AQA GCSE Biology Biology Paper 2 (Foundation), June 2022: Question 4

✅ Correct Answer

Tick (✓) Bacteria

🧠 Exam Technique

• "Most rapid increase" means the line with the steepest upward slope immediately following the sewage input point.
• Look directly at the vertical arrow marked "Sewage entered river". The line for bacteria shoots up almost vertically compared to the other two curves.

✅ Correct Answers (Any two from)

• Bacteria numbers increase before protozoa numbers increase (or protozoa numbers rise as bacteria numbers are high).
• As the number of protozoa increases, the number of bacteria decreases.
• After Site A, as bacteria numbers decrease, protozoa numbers also decrease (or when bacteria are low, protozoa are low).

💡 Key Knowledge

• This is a classic predator-prey curve:
  • Prey (bacteria) multiply first due to rich organic food in sewage.
  • Predator (protozoa) lag behind, then multiply as their food source expands.
  • High predator feeding causes the prey population to crash, which subsequently leads to a crash in predator numbers.

❌ Common Errors

• Vague statements: Saying "they both go up and down" without linking which happens first.
• Omitting time lag: Forgetting to mention that the increase in protozoa happens after the bacteria multiply.

✅ Correct Answer

(aerobic) respiration

❌ Common Errors

• Writing "breathing" (breathing is physical ventilation, not a cellular chemical process).
• Writing "anaerobic respiration" — the mark scheme explicitly states: do not accept anaerobic respiration because anaerobic means without oxygen!
• Writing "decomposition" without specifying the process — decomposers break down waste by respiring.

✅ Correct Answer

• (Green algae carry out) photosynthesis [1 mark]
• (Photosynthesis) produces oxygen (which dissolves in the water) [1 mark]

🧠 Exam Technique: "Explain why..."

Two-mark explanation structure:

Name the biological process → State what product it releases

Word equation: Carbon dioxide + Water → Glucose + Oxygen

📐 Step-by-Step Graph Plotting

• Scale check: 1 large grid square (10 small subdivisions) = 10 units. Therefore, 1 small square = 1 animal.
• Sludge worm:
  • Site A (white bar): Draw bar to exactly 80 (8 major lines up).
  • Site B (grey bar): Draw bar to exactly 2 (2 tiny squares up).
• Bloodworm:
  • Site A (white bar): Draw bar to exactly 36 (3 large squares + 6 small squares up).
  • Site B (grey bar): Draw bar to exactly 8 (8 small squares up, just below the 10 line).
• Shading: Keep Site A bars clear/white, and shade Site B bars grey to match the key.

❌ Common Errors

• Misreading scale: Plotting 36 at 33 or 38 due to miscounting small squares.
• Inverted shading: Shading Site A instead of Site B, losing the formatting mark.
• Drawing single bars: Forgetting to plot both sites for each animal.

✅ Correct Answer

Must include direct comparisons for both named animals:

• Sludge worms: There are more sludge worms at Site A than at Site B (or 80 at A compared to only 2 at B) [1 mark].
• Mayfly nymphs: There are no mayfly nymphs at Site A, but they are present / more at Site B (0 at A compared to 15 at B) [1 mark].

💡 Key Knowledge: Indicator Species

• Sludge worms & Bloodworms: Adapted to live in muddy, low-oxygen conditions (heavily polluted water). High numbers indicate low dissolved oxygen.
• Mayfly nymphs & Freshwater shrimps: Highly sensitive to pollution and require high oxygen levels. Their presence proves clean, well-oxygenated water.

🧠 Exam Technique: Always Be Comparative

• Examiners insist on comparative language when evaluating two sites.
• Writing "There are 80 sludge worms at A" is not enough on its own — you must say "There are more sludge worms at A than at B" or state both values explicitly.
• Make sure you write a sentence for each organism named in the question prompt.