Key Idea: Most marks in topic 2.3 come from carbon flows, the ocean and acidification, and at HL from human impacts on the nitrogen cycle. Learn those first.
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. Nitrogen and people (HL)
- Fertiliser adds reactive nitrogen: nitrates leach into rivers and cause eutrophication.
- Example: the Gulf of Mexico dead zone, fed by Mississippi farm runoff.
- Burning fuels makes nitrogen oxides: smog, ozone and acid deposition.
- N₂O traps about 270 times as much heat as CO₂.
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, fixation. |
| [2] to [4] Explain ocean acidification or the ocean sink | CO₂ dissolves, carbonic acid, lower pH; colder water holds more. |
| [4] Outline flows in the nitrogen cycle | Name the bacteria: fixing, nitrifying, denitrifying. |
| [7] to [9] Human impacts on the nitrogen cycle | Fertiliser, leaching, eutrophication, NOx; named places; management. |
| [7] Compare and contrast the carbon and nitrogen cycles | Similarities, then differences with 'whereas'. |
Key terms, one line each:
| Term | In one line |
|---|---|
| Sink / source | takes in more carbon than it releases / releases more than it takes in |
| Nitrogen fixation | turning nitrogen gas into ammonium, by bacteria or the Haber process |
| Nitrification | bacteria turning ammonium into nitrites and then nitrates |
| Denitrification | bacteria in waterlogged soil turning nitrates back into nitrogen gas |
| Leaching | nitrates washed down through the soil into groundwater and rivers |
| Planetary boundary | a safe limit for a global process; the nitrogen one has been crossed |
Last-minute check: answer, then tap to see.
Which bacteria turn nitrates back into nitrogen gas? Denitrifying bacteria, in waterlogged soils.
Which partnership fixes nitrogen in clover roots? Rhizobium bacteria in root nodules of legumes.
How much more heat does methane trap than CO₂ over 20 years? About 80 times.
Why does colder water take up more CO₂? Gases dissolve better in cold water.
What causes a coastal dead zone? Nitrate runoff, algal bloom, decomposers use up the oxygen.
Farms in the Mississippi basin spread large amounts of nitrogen fertiliser each spring. Explain how this leads to a dead zone in the Gulf of Mexico each summer.
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.
- In a nitrogen answer, name the bacteria and the store the nitrogen moves to.