AQA GCSE Biology Biology Paper 2 (Higher), June 2024: Question 2

✅ Mark Scheme Pathways (Give any 3 linked points)

Route 1: Oxygen Pathway

• Lack of oxygen for (aerobic) respiration [1]
• So less / no energy is released [1]
• For microorganisms / decomposers / bacteria / fungi [1]

Route 2: pH / Acidity Pathway

• Low pH denatures enzymes (or reduces enzyme activity) [1]
• So fewer chemical reactions / less respiration / less energy released [1]
• In microorganisms / decomposers / bacteria / fungi [1]

🧠 Exam Technique

• Name the organism: Decay is carried out by microorganisms / decomposers, not the dead plant itself. You must state who is respiring!
• Cause → Effect → Impact: State the physical condition (lack of O₂ or acidic pH) → biological mechanism (denatured enzymes or no aerobic respiration) → consequence (decomposer survival/growth inhibited due to lack of released energy).

❌ Common Errors & Examiner Traps

• "Energy is made / created / produced": Instant mark penalty! Energy cannot be created; it is only released or transferred.
• Saying the dead plants cannot respire: Plants are already dead; the organisms that need oxygen to respire are the decomposers.
• Stating only "there is no oxygen": This repeats the question stem without explaining why that prevents decay.

✅ Correct Answer

34 g/m²/year

📐 Step-by-Step Calculation

1. Recall the standard transfer efficiency: On average, only about 10% of biomass is transferred from one trophic level to the next (producers → primary consumers).
2. Apply the transfer rule:
   Transferred Biomass = 10% of 340 g/m²/year
   Transferred Biomass = 340 × 0.10 = 34 g/m²/year

✅ 02.3 Correct Answer (Phosphates)

DNA [1 mark]

✅ 02.4 Correct Answer (Nitrogen)

Protein [1 mark]

💡 Key Knowledge: Molecular Composition

• Phosphates (PO₄³⁻): Essential part of the sugar-phosphate backbone in DNA and RNA, as well as ATP and cell membranes (phospholipids). Starch, cellulose, and glycerol only contain carbon (C), hydrogen (H), and oxygen (O).
• Nitrogen (N): Essential for making amino acids, which join together to form proteins. Nitrogen is also found in DNA bases. Fatty acids, glucose, and lactic acid contain only C, H, and O.

✅ Mark Scheme Breakdown (4 marks max)

• Atmospheric effect: Increase in temperature / global warming / heat (energy) is trapped [1]
• Release from storage: Carbon dioxide is released (from the drained peat bog) [1]
• Release from usage: Carbon dioxide is produced by burning peat OR by decay of peat [1]
• Methane contribution: Methane is released (from the peat bog) [1]

💡 The Mechanism of the Greenhouse Effect

• Peat bogs are huge natural carbon sinks.
• When drained, oxygen enters → aerobic microbes decay the peat, releasing CO₂.
• When dried peat is burned as fuel:
  Peat (Carbon compounds) + O₂ → CO₂ + H₂O
• Both CO₂ and CH₄ trap infrared radiation (heat) reflected from the Earth's surface, enhancing the greenhouse effect.

❌ Common Errors

• Simply saying "it causes pollution": Too vague. Must specify increasing temperature or global warming.
• Confusing the origin of methane: The mark scheme specifically states: ignore methane is released from burning/decay of peat. Methane is already trapped and released from the waterlogged anaerobic peat bog itself.
• Mentioning the ozone layer: Global warming and ozone depletion are two completely different environmental issues. Never mention ozone when discussing CO₂ or global warming!

🧠 Top-Scorer Strategy

Structure your answer into two distinct parts:

1. The temperature change: State clearly that the Earth's atmospheric temperature increases (global warming).
2. The reasons (gases & origins):
   • Carbon dioxide released when peat is burned as fuel.
   • Carbon dioxide released when peat is dug up, aerated, and decayed by microbes.
   • Methane released directly from disturbed peat bogs.