AQA GCSE Biology Biology Paper 2 (Foundation), June 2025: Question 7

📐 Step-by-Step Calculation

1. Identify total percentage: A complete pie chart equals 100%.
2. Sum the given segments: Methane (60%) + Other gases (3%) = 63% [1 mark].
3. Subtract from 100%: 100% − 63% = 37% [1 mark].

✅ Correct Answer

37 %

🧠 Exam Technique

Always show your working out line even on simple mental arithmetic. If you make a slip like writing 38%, showing 100 − 63 still secures 1 out of the 2 marks.

❌ Common Errors

• Forgetting to subtract the small 3% "other gases" wedge and writing 40%.
• Attempting to measure angles with a protractor instead of reading the percentages provided on the labels.

✅ Mark Scheme Evidence

• Conclusion 1 (Chicken manure better than cow): Chicken manure produces more total biogas (600 cm³/g vs 400 cm³/g) OR produces biogas at a faster rate. [1 mark]
• Conclusion 2 (Fastest in first week): The line / graph is steepest in the first week OR the most biogas is produced during week 1 (e.g. 250 cm³/g for chicken). [1 mark]
• Conclusion 3 (Waste of money after 4 weeks): The curve levels off / plateaus OR no more (or very little) biogas is produced after 4 weeks. [1 mark]

💡 Key Knowledge

• Gradient = Rate: On a cumulative volume graph, the steeper the curve, the faster the rate of biogas production.
• Plateau = Reaction Finished: When the curve becomes horizontal (flat), microorganisms have consumed all digestible organic matter; production has stopped.

🧠 Top Tip: Use Data or Graph Shapes

When asked for "evidence from the graph", refer directly to line features (steepest gradient, plateau / horizontal line) or quote paired coordinates (e.g., "Chicken reaches 600 cm³/g while cow only reaches 400 cm³/g").

❌ Common Errors

• Writing "chicken produces more" without stating it produces more biogas or without quoting the numbers.
• Vague statements for week 1 such as "it goes up a lot" instead of using the term steepest.
• Stating "the gas runs out" rather than that production stops or levels off.

✅ Correct Completions

1. When fish fat is added, the volume of biogas produced: increases [1 mark]
2. When fish fat is added, the volume of methane produced: increases [1 mark]
3. When fish fat is added, the percentage of methane in the biogas: stays the same [1 mark]

📐 Reading Table 4

• Biogas volume: goes from 426 to 999 cm³/g → increases
• Methane volume: goes from 256 to 599 cm³/g → increases
• Methane %: 60% before vs 60% after → stays the same

❌ Common Trap: Volume vs Percentage

Many students confuse total amount (yield) with proportion (%). The volume of methane more than doubles, but because the total biogas volume increases at the exact same ratio, the percentage stays exactly at 60%.

✅ Acceptable Answers (Any ONE)

• Transport / Distance costs: Transport costs too much money / it is too far to transport (250 km) / takes too long.
• Economic cost: Buying fish fat is expensive / uneconomic.
• Environmental / Sensory: Fish fat has a very bad/unpleasant smell.
• Pests: Fish fat attracts vermin, pests, or wild animals.

🧠 Clue Hunting in the Prompt

The prompt explicitly notes: "A farmer has a farm 250 kilometres from the sea." Whenever examiners include an exact number or detail like that, it is an intentional clue! Relate distance directly to high fuel/transport costs.

❌ What Examiners Reject

Saying simply "fish fat is bad" or "the cows won't like it" (the fat goes into the biogas tank, not into the living animals!).

✅ Model Answer

So that the crops grow better / faster / have higher yield [1 mark], because the digested manure contains and releases mineral ions / minerals / salts [1 mark].

💡 Biological Mechanism

• Decomposition breaks down complex organic matter into soluble mineral ions (e.g. nitrates, phosphates, potassium).
• Root hair cells absorb these mineral ions by active transport to produce essential molecules like amino acids and proteins.

❌ Biological Error: "Plant Food" Trap

Never write that manure provides "food" or "energy" for plants! Plants make their own food through photosynthesis (glucose). Manure supplies mineral ions (or mineral salts). The mark scheme strictly states: "ignore nutrients / nitrogen / food / carbon dioxide".