AQA GCSE Biology Biology Paper 1 (Higher), November 2021: Question 4

✅ Correct Answer

Tick Box 4:

6O₂ + C₆H₁₂O₆ → 6H₂O + 6CO₂

🧠 Exam Technique

Reactants are oxygen and glucose; products are water and carbon dioxide. Notice that Option 1 is unbalanced on hydrogens, Option 2 shows water as a reactant, and Option 3 is photosynthesis in reverse without glucose on the right.

✅ Correct Answer

Mitochondria (or mitochondrion)

❌ Common Errors

Writing "nucleus", "ribosome", or "chloroplast". While plant cells have chloroplasts for photosynthesis, aerobic respiration occurs in the mitochondria in all eukaryotic cells.

✅ Acceptable Answers (Any Two)

• Movement / muscle contraction
• Keeping warm / maintaining body temperature
• Active transport (e.g. absorbing mineral ions against a concentration gradient)
• Building larger molecules (e.g. protein synthesis, making cellulose or glycogen)
• Cell division / mitosis

❌ Examiner Watch-outs

• Do not write vague terms like "growth" or "metabolism" alone without detail.
• Avoid simply saying "to survive" or "for energy" (circular argument).

✅ Correct Differences (Any Two)

• Anaerobic produces lactic acid, whereas aerobic does not.
• Aerobic produces carbon dioxide and water, whereas anaerobic does not.
• Anaerobic releases much less energy than aerobic (or aerobic releases more energy / ATP).
• Aerobic occurs in mitochondria; anaerobic occurs only in the cytoplasm.
• Anaerobic creates an oxygen debt; aerobic does not.

❌ Major Examiner Trap

Never say respiration "produces" or "creates" energy! Energy cannot be created—it is released or transferred. Also, mentioning oxygen scores 0 because the question explicitly says: "Do not refer to oxygen in your answer."

✅ Correct Boxes to Tick

• Carbon dioxide
• Ethanol

💡 Key Knowledge

Anaerobic respiration in plant and yeast cells is known as fermentation:

Glucose → Ethanol + Carbon dioxide

In contrast, anaerobic respiration in animal muscles produces only lactic acid.

✅ Marking Points

• Mark 1: Pondweed takes in CO₂ for photosynthesis.
• Mark 2: Both the snail and pondweed are respiring and producing CO₂.

🧠 Exam Technique

For a steady state, the rate of removal must equal the rate of addition. Mention that both organisms respire; plants respire continuously day and night, not just animals!

Compensation rule: If neither specific point is awarded, stating that the rate of photosynthesis equals the rate of respiration scores 1 mark.

✅ Full Answer (Needs Both Ideas)

With no light, there is no photosynthesis (the plant stops taking in CO₂), and both the snail and plant are still respiring and releasing CO₂.

❌ Common Error

Students often forget that the pondweed continues to respire in the dark. It is not just the snail releasing CO₂; both organisms release it, but none is absorbed because light is blocked.

✅ 3-Step Marking Breakdown

1. The snail's body is decayed / decomposed / broken down [1 mark]
2. By decomposers / bacteria / fungi / microorganisms in the water [1 mark]
3. These microorganisms respire, releasing CO₂ into the tube [1 mark]

🧠 Top Level Logic: The Decomposition Link

Students often lose marks by simply stating "the snail rotted and emitted gas". You must explain how decomposition works:

Dead biomass → Broken down by bacteria/fungi → Microbes respire aerobically → Releases CO₂.