temperature, 37 °C).

🧠 Exam Technique

Always specify how or when an action is performed. Never write single vague words. Say "sterilise equipment before use" rather than just "keep it clean".

❌ Common Errors

Writing simply "clean equipment" (Mark scheme specifically notes: ignore 'clean' unqualified).
Writing "wash hands" (this is good hygiene, but the mark scheme explicitly ignores washing hands as a specific culturing aseptic step).
Saying "tape the lid completely shut all the way round" — this prevents oxygen entry and encourages dangerous anaerobic bacteria!

Mark Allocation: 1 mark for each valid, distinct aseptic technique described. Max 2 marks (AO1).

Question 02.2

Identifying Antibiotic Effectiveness

Which antibiotic is the least effective? Give a reason for your answer. [1 mark]

✅ Correct Answer

Antibiotic: B

Reason: It has the smallest clear area (smallest zone of inhibition) / it kills the fewest bacteria.

💡 Key Knowledge

The clear region surrounding a disc is called the zone of inhibition. The antibiotic diffuses outwards through the agar:

Larger clear zone: Highly effective (bacteria killed/inhibited at lower concentrations).
Smaller clear zone: Less effective.
No clear zone: Bacteria are completely resistant to that antibiotic.

❌ Common Errors

Naming B without giving a comparison or reason (both antibiotic and reason are required for this 1 mark).
Confusing "least effective" with "most effective" and accidentally choosing C.

Mark Allocation: 1 mark for correctly identifying B accompanied by an acceptable comparative reason (AO3).

Question 02.3

Calculating the Zone of Inhibition

Calculate the area where no bacteria were growing for antibiotic C. Use π = 3.14. Give the unit. [5 marks]

Examiner Diagram Measurement Note:
Using a standard millimetre ruler on the exam paper for the clear circular zone around disc C:
• Measured Total Diameter = 22 mm (or 2.2 cm) [allowed tolerance: ±1 mm]
• Measured Radius (r) = 11 mm (or 1.1 cm)

📐 Step-by-Step Calculation Breakdown

Step 1: Measure the radius (or diameter) [1 Mark]
Measure across the centre of the clear zone for disc C.
Diameter = 22 mm → Radius r = 11 mm (or r = 1.1 cm ).
Step 2: Recall the formula for the area of a circle [1 Mark]
Area = πr²
Step 3: Substitute your values using π = 3.14 [1 Mark]
Using millimetres: Area = 3.14 × (11)² = 3.14 × 121
OR Using centimetres: Area = 3.14 × (1.1)² = 3.14 × 1.21
Step 4: Calculate the final value [1 Mark]
In mm: 379.94 (or rounded to 380 )
OR In cm: 3.7994 (or rounded to 3.8 )
Step 5: Provide the matching unit [1 Mark]
If you measured in mm → mm²
If you measured in cm → cm²

🧠 Exam Technique & Error Carry Forward

Always show your formula and working: If you misread your ruler (e.g. 12 mm instead of 11 mm), you lose Step 1, but you can still score the remaining 4 marks through Error Carried Forward (ecf)!
Value required: You must use the value given in the question ( π = 3.14 ), not the calculator's π button, to avoid small rounding discrepancies.

❌ Common Calculation Traps

Using diameter instead of radius: Squaring 22 mm directly ( 3.14 × 22² ) is the single most common student mistake. Always halve the diameter first!
Unit mismatch: Writing a numerical answer in mm² but stating cm² as the unit, or writing cm instead of area units cm² .
Note: You do not need to subtract the small paper disc area unless the question explicitly asks you to.

Mark Breakdown: [1] Radius measurement • [1] Recall πr² • [1] Correct substitution • [1] Correct numerical calculation • [1] Correct unit matching answer (AO2).

Question 02.4

Improving the Investigation

Suggest one way the student could improve the investigation. [1 mark]

✅ Correct Answers (Choose One)

Repeat the experiment and calculate a mean.
Repeat the experiment and identify / eliminate anomalies.
Add a control disc soaked in sterile water (to prove that filter paper alone does not kill bacteria).
Test on different species / types of bacteria.

🧠 Exam Technique: The "Repeat" Rule

Never write just the single word "Repeat" on GCSE exam papers. Examiners will not award the mark.

Always complete the full phrase: "Repeat and calculate a mean" or "Repeat to identify anomalies".

❌ Common Errors

Writing "set up a control" without qualification (Must explain what the control is, e.g. "disc soaked in sterile water").
Suggesting changes that alter the premise entirely (e.g. "use different antibiotics" — the aim was to test these three).

Mark Allocation: 1 mark for any valid scientific improvement (AO3).

Topics

Biology · Required Practicals · B1: Cell Biology · B3: Infection and Response · Required Practicals

Question and mark scheme from the AQA GCSE Biology examination, Biology Paper 1 (Higher), November 2021. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.