AQA GCSE Biology Biology Paper 2 (Higher), June 2023: Question 4

💡 Key Knowledge (Indicative Content)

• Breakdown: Microorganisms (bacteria and fungi) digest leaves by releasing extracellular enzymes to break down large organic molecules into smaller ones.
• Carbon cycle: Microorganisms respire, releasing carbon as CO₂ into the air. Leaves take in CO₂ through stomata for photosynthesis to make glucose/cellulose.
• Nitrogen cycle: Decomposition releases nitrate ions into the soil. Roots absorb nitrates via active transport to make amino acids and proteins.

🧠 Exam Technique & Level Criteria

• Level 2 Rule (4–6 marks): You must refer to both breakdown and the reuse of both carbon and nitrogen. If you miss nitrogen entirely, you are capped at Level 1 (max 3 marks).
• Structure your answer into three clear mini-paragraphs:
  1. How leaves are broken down
  2. How carbon is recycled and used
  3. How nitrogen is recycled and used

✅ Model Answer Plan (Full 6 Marks)

"Leaves are broken down by decomposers such as bacteria and fungi, which secrete enzymes to digest the leaf tissue.

Microorganisms respire, producing and releasing carbon dioxide into the atmosphere. This CO₂ enters living trees through stomata and is used in photosynthesis to make glucose.

Nitrogen compounds in the leaves decay into nitrate ions in the soil. Tree roots absorb these nitrates by active transport and use them to synthesize proteins and DNA for growth."

❌ Common Errors

• Claiming plants absorb carbon directly from the soil through roots (carbon enters leaves as gaseous CO₂).
• Confusing nitrogen gas (N₂) with nitrate ions (plants absorb nitrates from the soil, not nitrogen from air).
• Forgetting to state what the recycled compounds are used for (e.g., glucose/cellulose for carbon; amino acids/proteins for nitrogen).

✅ Acceptable Improvements (Choose One)

• Repeat the experiment at each temperature and calculate a mean (or identify anomalies).
• Test more temperatures (e.g., 10 °C, 20 °C, 30 °C) to find the optimum temperature.
• Take pH measurements at smaller time intervals (e.g., every 6 hours or 12 hours).
• Use a data logger for continuous, automated pH recording.
• Ensure the same type/fat content of milk is used in every beaker.
• Cover (or uncover) all beakers with lids to control exposure.

❌ Ignored by Examiners

• "Use the same pH probe" (this is standard procedure, not a method improvement).
• "Extend beyond 4 days" (doesn't improve the reliability of the trend observed).
• "Keep milk at same age" (method already specifies 'fresh milk').

💡 Coordinates from Table 1

• Day 0 = 6.8 (already plotted)
• Day 1 = 5.3
• Day 2 = 4.9
• Day 3 = 4.8
• Day 4 = 4.8 (already plotted)

🧠 Graph Rules & Diagram Description

What your graph should look like:

• Plot points for Days 1, 2, and 3 using small neat crosses ( × ) precisely on the grid lines (tolerance ±½ small square).
• Draw a single, smooth, continuous curved line of best fit descending sharply from (0, 6.8) through (1, 5.3) and (2, 4.9), flattening out smoothly along pH 4.8 between Day 3 and Day 4.

❌ Common Errors

• Joining points with straight ruler lines (dot-to-dot).
• Drawing a single straight line of best fit (the rate clearly changes, requiring a curve).
• "Feathering" or drawing multiple overlapping pencil lines.

✅ Final Answer

Rate at 15 °C is 1.67 (or 1.7) times faster.

❌ Common Traps

• Adding units to a ratio: "1.67 pH/day" loses the final mark. A "how many times faster" question produces a dimensionless number!
• Making the tangent triangle too small; examiners penalise triangles smaller than half the drawn tangent line.

💡 Biological Chemistry of Milk Decay

• Milk contains lipids (fats).
• Decomposers produce lipase enzymes that digest lipids into fatty acids and glycerol.
• Fatty acids are acidic; as they accumulate, hydrogen ion concentration rises, causing the pH to drop.

✅ Marking Breakdown (3 Points Required)

1. Enzyme activity / Bacteria: At higher temperatures, enzymes have more kinetic energy (resulting in more frequent successful collisions) OR bacteria reproduce faster.
2. Lipid breakdown: Lipids / fats are broken down more quickly.
3. Acid production: Fatty acids are produced at a faster rate (which lowers the pH).

❌ Common Errors

• Saying "amino acids" are produced (lipids break down into fatty acids; proteins break down into amino acids).
• Stating vaguely that "enzymes work better" (must state: more active / work faster / have more kinetic energy).
• Failing to explicitly name fatty acids as the specific chemical causing the drop in pH.