AQA AS Level Biology Paper 2, June 2022: Question 3

✅ Correct Answers (Any 2)

• Microvilli: Increase surface area for diffusion, facilitated diffusion, active transport, or co-transport. Also provide more room for channel/carrier proteins.
• Mitochondria: Many mitochondria release energy (ATP) via aerobic respiration for active transport.

💡 Key Knowledge

• Link ultrastructure directly to Fick's Law: larger surface area increases the rate of diffusion.
• Always pair a structural description with its functional consequence (e.g., mitochondria producing ATP for active transport).

🧠 Exam Technique

• This is a "describe and explain" command word. You must state what the structure does (describe) and why it aids absorption (explain).
• Accept alternative terms like "brush border" or "folded cell surface membrane".

❌ Common Errors

• Saying mitochondria "create" or "produce" energy (violates the conservation of energy; use "release energy" or "synthesize ATP").
• Vague references to "more absorption" without linking to a specific transport mechanism or protein.

📐 Step-by-Step Calculation

1. Find radius ($r$) and length ($l$) of one microvillus:
   Diameter = 0.1 µm, so radius ($r$) = 0.05 µm. Length ($l$) = 1 µm.
2. Calculate surface area of one microvillus:
   Formula given: 2πrl + πr²
   = 2 × 3.14 × 0.05 × 1 + 3.14 × (0.05)²
   = 0.314 + 0.00785 = 0.32185 µm² (Accept 0.32 to 0.33)
3. Calculate total surface area contributed by 1000 microvilli:
   0.32185 × 1000 = 321.85 µm²
4. Add base surface area without microvilli to get total surface area with microvilli:
   321.85 + 7.85 = 329.7 µm²
   *(Note: If using rounded 0.32 or 0.33, total cell surface area equals 327.85 to 337.85 µm²)*
5. Calculate ratio (With : Without):
   329.7 / 7.85 = 42.0
   Acceptable mark scheme ranges give a final ratio of 40–41 : 1 .

✅ Correct Answer

Ratio: 41 : 1 (or 40 : 1)

Partial credit (1 mark) awarded for correctly calculating the surface area of a single microvillus (0.32–0.33) or the total cell surface area (321–322) if the final ratio is incorrect.

❌ Common Calculation Traps

• Forgetting to add the flat base surface area (7.85 µm²) to the total microvilli area.
• Confusing diameter with radius (using 0.1 instead of 0.05 in the formula).
• Failing to format the final answer in the required : 1 ratio format.

✅ Correct Answers (Mark Scheme Points)

• ZO-1 protein is located on the cell (surface) membrane of ileum cells.
• The antibody has a variable region with a specific tertiary structure that is complementary to the ZO-1 protein.
• The antibody binds/attaches to the ZO-1 protein, forming an antigen-antibody complex.
• A dye, stain, or fluorescent marker is linked to the antibody, allowing the ileum cells to be visualized under an optical microscope.

💡 Key Knowledge

• Monoclonal antibodies are identical antibodies produced from a single clone of hybridoma cells. They target a single, specific epitope (in this case, the ZO-1 protein).
• To make something visible under a light microscope using antibodies, the antibody must be conjugated to a fluorophore or coloured stain.

🧠 Exam Technique

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• Use precise biological terminology: talk about complementary shapes and variable regions, never use "active site" for antibodies (active sites belong to enzymes!).
• Ensure you explicitly state how the visualization happens (mentioning the dye/fluorescent tag linked to the antibody).

❌ Common Errors

• Referring to an antibody "active site" instead of a binding site or variable region.
• Stating that the antibody itself provides colour without mentioning a conjugated dye, stain, or fluorescent marker.