Flashcards · Biology · Material Cycles
The Nitrogen Cycle
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What percentage of the air is nitrogen?
About 78 percent.
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Why can plants not use nitrogen gas from the air?
It is very unreactive, so plants cannot take it in and use it.
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What do plants make from nitrates?
Amino acids, and then proteins.
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How do root hair cells take up nitrates?
By active transport.
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What are the signs of a plant with too few nitrates?
Slow growth and yellow leaves.
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How do animals get their nitrogen?
By eating plants or other animals.
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What do nitrogen-fixing bacteria do?
Change nitrogen gas into ammonia and other nitrogen compounds.
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Where do nitrogen-fixing bacteria live?
In the soil, and in root nodules of legumes.
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What do decomposers do in the nitrogen cycle?
Break down proteins and urea in dead material and waste, releasing ammonium compounds.
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What do nitrifying bacteria do?
Change ammonium compounds into nitrates.
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What do denitrifying bacteria do?
Change nitrates into nitrogen gas.
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Why does waterlogged soil lose nitrates?
Denitrifying bacteria thrive where oxygen is scarce and turn nitrates into nitrogen gas.
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How does lightning make nitrates?
It makes nitrogen and oxygen combine into nitrogen oxides, which dissolve in rain as nitrates.
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Why do farmers add fertilisers?
To replace the nitrates removed when crops are harvested.
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What is a problem with too much fertiliser?
It is washed into rivers and lakes and can cause eutrophication.
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What is crop rotation?
Growing a different crop each year, in a repeating order.
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Why include a legume in a crop rotation?
Its root nodule bacteria fix nitrogen, which adds nitrates to the soil.
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Name two legumes.
Peas, beans or clover.
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What is mutualism between a legume and its bacteria?
The plant gives sugars to the bacteria and the bacteria give nitrogen compounds to the plant.
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Name each box and each bacterium shown in the nitrogen cycle. 
The ring is plants and animals, dead organisms and waste, ammonium compounds and nitrates. Decomposers make ammonium compounds, nitrifying bacteria make nitrates, plants take up nitrates, denitrifying bacteria return nitrogen to the air, and nitrogen-fixing bacteria and lightning bring it from the air.
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Which bacteria make nitrates, and which remove them?
Nitrifying bacteria make nitrates from ammonium compounds. Denitrifying bacteria remove nitrates by changing them to nitrogen gas.