AQA AS Level Biology Paper 2, June 2024: Question 8

✅ Correct Answer

• Unbound secondary antibody with enzyme remains (is not washed away). [1 mark]
• Enzyme reacts with substrate, converting it to a coloured product (even without target antibodies present). [1 mark]

💡 Key Knowledge

The ELISA in Figure 5 is an indirect ELISA:

• Step 3: Unbound primary antibodies are washed away. Second antibody (carrying enzyme) binds to primary antibody.
• Step 4: Essential to wash away any unbound secondary enzyme-linked antibodies.
• If unbound enzyme stays in the dish, it will hydrolyse the substrate, causing a colorimetric signal regardless of whether the patient actually has B. burgdorferi antibodies.

🧠 Exam Technique

Always state which antibody carries the enzyme. Don't simply write "the antibody remains". Specify that it is the second antibody or the antibody with enzyme attached.

❌ Common Errors

Confusing the Step 3 wash with the Step 4 wash. In Step 4, the primary antibody has already bound; the wash is solely to clear free, unbound enzyme-linked secondary antibodies.

✅ Correct Answer

Any one of the following:

• Antibodies produced against syphilis and B. burgdorferi have a similar structure / similar antigen-binding sites. [1 mark]
• Antibodies produced against syphilis are complementary to the antigens of B. burgdorferi. [1 mark]
• Antigens of the syphilis bacterium and B. burgdorferi are similar in structure. [1 mark]

💡 Key Knowledge

Antibody-antigen binding depends entirely on tertiary structure complementarity.

Both Borrelia burgdorferi and Treponema pallidum (syphilis) are spirochaete bacteria. They share evolutionary history and may express surface antigens with very similar tertiary epitopes. Syphilis antibodies can therefore bind to the immobilised B. burgdorferi antigens via molecular cross-reactivity.

❌ Common Errors

Writing that "the diseases are similar" or that "the bacteria are the same". Marks require reference to antibodies or antigens having a similar structure, or syphilis antibodies being complementary to B. burgdorferi antigens.

✅ Correct Answer

• Low concentration of antibodies in the blood / no antibodies yet produced (against B. burgdorferi). [1 mark]
• Because it is only the primary immune response / plasma cells have not yet developed / B cells have not yet undergone clonal selection and divided into plasma cells. [1 mark]

💡 Key Knowledge

The primary immune response has a pronounced latent period (lag phase):

1. Antigen presentation by APCs.
2. Selection and activation of specific T helper and B cells.
3. Clonal expansion (mitosis) and differentiation into plasma cells.
4. Synthesis and secretion of antibodies.

Within the first 14 days, the antibody titre is below the minimum detection threshold of the ELISA.

🧠 Exam Technique

Ensure you link the observation (low antibody titre) directly to the cellular mechanism (primary response / plasma cells not yet formed / B cells not cloned). Both elements are needed for the 2 marks.

❌ Common Errors

Claiming "memory cells haven't produced antibodies". Memory cells are responsible for the secondary response; in an initial infection, antibodies are secreted by freshly differentiated plasma cells.

✅ Correct Answer (Award up to 4 marks max)

Evidence Supporting the Conclusions:

• For symptom occurrence: A higher percentage of PTLDS patients experienced all five symptoms compared to the control group (e.g., fatigue 100% vs 57%, joint pain 96% vs 32%). [1 mark]
• For symptom intensity: There is a statistically significant difference / increase in intensity for all symptoms in the PTLDS group, because all P values are < 0.05 (less than 5% probability that the difference in intensity is due to chance). [1 mark]
• Nuance in significance: The difference in intensity is most significant for fatigue, joint pain, and muscle pain (P < 0.001) compared to depression and fever (P < 0.005). [1 mark]

Evidence Questioning / Limiting the Conclusions:

• Duration limitation: Investigation was only carried out for 2 weeks, which is very short considering PTLDS symptoms persist for 6 months or longer. [1 mark]
• Subjectivity: Assessing "intensity" of symptoms is subjective / difficult to measure objectively and relies on self-reporting. [1 mark]

🧠 Critical Evaluation Rules

• Never just say "the results are significant": The mark scheme explicitly states: Reject 'results are significant'. You must say the difference in intensity is significant!
• Address BOTH parts of the conclusion:
  Part 1: More patients experienced symptoms → Back this up with percentages from Table 3.
  Part 2: Greater intensity → Back this up with P-values and statistical significance.
• Look for experimental weaknesses: Ask yourself: How long was the trial? (Only 2 weeks). How was the data gathered? (Self-reported rating scale → subjective).

📐 Interpreting the P-Values

Since all P < 0.05, we reject the null hypothesis of no difference in intensity between the two groups.

❌ Common Errors

• Confusing the percentages (which show how many people had the symptom) with the P-value (which evaluates the intensity of the symptom).
• Stating that "P < 0.05 proves the hypothesis is true". In science, statistics never prove; they show that the probability of differences arising by chance is extremely low.
• Failing to provide counter-arguments (e.g. failing to mention the 2-week duration or subjectivity). An "Evaluate" question always demands both sides!