AQA AS Level Biology Paper 2, June 2019: Question 2

✅ Correct Answer

• Point 1: Increases / more oxygen dissociation / unloading (or releases more O₂)
• Point 2: Decreases haemoglobin's affinity for O₂ by decreasing blood pH / increasing acidity.

💡 Key Knowledge

• High respiratory rates produce high levels of CO₂, dissolving to form carbonic acid (lowering blood pH).
• This changes the tertiary structure of haemoglobin, reducing its affinity for oxygen and promoting off-loading in actively respiring tissues.

🧠 Exam Technique

• Be precise with terminology: use terms like affinity and dissociation/unloading rather than vague phrases like "let go of oxygen".
• Ensure you link the physical factor (CO₂ / acidity) directly to haemoglobin's binding capability.

❌ Common Errors

• Stating that CO₂ binds to the "active site" of haemoglobin (haemoglobin is a transport protein with binding sites/haem groups, not active sites which belong to enzymes).

✅ Correct Answer

• Point 1: Myoglobin has a higher affinity for O₂ than haemoglobin (or dissociates oxygen less readily / associates more readily).
• Point 2: Allows aerobic respiration to continue when diving at low partial pressures of O₂, acts as an oxygen store, or delays anaerobic respiration/lactate production.

💡 Key Knowledge

• Myoglobin's dissociation curve is shifted far to the left of haemoglobin. This means it stays saturated with oxygen even when partial pressures of oxygen (pO₂) drop very low in resting or exercising muscle tissue during a dive.

🧠 Exam Technique

• Always refer directly to the visual evidence in the graph (e.g., noting that the curve is positioned to the left, showing high saturation at low pO₂).
• Connect the molecular adaptation (high affinity/storage) to the survival benefit for the organism (delaying harmful anaerobic respiration).

❌ Common Errors

• Confusing the curves for myoglobin and haemoglobin. Students often mistakenly describe haemoglobin as having the higher affinity curve.

✅ Correct Answer

• Final Answer: 10.8 to 11 minutes (or 10 minutes and 48 seconds ).

📐 Step-by-Step Calculation

1. Step 1: Calculate total oxygen used per minute by the whole seal.
   Rate = 5.2 cm³ min⁻¹ per kg × 190 kg = 988 cm³ min⁻¹
2. Step 2: Calculate maximum dive time. Total oxygen / oxygen use rate.
   Time = 1.07 × 10⁴ cm³ ÷ 988 cm³ min⁻¹ = 10.829... minutes
3. Step 3: Convert fractional minutes to seconds if needed.
   0.829 minutes × 60 seconds = ~49.8 seconds (Accepting standard range 10.8 to 11 mins or 10m 48s ).

🧠 Exam Technique

• Show all working out clearly. If your final arithmetic slips, intermediate scoring steps (like calculating oxygen use per kg correctly) can still secure 1 mark.
• Pay close attention to standard form numbers ( 1.07 × 10⁴ equals 10,700).

❌ Common Errors

• Forgetting to multiply the oxygen consumption rate ( 5.2 cm³ ) by the seal's total body mass ( 190 kg ), leading to a massive miscalculation of total oxygen depletion rates.
• Incorrectly converting decimal minutes into seconds (e.g., writing 10.48 minutes instead of converting 0.83 of a minute correctly into seconds).