AQA AS Level Biology Paper 2, June 2025: Question 7

✅ Correct Answer

Any three from the structures labelled in Figure 11:

• Flagellum (or flagella)
• Plasmid (or plasmids)
• Capsule (slime layer)

💡 Key Knowledge

Universal to all bacteria:

• 70S Ribosomes
• Cell-surface membrane
• Cell wall (murein/peptidoglycan)
• Circular DNA molecule (nucleoid)
• Cytoplasm

Plasmids, capsules, and flagella are facultative (optional) adaptations.

🧠 Exam Technique

Always inspect the provided diagram carefully. The question asks for structures "shown in Figure 11". Do not name features absent from the diagram (e.g. pili or endospores), as examiners will reject items outside the provided figure.

❌ Common Errors

• Listing cell wall or circular DNA (all bacterial species possess these).
• Writing "nucleus" or "mitochondria" (prokaryotes lack membrane-bound organelles altogether).

✅ Correct Answer

1. The toxin has a specific tertiary structure (or specific 3D shape). 1 Mark
2. Its shape is complementary to the binding site/receptor present only on the cell-surface membrane of epithelial cells in the ileum. 1 Mark

❌ Common Examiner Traps (Automatic Loss of Mark)

Reject references to:

• "Active site"
• "Enzyme" / "Substrate"
• "Induced fit model"
• "Antibody / Antigen"

Examiner Note: A toxin binding a membrane receptor is not an enzymatic catalytic event. Using enzyme vocabulary here demonstrates poor conceptual understanding.

💡 Key Knowledge

Proteins achieve binding specificity via unique sequences of amino acids folding into precise tertiary structures held together by hydrogen bonds, ionic bonds, and disulfide bridges. Receptors are typically transmembrane glycoproteins or glycolipids found exclusively on specific target cell lineages.

🧠 Exam Technique

Never write just "shape" or "they fit together like a lock and key". Use the precise A-Level terminology: specific tertiary structure and complementary shape.

✅ Correct Answer

1. The accumulation of chloride ions (Cl⁻) lowers the water potential of the lumen / ileum (making it more negative)
   OR loss of Cl⁻ increases the water potential inside epithelial cells. 1 Mark
2. Water moves out of the epithelial cells and into the lumen / ileum down a water potential gradient by osmosis. 1 Mark

💡 Key Knowledge: Osmotic Flow in Cholera

Pure water has a water potential (Ψ) of 0 kPa. Adding solutes (like Cl⁻) always lowers Ψ (making it more negative). Water constantly moves from a region of higher (less negative) Ψ to a region of lower (more negative) Ψ across a partially permeable membrane via osmosis.

🧠 Exam Technique

• You may write the symbol Ψ instead of "water potential", but never use abbreviations like "wp" or "WP" (these are strictly not accepted by AQA examiners).
• Always state the name of the process: osmosis. Saying "water diffuses" or "water moves down a concentration gradient" without specifying osmosis often forfeits MP2.

❌ Common Errors

• Stating that chloride ions "increase water potential" (solutes always lower water potential).
• Forgetting to state direction: water leaves the cells/blood and enters the ileum/lumen.
• Confusing water concentration with water potential. Stick to water potential terminology.

Final Answer: Difference in the number of deaths = 108 2 Marks
Partial credit: If final answer is incorrect, 1 mark is awarded for either showing 1 260 or 1 152, or giving all 4 correct graph readings (350 000, 0.36, 90 000, 1.28), or showing correct formula with misread values.

🧠 Exam Technique

Always write down each intermediate number ( 1260 and 1152 ) in the working space. If you make an arithmetic slip on the final subtraction, you still secure 1 mark for the working.

❌ Common Errors

• Forgetting to divide the percentage by 100 (multiplying directly by 0.36 instead of 0.0036).
• Misreading the right-hand axis (e.g. assuming each square is 0.02 or 0.1 rather than 0.04).

✅ Correct Answer

Any one valid reason:

• The data only records reported cases / not all cases are reported. 1 Mark
• Misdiagnosis of cholera / cholera was not diagnosed (e.g. recorded as general gastroenteritis or dehydration). 1 Mark
• People die before reaching medical help / without being diagnosed in clinical facilities. 1 Mark

🧠 Exam Technique: "Using the information..."

Look at the wording of Figure 12: "Number of reported cases". Whenever the word "reported" appears in health statistics, it is a deliberate prompt indicating that non-reporting or under-reporting is a key limitation.