AQA GCSE Biology Biology Paper 1 (Foundation), June 2022: Question 6

📐 Step-by-Step Calculation

1. Identify the formula:
   SA:V ratio = Surface Area ÷ Volume
2. Extract data from Table 4 for Jellyfish A:
   Surface area = 3 600 mm²
   Volume = 1 200 mm³
3. Divide to find X (relative to 1):
   X = 3 600 ÷ 1 200 = 3

❌ Common Errors

• Adding units: Ratios are unitless comparisons. The mark scheme explicitly states: do not accept if a unit is given (e.g. "3 mm²" scores 0 for the answer mark).
• Inverting the fraction: Dividing volume by surface area (1 200 ÷ 3 600 = 0.33) rather than surface area by volume.

✅ Correct Answer

As the size of the jellyfish increases, its surface area to volume ratio decreases.

(Alternative accepted: As size decreases, the ratio increases.)

🧠 Exam Technique: Describing Trends

Always use a comparative statement structure: "As [variable X] increases, [variable Y] decreases". Avoid just quoting single figures without describing the overall pattern.

✅ Acceptable Answers (Any One)

• Carbon dioxide (CO₂)
• Glucose / sugar
• Water (H₂O)
• Ions / mineral salts (e.g. sodium ions, potassium ions)
• Amino acids / fatty acids / glycerol

❌ Pitfalls & Exclusions

• Oxygen: The question explicitly says "Do not refer to oxygen in your answer". Mentioning oxygen gains zero.
• Vague food terms: Words like "nutrients" or "food" are ignored because they are too general. Name the specific molecule.

💡 Key Knowledge: Factors Affecting Diffusion

According to Fick's Law and core diffusion principles, the rate is determined by:

• Concentration gradient (difference in oxygen levels inside vs outside)
• Surface area (or surface area to volume ratio)
• Diffusion distance / membrane thickness (thickness of the jellyfish body wall/skin)
• Temperature (higher temperature = more kinetic energy)

❌ Common Errors

• Writing simply "size" unqualified. "Size" is not a factor—it must be qualified as surface area, thickness of surface, or surface area to volume ratio.

✅ Correct Answer

Gills

💡 Context Note

Fish pump water through their mouth and over their gills, where oxygen diffuses into the blood capillaries within gill filaments/lamellae.

✅ Full-Mark Model Explanation (Feature + Reason)

• Large Surface Area:
  The lungs contain millions of tiny alveoli, which provide an enormous surface area for faster diffusion of oxygen and carbon dioxide.
• Short Diffusion Pathway:
  The walls of both the alveoli and the capillaries are extremely thin (one cell thick) and in close contact, meaning gases only diffuse across a tiny distance.
• Steep Concentration Gradient (Blood Flow):
  There is an extensive network of capillaries (rich blood supply) with constantly flowing blood. This rapidly removes oxygen and continuously delivers carbon dioxide, maintaining a steep concentration gradient.
• Steep Concentration Gradient (Ventilation):
  Breathing (ventilation) continually brings in fresh air rich in oxygen and expels air with high carbon dioxide levels, maintaining the steep gradient.

❌ Mistakes That Cap Responses at Level 1 or 2

• "Alveoli have thin cell walls": Animal cells do NOT have cell walls! Say "walls that are one cell thick" or "thin walls". Saying "thin cell wall" loses credit.
• Listing features without explanations: Saying "it has alveoli, thin walls, and capillaries" without explaining why each feature speeds up diffusion will restrict you to Level 1 (1–2 marks).
• Confusing respiration and breathing: Ventilation/breathing moves air into the lungs; respiration is the chemical process releasing energy inside mitochondria.