AQA GCSE Combined Science: Trilogy Biology Paper 2 (Higher), 2019: Question 2

💡 Key knowledge

• Greenhouse gases include methane, water vapour, carbon dioxide, CFCs, nitrous oxide and ozone.
• Climate change can lead to sea level rise, melting ice caps, extreme weather, habitat loss and biodiversity loss.
• In the carbon cycle, carbon is taken in by plants, passed to animals by feeding, returned by respiration, and recycled by microorganisms after death.

🧠 Exam technique

• For 1 mark, name one correct greenhouse gas exactly.
• For 2 marks, give two different effects of climate change.
• For the 6-mark explanation, link each process clearly: CO₂ → plants → animals → microorganisms → CO₂ .

❌ Common errors

• Saying “global warming” instead of a greenhouse gas.
• Repeating the same effect twice, e.g. “flooding” and “sea level rise” without making them distinct enough.
• Only describing photosynthesis and forgetting feeding, respiration or decay in the carbon cycle.

✅ Correct answers

Methane

Also allowed: CH₄ , water vapour, H₂O , CFCs, nitrous oxide, ozone.

🧠 How the mark is awarded

One correct greenhouse gas = 1 mark. The examiner is looking for a specific substance, not a description.

❌ Common errors

Do not write “heat”, “pollution”, or “global warming”. These are not greenhouse gases.

✅ Correct answers

• Ice caps melting
• Rise in sea levels
• Desertification
• Extreme weather
• Change in species distribution
• Change in migration patterns
• Loss of biodiversity
• Coral bleaching
• Crop failure / food insecurity
• Loss of habitat

Example full-marks response: ice caps melting and rise in sea levels.

💡 Key knowledge

Acceptable examples include storms, droughts and flooding. You can also mention some species becoming extinct or endangered, or polar bears losing ice as a habitat example.

❌ Common errors

The mark scheme says to ignore references only to increased temperature or to “more greenhouse effect”. That is too vague.

✅ Top-level answer idea

Carbon dioxide is taken from the atmosphere by green plants through their leaves and stomata. The carbon dioxide is used in photosynthesis to make glucose, which can be turned into other carbon compounds or stored as starch. Animals eat the plants and use the carbon compounds to make their own biomass. When plants, animals and microorganisms respire, carbon dioxide is released back into the atmosphere. When plants and animals die, microorganisms decompose them and use the carbon compounds, and respiration by microorganisms also returns carbon dioxide to the air.

💡 Key knowledge to include

• Photosynthesis: CO₂ enters plants through stomata/ leaves.
• CO₂ is used to make glucose / carbohydrates.
• Glucose is used to make other carbon compounds, such as proteins and lipids, or stored as starch.
• Feeding: plants are eaten by animals, which use the carbon compounds to build biomass.
• Decay: dead plants and animals are broken down by microorganisms.
• Respiration: plants, animals and microorganisms release CO₂ back into the atmosphere.

🧠 How to score 4–6 marks

• Make a chain of events, not just a list of facts.
• Use “because” or “which means” to show links between stages.
• Include at least two processes beyond photosynthesis for Level 2.
• Use the organism names in the figure: green plants, animals, microorganisms.

❌ Common errors where marks are lost

• Saying only “plants take in carbon dioxide” without saying what happens next.
• Forgetting that carbon moves to animals by feeding.
• Not mentioning respiration returning CO₂ to the atmosphere.
• Writing that decomposers “create” carbon dioxide without linking it to decay and respiration.
• Confusing the carbon cycle with the water cycle.

Carbon dioxide from the atmosphere enters green plants through the leaves and stomata. The carbon dioxide is used in photosynthesis to make glucose. The glucose can be turned into other carbon compounds such as proteins and lipids, or stored as starch. Animals eat the plants and take in these carbon compounds, which they use to make their own biomass. When plants, animals and microorganisms respire, carbon dioxide is released back into the atmosphere. When plants and animals die, microorganisms decompose them and use the carbon compounds, and respiration by microorganisms also returns carbon dioxide to the air.

📐 Diagram guidance

If you were asked to draw the cycle, you should show arrows between:

• carbon dioxide in the atmosphere → green plants
• green plants → animals
• plants/animals → microorganisms
• all living organisms → carbon dioxide in the atmosphere

Use arrows to show movement of carbon, not just labels.

🧠 Examiner insight

Top answers did not just name processes. They explained what carbon is used for in each stage, especially that plants make glucose and that animals and microorganisms use carbon compounds to build biomass. That extra detail pushed responses into Level 2.

💡 Memory chain

CO₂ is taken in by plants → used in photosynthesis → passed to animals when eaten → returned by respiration → recycled by microorganisms during decay.

✅ One-line checklist for full marks

Greenhouse gas named correctly + two clear climate effects + a linked carbon cycle explanation with photosynthesis, feeding, decay and respiration.