1[2 marks]
Explain why materials such as carbon and nitrogen need to be cycled in an ecosystem.
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- the supply of each material is limited (1)
- so they must be returned to the abiotic environment to be used again by new organisms (1)
2[3 marks]
Explain the role of decomposers in the carbon cycle.
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- they break down dead plants, dead animals and waste (1)
- they respire, releasing carbon dioxide into the air (1)
- so carbon dioxide is available again for photosynthesis (1)
3[2 marks]
(a) Name the process by which plants lose water vapour from their leaves. (1) (b) Name the process that turns water vapour into liquid water droplets in clouds. (1)
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- transpiration (1)
- condensation (1)
4[3 marks]
In areas of drought, desalination of sea water can be used to produce potable water. Explain what is meant by 'potable water' and 'desalination', and give one disadvantage of desalination.
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- potable water is water that is safe to drink (1)
- desalination removes the dissolved salt from sea water (1)
- needs large amounts of energy / expensive (1)
5[1 mark]
Which type of bacteria converts ammonium ions in the soil into nitrates?
- Nitrifying bacteria
- Denitrifying bacteria
- Decomposers
- Nitrogen-fixing bacteria in root nodules
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Answer: A nitrifying bacteria (1)
6[6 marks]
Explain how nitrates are made available for plants to take up from the soil, including the use of fertilisers and crop rotation.
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| Level | Marks | What the answer does |
|---|---|---|
| 3 | 5–6 | A clear, well-structured explanation covering the roles of several types of bacteria in the nitrogen cycle as well as fertilisers and crop rotation, with correct scientific terms. |
| 2 | 3–4 | An explanation that covers some of the processes or methods, with some correct scientific terms but with gaps in the detail. |
| 1 | 1–2 | Some relevant statements are made but with little explanation. |
Indicative content
- decomposers break down proteins and urea in dead material and waste into ammonia / ammonium ions
- nitrifying bacteria convert ammonium ions into nitrates
- nitrogen-fixing bacteria in the soil or root nodules convert nitrogen gas into nitrogen compounds, which become nitrates
- plants absorb nitrates through their roots and use them to make protein
- denitrifying bacteria remove nitrates from the soil by converting them into nitrogen gas
- fertilisers add nitrates to the soil to replace those removed in crops
- crop rotation: growing a legume (such as clover) increases nitrates in the soil because of nitrogen-fixing bacteria in its root nodules, then a crop needing nitrates is grown