AQA A2 Biology: The Nitrogen Cycle

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  • Created on: 20-09-13 17:50
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The Nitrogen Cycle
Plants and animals need a source of nitrogen in order to make proteins, nucleic acids and other nitrogen-
containing compounds.
The atmosphere is made up of about 78% nitrogen.
There are very few organisms that can use nitrogen gas directly.
Most organisms need bacteria to convert it into nitrogen compounds first.
The nitrogen cycle shows how nitrogen is converted into a useable form and then passed on between
different living organisms and the non-living environment.
There are four main stages in the nitrogen cycle:
Nitrogen fixation
Atmospheric Nitrogen
Nitrogen compounds Nitrogen compounds
in plants in animals
Nitrates Nitrites Ammonium Compounds

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Nitrogen Fixation
This is a process by which nitrogen gas is converted into nitrogen-containing compounds.
It can be carried out industrially and also naturally when light passes through the atmosphere.
The most important form of nitrogen fixation is carried out by microorganisms, of which there are two main types:
Type of microorganism Role
Free-living Reduces gaseous nitrogen to ammonia, which they then use to manufacture amino
nitrogen-fixing bacteria acids.
Nitrogen-rich compounds are released from them when they die and decay.…read more


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