AQA AS Level Biology Paper 2, June 2025: Question 8

5 marks · Medium difficulty · Short Answer

Explain why infections are more likely after HIV infection based on helper T cell data, and suggest and justify an aseptic technique during bacterial culture.

Practise this question

Question

Question 08 contains Table 4 showing time after infection in years (0 to 12), HIV RNA particles per cm cubed of blood (0 to 135 000), and TH cell count per mm cubed of blood (dropping from 1000 at year 0 to 150 at year 11 and 5 at year 12). Part 08.1 asks to use data and knowledge of the immune response to explain why other infections are more likely to develop after 11 years (3 marks). Part 08.2 describes steps taken by a trainee microbiologist culturing Streptococcus pneumoniae and asks to suggest and justify one other aseptic action taken (2 marks).
Question text

08 Table 4 shows the changes in the number of HIV RNA particles and helper T cells

(TH cells) in a person infected with HIV who was not receiving any treatment.

Table 4

Time after HIV RNA particles per TH cell count / cells per

infection / years cm3 of blood mm3 of blood

00 1000

1 80 000 305

2 20 000 854

6 22 000 812

10 70 000 597

11 120 000 150

12 135 000 5

08.1 A scientist concluded that other infections are more likely to develop in this person

after 11 years.

Use the data in Table 4 and your own knowledge of the immune response to explain

the scientist’s conclusion.

[3 marks]

One type of infection is pneumonia, caused by the bacterium

Streptococcus pneumoniae.

A trainee microbiologist cultured S. pneumoniae on an agar plate.

The trainee microbiologist:

• disinfected the laboratory bench

*28•*used a sterile pipette to transfer S. pneumoniae from a broth bottle to an agar plate

• took a glass spreader and spread the S. pneumoniae across the agar plate

• sealed the lid with four strips of tape.

08.2 Suggest one other action the trainee microbiologist took during this method to ensure

the technique was aseptic.

Justify why this action was needed.

Do not refer to gloves, masks or safety glasses in your answer.

[2 marks]

Action

Justification

Mark scheme

Show the mark scheme Mark scheme for 08.1 lists three points: 1. Large decrease or low number of helper T cells after 11 years; 2. Fewer B cells divide by mitosis or undergo clonal selection/expansion; 3. Fewer plasma cells produce antibodies (3 marks). For 08.2, pairs of actions and matching justifications are given (2 marks max): lifting lid at an angle, flaming the spreader/neck of bottle, working close to a Bunsen flame to create convection currents, or placing used tools into disinfectant.

Question Marking Guidance Mark Comments

1. (After 11 years) there is a large decrease in the 1. Allow other words

number of helper T-cells for large eg

significant/big

OR

(After 11 years) there are few/low number of

helper T-cells;

08.1 2. (So) fewer B cells divide by mitosis (3 x 2.and 3. Accept ‘no’

AO3) for fewer

OR 2. Accept clonal

expansion

(So) fewer B cells undergo clonal selection;

3. (So) fewer plasma cells produce antibodies;

Mark as pairs 1 and 2 or 3 and 4 or 5 and 6 or 7

and 8. Justification must match action.

1. Accept lift lid

1. Lift lid of (agar) plate at an angle; slightly OR keep lid

over plate OR open lid

2. To prevent contaminating the dish of petri dish as little as

possible

OR

Prevent spread of bacteria outside the dish;

3. Flame the spreader 3. Accept loop for

spreader

OR

Use a sterile spreader Ignore ‘wash hands’

OR

Flame the neck of the container of the culture;

4. To maintain a pure culture

2 max

OR

08.2 (2 x

AO3)

To prevent contaminating the dish

OR

Prevent spread of bacteria outside the dish;

5. Work close to a Bunsen burner/flame;

6. To create a convection current to carry other

contaminants away/prevent them landing on the

dish

OR

To create upward air movement to carry other

contaminants away/prevent them landing on the

dish;

7. Place used pipette/spreader into (container of)

disinfectant;

8. To ensure no transfer of bacteria to bench;

How to answer it

HIV Progression & Aseptic Culture Techniques

📋 What this question tests

This question assesses your ability to combine quantitative data analysis with your theoretical knowledge of the humoral immune response, alongside practical application of aseptic microbiological techniques:

  • Interpreting clinical data (HIV RNA load vs TH cell count over time).
  • Explaining the link between Helper T cells (TH), clonal selection/expansion of B cells, and antibody production by plasma cells.
  • Proposing standard aseptic microbiological precautions and providing clear biological justifications (preventing contamination vs maintaining pure cultures).
Question 08.1 3 Marks (AO3)

HIV Infection & Vulnerability to Secondary Diseases

Data interpretation and mechanism of secondary infection development

✅ Correct Answer / Marking Points

  1. Mark 1: (After 11 years) there is a large decrease / low number of helper T cells (TH cells).
  2. Mark 2: (So) fewer B cells divide by mitosis OR fewer B cells undergo clonal selection / clonal expansion.
  3. Mark 3: (So) fewer plasma cells produce antibodies.

