AQA GCSE Biology Biology Paper 2 (Foundation), June 2024: Question 10

✅ Correct Answer Routes

You can explain this using either oxygen limitation or pH/enzymes:

Route 1 (Oxygen):

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

Route 2 (Acidity):

• Low pH denatures enzymes (or reduces enzyme activity) [1]
• So fewer/no chemical reactions occur OR less/no energy is released [1]
• In microorganisms / decomposers / bacteria / fungi [1]

💡 Key Knowledge

• Decomposition requires: Oxygen (for aerobic respiration), moisture, warmth, and a suitable pH for enzymes.
• Layer B is waterlogged and deep: it lacks oxygen and is highly acidic.
• Decomposers cannot survive or function efficiently under these extreme conditions.

❌ Common Errors & Traps

• "Energy is made/produced": Never say respiration "produces" or "creates" energy! Energy is only transferred or released.
• Blaming the dead plants: Stating "the dead plants cannot respire" gets zero marks. It is the decomposers (bacteria/fungi) that respire.
• Vague comments: Simply saying "it is too acidic" without mentioning enzymes or microorganisms will not earn full marks.

🧠 Exam Technique

Notice the prompt gave bullet points for Layer B (low pH, very little oxygen). Pick one bullet point and complete the full chain of consequence:

Condition → Effect on Decomposers/Enzymes → Consequence for Respiration/Energy

✅ Correct Answer

34 g/m²/year

(Option 3 ticked)

📐 Step-by-Step Calculation

1. Identify rule of thumb: Approximately 10% of biomass from one trophic level is transferred to the next.
2. Producer biomass: 340 g/m²/year.
3. Calculation: 10% of 340 = 340 × 0.10 = 34 g/m²/year .

🧠 Exam Technique

When an exam paper does not supply a specific percentage for a biomass pyramid calculation, recall the standard textbook value: roughly 10% of biomass is transferred from producer to primary consumer.

❌ Common Errors

• Selecting 340 g/m²/year (assuming 100% transfer efficiency).
• Selecting 3.4 g/m²/year (accidentally calculating 1% instead of 10%).

✅ Correct Choices

10.3 Using Phosphates:

DNA

10.4 Using Nitrogen:

Protein

💡 Key Knowledge

• Phosphates: Essential for building nucleic acids (DNA and RNA) and ATP. Plant cell walls (cellulose) and storage carbs (starch) only contain Carbon, Hydrogen, and Oxygen (C, H, O).
• Nitrogen: Combined with glucose to form amino acids, which polymerise into proteins.
• Carnivorous plants like sundews trap insects specifically to obtain nitrogen and phosphorus from poor, boggy soil.

❌ Common Errors

• Confusing glucose or starch with nitrogen compounds (both are carbohydrates containing only C, H, and O).
• Selecting fatty acids or glycerol (lipids contain C, H, and O only).

🧠 Quick Element Memory Check

Keep these elemental combinations clear in your mind:

• Carbohydrates & Lipids: C, H, O
• Proteins: C, H, O, N (sometimes S)
• DNA / RNA: C, H, O, N, P

✅ Correct Answer (4 Marks)

Award 1 mark for each clear point made:

1. Atmospheric effect: Increases atmospheric temperature / causes global warming / traps more heat [1]
2. Direct release: Carbon dioxide is released when the bog is drained/cleared [1]
3. Burning / Decay: Carbon dioxide is produced by burning peat as fuel OR by decay of peat compost by microbes [1]
4. Methane: Methane is released from the peat bog [1]

💡 Key Knowledge: The Peat Bog Story

• Peat is a massive carbon store (carbon sink) locked underground for thousands of years.
• When drained, oxygen penetrates the soil, allowing aerobic decomposers to rapidly break down dead organic matter, releasing CO₂.
• When dried peat 'bricks' are burned for fuel, combustion releases CO₂: C + O₂ → CO₂ .
• CO₂ and methane (CH₄) are greenhouse gases that absorb infrared radiation and warm the atmosphere.

🧠 Structuring a 4-Mark Explanation

Ensure you cover both what happens to the temperature and the specific causes:

• State the direct temperature change: Temperature increases.
• Name gas 1 and its origin: CO₂ released by burning or decomposition.
• Name gas 2 and its origin: Methane released.
• Mention less photosynthesis: Fewer plants to absorb CO₂ (alternative accepted mark).

❌ Common Misconceptions

• Confusing ozone with global warming: Never mention the "ozone layer" or UV rays when explaining peat destruction. Peat destruction is purely about greenhouse gases trapping infrared heat.
• Ignoring methane from the diagram: The question explicitly told you Layer B contains methane—use this clue to secure an easy mark!
• Only stating "greenhouse gases increase": The mark scheme specifically notes: ignore vague references to greenhouse gases—you must name carbon dioxide and/or methane explicitly.