AQA GCSE Biology Biology Paper 2 (Foundation), November 2021: Question 3

✅ Correct Answer

• Carbon dioxide 1 mark
• Water 1 mark

💡 Key Knowledge

Word equation:
carbon dioxide + water → glucose + oxygen

Algae are aquatic photosynthetic organisms (protists/plants) that take in dissolved CO₂ and H₂O directly from their surroundings.

❌ Common Errors

• Oxygen: This is a product, not a reactant.
• Starch: Glucose is converted to starch for storage; it is not taken in for photosynthesis.
• Nitrogen: Used to make amino acids, but not a reactant in photosynthesis.

✅ Correct Answer

• Light 1 mark

💡 Key Knowledge

Photosynthesis is an endothermic reaction. Chlorophyll in chloroplasts absorbs light energy to drive the chemical conversion of carbon dioxide and water into glucose.

🧠 Exam Technique

Do not confuse chemical substances with the energy source. Mineral ions, water, and proteins are material substances, not sources of radiant energy.

✅ Correct Answer

• The standard upright stepped pyramid (Box 3). 1 mark

💡 Golden Rule of Biomass

A pyramid of biomass is ALWAYS pyramid-shaped (broadest at the base and progressively narrower at each trophic level).

Only ~10% of biomass is transferred from one level to the next because of respiration, excretion, and unconsumed parts. Unlike pyramids of numbers, it can never be inverted.

Step 1: Read graph at 1960 2.3 million tonnes (allow 2.25–2.30) 1 mark

Step 2: Read graph at 1970 0.5 million tonnes (allow 0.50–0.55)

Step 3: Calculate difference 2.3 - 0.5 = 1.8 million tonnes

Step 4: Substitute into formula (1.8 ÷ 2.3) × 100 1 mark

Step 5: Unrounded result 78.2608...% 1 mark

Step 6: Round to nearest whole number 78% 1 mark

❌ Common Calculation Traps

• Dividing by the wrong year: Students often divide by the final year (0.5) instead of the initial year (2.3). The denominator must be the original value (1960).
• Forgetting to round: Leaving the answer as 78.26% or 78.3% loses the 4th mark. Always check the required format ("nearest whole number").
• Misreading grid lines: Each small square on the y-axis represents 0.05 million tonnes.

🧠 Exam Technique

Even if your graph reading is slightly inaccurate, write down your working clearly! Examiners award error carried forward (ECF) marks for correct substitution and correct rounding based on your values.

✅ Mark Scheme Breakdown

• Trend: Biomass increased 1 mark
• Data citation (any one):
  • From 0.1 to 1.8 (million tonnes) 1 mark
  • Increased by 1.7 (million tonnes) 1 mark
  • Increased by 1700% 1 mark

🧠 Top Tip for "Describe Data" Questions

A 2-mark "describe the change with data" question almost always follows this formula:

1. Word description of the trend: (e.g. increased / decreased / stayed level).
2. Quantify with figures: Give the starting value AND the ending value, including the units (million tonnes).

✅ Model Answer

• Smaller / younger / 4-year-old fish are not caught (they escape through the larger mesh holes). 1 mark
• These escaping fish survive to reach breeding age and reproduce (increasing the population). 1 mark

💡 Biological Mechanism

Herring reach sexual maturity at 3–4 years old.

If nets catch juveniles before they can reproduce, the birth rate drops below the death rate, causing population collapse. Regulating mesh size ensures individuals reproduce at least once before they are liable to capture.

❌ Lost Mark Warning

Simply saying "so smaller fish don't die" is not enough for the second mark. You must explicitly link survival to reproduction / breeding / replenishing the population.