Key Idea: Most marks in topic 2.3 come from carbon flows between stores and from ocean acidification. Learn those first, then sinks and sources.
1. Carbon flows (most marks)
- Photosynthesis moves carbon from the air into biomass.
- Respiration, decomposition and combustion move it back to the air.
- Feeding moves it along food chains; fossilization locks it in rocks.
- Name the flow AND the two stores it links.
2. Oceans and acidification
- The ocean takes up about a quarter of the CO₂ we release.
- CO₂ dissolves and forms carbonic acid, so the water becomes more acidic.
- Surface pH has fallen from about 8.2 to 8.1.
- Less carbonate: corals and shelled animals struggle to build calcium carbonate.
3. Sinks and sources
- A sink takes in more carbon than it releases; a source releases more than it takes in.
- A growing forest is a sink; a burning or cleared forest is a source.
- Only about half of the CO₂ we release is taken up by land and ocean sinks.
- The rest stays in the air, so the atmospheric store keeps growing.
How it is examined:
| Question | How to score |
|---|---|
| [1] to [2] Identify a flow or a store on a diagram | Use the exact word: photosynthesis, respiration, combustion. |
| [2] to [3] Draw or complete a carbon cycle diagram | Boxes for stores, labelled arrows for flows, arrows the right way. |
| [1] Calculate a change in a store | Inputs minus outputs, with units (billion tonnes of carbon a year). |
| [2] to [4] Explain ocean acidification and its impacts | CO₂ dissolves, carbonic acid, lower pH, less carbonate for shells. |
| [2] Outline ways to reduce carbon emissions | A named method and how it cuts a flow or grows a sink. |
Key terms, one line each:
| Term | In one line |
|---|---|
| Store | a place where carbon is held, such as the atmosphere, oceans, biomass or fossil fuels |
| Flow | a movement of carbon between two stores, such as photosynthesis |
| Sink | a store that takes in more carbon than it releases |
| Source | a store that releases more carbon than it takes in |
| Sequestration | taking carbon out of the air and storing it for a long time |
| Ocean acidification | the fall in ocean pH as it absorbs more carbon dioxide |
Last-minute check: answer, then tap to see.
Which flow moves carbon from the air into plants? Photosynthesis.
Name three flows that return carbon to the air. Respiration, decomposition and combustion.
Is a newly planted forest a sink or a source? A sink: it takes in more carbon than it releases as it grows.
Why does ocean pH fall as CO₂ rises? CO₂ dissolves and forms carbonic acid.
Why do corals and shellfish suffer in more acidic water? There is less carbonate to build calcium carbonate shells.
Since 1988, pH measured in the Pacific near Hawaii has fallen as carbon dioxide at Mauna Loa has risen. Explain how rising carbon dioxide in the air could harm coral reefs.
Model answer plan
See the mark-by-mark plan — for / against / judgement, with marking guidance — in study mode.
Exam tips
- On a diagram, stores are boxes and flows are labelled arrows pointing the right way.
- Name the flow and the two stores it links, not just the process.
- For acidification, go step by step: dissolves, carbonic acid, lower pH, less carbonate.
- For a sink or a source, say which is bigger: carbon in or carbon out.