7.8 The nitrogen cycle
5 exam questions worth 11 marks. Write your answer, open the mark scheme, then give yourself the marks you earned. Your answers are saved on this device.
0 / 11
Why do plants need nitrates?
Mark scheme
to make proteins / amino acids (and DNA)
Model answer
To make proteins.
Describe the role of nitrifying bacteria and denitrifying bacteria.
Mark scheme
- nitrifying: turn ammonium compounds into nitrates (1)
- denitrifying: turn nitrates into nitrogen gas (1)
Model answer
Nitrifying bacteria convert ammonium compounds into nitrates; denitrifying bacteria convert nitrates back into nitrogen gas.
Explain why farmers grow a crop of clover or beans every few years.
Mark scheme
- legumes have nitrogen-fixing bacteria in root nodules (1)
- which increase the nitrates / nitrogen compounds in the soil (1)
Model answer
These legumes have nitrogen-fixing bacteria in their root nodules, which add nitrogen compounds to the soil, so the next crop has more nitrates.
Explain why waterlogged soil has fewer nitrates.
Mark scheme
- little oxygen (1)
- denitrifying bacteria thrive / nitrifying bacteria cannot work (1)
Model answer
Waterlogged soil has little oxygen, so denitrifying bacteria turn nitrates into nitrogen gas, and nitrifying bacteria, which need oxygen, cannot make more nitrates.
Describe how nitrogen in the protein of a dead animal can become part of a plant.
Mark scheme
- decomposers break down proteins (1)
- into ammonium compounds (1)
- nitrifying bacteria convert these to nitrates (1)
- plant absorbs nitrates (through roots) and makes protein (1)
Model answer
Decomposers break down the proteins in the dead animal into ammonium compounds. Nitrifying bacteria convert these into nitrates, which the plant absorbs through its roots and uses to make proteins.
Found a mistake?