💡 Key Knowledge

  • Role of TH Cells: Helper T cells release cytokines/chemical signals that stimulate specific B cells to divide by mitosis.
  • Humoral Response: Clonal selection and clonal expansion lead to plasma cells, which secrete antibodies specific to foreign antigens.
  • AIDS: Once TH cell count falls below a critical threshold (e.g. from 1000 down to 5 cells mm⁻³ at year 12), opportunistic pathogens like Streptococcus pneumoniae cannot be effectively destroyed.

🧠 Exam Technique

  • Read the trigger phrase: "Use data in Table 4 and your own knowledge". You must start by stating the observation from the table before chaining your biological explanation.
  • Quantify / describe the trend: Saying "T cells change" earns 0 marks. You must state they decrease significantly / reach a very low level after year 11.
  • Trace the cascade fully: TH cells ↓ → B cell mitosis / clonal expansion ↓ → plasma cells produce fewer antibodies.

❌ Common Errors

  • Confusing cell types: Stating that "T cells produce antibodies" (a major biological error; only plasma cells derived from B cells secrete antibodies).
  • Vague data comments: Writing "HIV increases" without relating it to why other infections happen (the mark requires focusing on the helper T cell drop).
  • Stopping short: Stating "fewer antibodies are produced" without explaining why (missing B cell division/mitosis or plasma cells).
Examiner Insight: For Mark 2 & 3, examiners accept "no" instead of "fewer" (e.g., "no antibodies produced"). Top students systematically linked the fall in TH numbers directly to the failure of B cell clonal selection and consequent failure of antibody secretion.
Question 08.2 2 Marks (AO3)

Aseptic Culturing of Streptococcus pneumoniae

Identifying an additional aseptic action and providing its biological justification

✅ Paired Correct Answers (Choose ONE pair)

  • Action: Lift the lid of the agar plate at an angle / keep lid over plate / open lid as little as possible.
    Justification: To prevent contaminating the dish OR prevent airborne microbes entering OR prevent spread of bacteria outside the dish.
  • Action: Flame the spreader / use a sterile spreader / flame the neck of the culture container.
    Justification: To maintain a pure culture / prevent contaminating the dish / kill unwanted bacteria.
  • Action: Work close to a Bunsen burner flame.
    Justification: Creates an upward convection current that carries airborne contaminants away from the petri dish.
  • Action: Place used pipette/spreader into a container of disinfectant.
    Justification: To prevent transfer of bacteria to the laboratory bench / prevent contamination of surroundings.

💡 Key Knowledge

  • Aseptic technique has two distinct purposes:
    1. Preventing contamination of the culture with unwanted environmental organisms (maintaining purity).
    2. Preventing contamination of the worker and environment with the cultured pathogen (safety).
  • Bunsen convection: The hot flame heats the surrounding air, creating an updraft that keeps falling microbes away from open plates.

🧠 Exam Technique

  • Notice constraints: The prompt explicitly states: "Do not refer to gloves, masks or safety glasses." Offering PPE immediately scores 0.
  • Already stated actions: Do not repeat what the trainee already did (disinfecting bench, sterile pipette, glass spreader, four strips of tape).
  • Marked in strict pairs: Your justification must accurately match your specific action (e.g. flaming the spreader matches killing bacteria on the tool, not convection currents).

❌ Common Errors

  • Ignoring instructions: Writing "wear gloves" or "wear a face mask" despite explicit prohibition.
  • Ignoring "wash hands": Mark schemes explicitly state "Ignore wash hands" as an aseptic culturing action here.
  • Incomplete justification: Saying just "for hygiene" or "for safety" without specifying the biological reason: maintaining a pure culture or preventing airborne microbe entry.
Examiner Insight: This question is marked as matching pairs (1 & 2, 3 & 4, 5 & 6, or 7 & 8). A correct justification with an unmatched or incorrect action receives 0 marks.

Topics

Biology · Required Practicals · 3.2 Cells · AS practicals (1–6)

Question and mark scheme from the AQA AS Level Biology examination, Paper 2, June 2025. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.