AQA GCSE Biology Biology Paper 2 (Higher), June 2023: Question 6

✅ Mark Scheme Breakdown (4 Marks)

From 0 to 6 hours (Night / Darkness):

• It is dark / night, so only respiration occurs (no photosynthesis) [1 mark].
• Respiration releases / produces CO₂ into the air [1 mark].

From 8 to 12 hours (Daylight / Midday):

• In the light, the rate of photosynthesis is faster than respiration [1 mark].
• There is a net uptake / removal of CO₂ from the air [1 mark].

💡 Key Knowledge

• Respiration: Occurs continuously (24 hours a day) in all living plant cells:
  Glucose + Oxygen → CO₂ + H₂O.
• Photosynthesis: Requires light energy, so it only happens during daytime:
  CO₂ + H₂O → Glucose + Oxygen.
• Net Gas Exchange: Around midday, sunlight is intense, so the rate of photosynthesis easily exceeds respiration, causing atmospheric CO₂ around the canopy to drop.

🧠 Exam Technique

Always structure your answer clearly by splitting it under the two distinct headings provided in the exam paper:

• State the light condition (night vs. day).
• Name which process is happening or which is faster.
• State the overall effect on CO₂ (release vs. net uptake).

❌ Common Errors

• Never say: "Respiration stops during the day" or "Plants only respire at night." Plants respire constantly! Marks are lost if you write "no respiration occurs in the light".
• Failing to mention the word faster or rate when comparing photosynthesis and respiration during 8–12 hours.

✅ Model Answer (2 Marks)

• Seasonal uptake: The maize field only grows and absorbs CO₂ for 6 months of the year, whereas the rainforest absorbs CO₂ all year round [1 mark].
• Daily rate: The field of maize takes in less CO₂ per day than the rainforest (maize peak decrease is only −5 arbitrary units compared to −15 for rainforest) [1 mark].

🧠 Reading the Graphs

• Rainforest (Fig 9): Dips to −15 units during midday. Active all 12 months.
• Maize (Fig 10): Dips to only −5 units during midday. Active for only 6 months.
• Comparative rule: When an exam question says "Use information from Figure 9 and Figure 10", look at the numbers/axes and directly contrast both figures!

✅ Model Answer (2 Marks)

• To allow for valid comparison between different study areas [1 mark].
• Because different control / study areas started with different numbers of tree species [1 mark].

🧠 Standard "Why Use Percentages?" Formula

Whenever GCSE papers ask why results are converted to percentages or percentage change, the answer almost always follows this two-part structure:

1. "To allow comparison..." (1st mark)
2. "...because the starting values were different." (2nd mark — state what starting value specifically varied in the study).

✅ Acceptable Conclusions (Choose Any ONE)

• The number of tree species increases rapidly at first, then increases more slowly later.
• The rate of increase of tree species decreases over time.
• It takes roughly 45 to 55 years (or ~50 years) for the forest species count to recover to control levels (100%).
• Beyond ~50 years, there were more tree species than in the original / control rainforest (>100%).
• There was great variation / scatter in the number of tree species between different study areas of the same age.

❌ Common Errors

• Repeating what is already given: "Biodiversity increases over the 100 years" scores 0 marks because it was explicitly stated in the question stem.
• Vague statements like "Trees grow back" without referring to the rate of change or time scale.

✅ Model Answer (2 Marks)

• Reason 1: More / different types of food available (e.g. different fruits, seeds, leaves, or prey species) [1 mark].
• Reason 2: More / different types of habitats, shelter, or nesting sites available [1 mark].

💡 Ecological Understanding

• Different animal species are adapted to eat specific foods and live in specific niches.
• A monoculture (like a single crop field) provides only one food type and limited nesting architecture.
• A rich variety of tree species creates diverse vertical layers (canopy, understory, forest floor) and a continuous supply of diverse food sources.