AQA GCSE Combined Science: Trilogy Biology Paper 1 (Higher), 2023: Question 5
6 marks · High Demand difficulty · Extended Answer
Explain the overall exchange of carbon dioxide and oxygen in a tree at bright light, dim light, and no light.
Practise this questionQuestion
Question text
05 Light intensity varies during 24 hours.
Figure 6 shows the overall exchange of carbon dioxide and oxygen for a tree at
three different light intensities.
Figure 6
Explain the overall exchange of carbon dioxide and oxygen at the three different
light intensities.
[6 marks]
Mark scheme
Show the mark scheme
Question 5
AO /
Question Answers Mark
Spec. Ref.
Level 3: Relevant points (reasons/causes) are identified, given in
05 5–6 AO2
detail and logically linked to form a clear account.
Level 2: Relevant points (reasons/causes) are identified, and there 3–4 AO1
are attempts at logical linking. The resulting account is not fully
clear.
Level 1: Points are identified and stated simply, but their relevance 1–2 AO1
is not clear and there is no attempt at logical linking.
No relevant content 0
Indicative content
4.4.1.1
Bright light: 4.4.1.2
• rate of photosynthesis is high due to high light intensity 4.4.2.1
• (extra) carbon dioxide is absorbed from the air
• (so) oxygen is released into the air
• carbon dioxide produced in respiration is used for photosynthesis
• (so) no carbon dioxide released from plant or into the air
• sufficient oxygen for respiration is produced by photosynthesis
• (so) no extra oxygen absorbed from the air
• rate of photosynthesis is greater than rate of respiration
Dim light:
• no net / overall flow / exchange of carbon dioxide or oxygen with
the air
• (because) rate of respiration equals rate of photosynthesis
• carbon dioxide produced in respiration is used for photosynthesis
• no (extra) carbon dioxide is absorbed from the air
• oxygen produced by photosynthesis is used in respiration
• no (extra) oxygen is absorbed from the air
No light:
• no photosynthesis occurs as no light
• (so) no carbon dioxide is absorbed and no oxygen released /
produced
• plant respires so takes in oxygen and releases carbon dioxide
into the air
General points:
• plants photosynthesise when there is light using carbon dioxide
and producing oxygen
• rate of photosynthesis depends on light intensity
• respiration occurs at all times of the day
• respiration uses oxygen and produces carbon dioxide 19
Level 3 response must include explanation at all three light
intensities, demonstrating a good understanding of both
photosynthesis and respiration.
For Level 2 correct reference to both processes and to both
gases
Total Question 5 6
How to answer it
Tree gas exchange at different light intensities
Explaining how photosynthesis and respiration affect the movement of carbon dioxide and oxygen in a tree in bright light, dim light, and no light. To score well, you must compare the two processes, use the correct gas changes, and link your ideas logically.
Exam overview
Question 05 — 6 marks
✅ Big idea
- Photosynthesis happens only when there is light.
- Respiration happens all the time, day and night.
- Photosynthesis takes in CO₂ and releases O₂.
- Respiration takes in O₂ and releases CO₂.
🧠 How marks are awarded
- Top answers explain all three light conditions.
- They show the link between rate of photosynthesis and rate of respiration.
- They use both gases correctly and make the exchange clear.
Part (a) — Explain the overall exchange of carbon dioxide and oxygen
✅ Correct answer for full marks
Bright light: photosynthesis happens at a high rate, so the tree absorbs carbon dioxide from the air and releases oxygen. The rate of photosynthesis is greater than respiration, so there is a net exchange of CO₂ into the tree and O₂ out of the tree.
Dim light: photosynthesis and respiration happen at about the same rate, so there is no net exchange of either gas with the air. The carbon dioxide made in respiration is used in photosynthesis, and the oxygen made in photosynthesis is used in respiration.
No light: photosynthesis stops because there is no light. Respiration still happens, so the tree takes in oxygen and releases carbon dioxide.
💡 Key knowledge
- Photosynthesis: uses CO₂ and produces O₂.
- Respiration: uses O₂ and produces CO₂.
- Light intensity affects the rate of photosynthesis, not respiration.
- In dim light, the two rates can balance, so there is no overall gas exchange.
🧠 Exam technique
- Make sure you mention both processes.
- Always describe direction of gas movement: absorbed or released.
- Use comparative phrases like greater than , equals , and stops .
- A strong 6-mark answer is linked and organised, not just a list of facts.
❌ Common errors
- Saying respiration stops in light — it does not.
- Mixing up the gases: photosynthesis takes in CO₂ and releases O₂.
- Only describing bright light and forgetting dim light or no light.
- Writing “the plant breathes in oxygen” without explaining respiration properly.
📐 No calculation here, but use this structure
- State the condition — bright, dim, or no light.
- State photosynthesis — fast, equal, or absent.
- State respiration — always happening.
- Link to gas exchange — CO₂ in/out and O₂ in/out.
What the examiner wanted to see
💡 Level 3 features
- Relevant points in detail.
- Clear links between cause and effect.
- Explanation of all three light intensities.
- Good understanding of photosynthesis and respiration.
🧠 What lifted answers to the top band
- Students compared the three conditions clearly.
- They explained that respiration occurs at all times.
- They used the idea of net exchange correctly.
Model answer you could adapt in the exam
In bright light, the rate of photosynthesis is high, so the tree absorbs carbon dioxide from the air and releases oxygen. Photosynthesis is happening faster than respiration, so there is a net uptake of CO₂ and a net release of O₂.
In dim light, the rate of photosynthesis is about the same as the rate of respiration. The carbon dioxide produced by respiration is used in photosynthesis, and the oxygen produced by photosynthesis is used in respiration, so there is no overall exchange of either gas.
In no light, photosynthesis cannot happen, but respiration still continues. Therefore the tree takes in oxygen and releases carbon dioxide.
Quick recall checklist
✅ Remember these facts
- Light is needed for photosynthesis.
- Respiration happens all day and night.
- Photosynthesis uses CO₂ and gives out O₂.
- Respiration uses O₂ and gives out CO₂.
❌ Don’t say
- “No light means no respiration.”
- “Dim light means no gas exchange because nothing happens.”
- “Oxygen is always absorbed by plants.”
How to turn this into 6 marks
- 1 mark: correct statement about bright light.
- 1 mark: correct statement about dim light.
- 1 mark: correct statement about no light.
- 1–3 marks: developed explanation linking photosynthesis and respiration, including both gases and overall net exchange.
Best strategy: write 3 short paragraphs, one for each light level, and make sure each one contains both what happens and why it happens.
Topics
Biology · B4: Bioenergetics
Question and mark scheme from the AQA GCSE Combined Science: Trilogy examination, Biology Paper 1 (Higher), 2023. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.