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

📐 Step-by-Step Calculation

1. Measure image length (P to Q): Using a ruler on the original print gives approximately 39 mm to 41 mm (accepting 38.5–41.5 mm).
2. Convert mm to µm: Multiply by 1000.
   40 mm × 1000 = 40,000 µm
3. Rearrange formula:
   Actual = Image ÷ Magnification
4. Calculate actual size:
   40,000 µm ÷ 5500 = 7.27 µm

✅ Mark Scheme Breakdown

• 2 marks: Correct final answer within the range of 7 to 7.5 µm [2 marks]
• 1 mark (compensatory):
  • Clear evidence of dividing measured size by 5500.
  • OR correct digits with decimal place error (e.g. 70, 72.7, 0.72).

🧠 Exam Technique

Always measure in millimetres first, then immediately convert to micrometres by multiplying by 1000 before dividing by magnification. Writing the formula and unit conversions on the page guarantees working marks even if a minor ruler slippage occurs.

❌ Common Errors

• Dividing image mm directly by 5500 without converting units, giving 0.0073 (forgetting that 1 mm = 1000 µm).
• Misaligning the ruler between calipers P and Q.

📐 Step-by-Step Calculation

1. Account for dilution:
   21 cells are in 0.004 mm³ of a 10⁻³ diluted sample.
   Concentration in undiluted blood = 21 ÷ 10⁻³ = 21,000 cells per 0.004 mm³.
2. Find cells per 1 mm³:
   21,000 ÷ 0.004 = 5,250,000 cells mm⁻³ ( 5.25 × 10⁶ )
3. Convert volume units (dm³ to mm³):
   1 dm = 100 mm → 1 dm³ = 100³ = 1,000,000 mm³ ( 10⁶ mm³ ).
   Adult blood volume = 4.8 × 10⁶ mm³ .
4. Multiply concentration by total volume:
   5.25 × 10⁶ cells mm⁻³ × 4.8 × 10⁶ mm³ = 2.52 × 10¹³ cells

✅ Mark Scheme Breakdown

• 2 marks: Correct answer of 3 × 10¹³, 2.5 × 10¹³, or 2.52 × 10¹³ in any valid mathematical notation.
• 1 mark (compensatory):
  • Incorrect power of 10, but shows 5250 (or 5.25) in working.
  • OR answer shows the correct starting digits 252 with power of ten errors.

❌ Common Errors & Traps

• Volume conversion trap: Incorrectly assuming 1 dm³ = 1000 mm³. Remember, volume conversion is cubed: (10²)³ = 10⁶ .
• Inverting dilution: Multiplying by 10⁻³ instead of dividing, resulting in an impossible microscopic cell count.

🧠 Top Tip: Biology Sense Check

A typical human body contains trillions of red blood cells. An answer around ~10¹³ makes physiological sense. If your answer is 25,000 or 10⁷, re-check your unit conversions immediately!

✅ Mark Scheme Requirements

• Mark 1: Osmosis does not occur
  (Accept: No net movement / no net flow of water).
• Mark 2: (Red blood) cells do not burst / lyse / shrink
  (Accept: crenation; accept cells do not change size).

💡 Key Knowledge

Red blood cells are animal cells surrounded only by a fragile plasma membrane (no cell wall):

• In hypotonic solution (higher Ψ): Water enters by osmosis down the water potential gradient → cell swells and undergoes osmotic lysis (bursts).
• In hypertonic solution (lower Ψ): Water leaves by osmosis → cell shrivels (crenation).
• Isotonic solution (equal Ψ): Dynamic equilibrium → no net water flux.

✅ 02.4: Donor A to Recipient B (2 Marks)

• Mark 1: Binding / antigen-antibody complex formed between antigen A (on donor RBCs) and anti-A antibodies (in recipient plasma).
• Mark 2: Causes red blood cells to clump / join together.

✅ 02.5: Recipients for Group O Blood (1 Mark)

A, B, AB and O (Accept: all blood groups)

🧠 Exam Technique: Agglutination vs Clotting

Examiners penalise students who conflate immunological agglutination with haemostatic blood clotting:

• Agglutination: Bivalent/multivalent antibodies cross-link antigens on foreign cell surfaces, clumping them together.
• Blood Clotting: An enzymatic cascade producing insoluble fibrin strands to plug a wound.

💡 Donor vs Recipient Perspective

When considering blood transfusions, focus entirely on:

1. What antigens are present on the donor's red blood cells?
2. What antibodies are circulating in the recipient's plasma?

Donor plasma antibodies are greatly diluted upon transfusion and generally ignored at AS level.