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Science

Natural polymers

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Nitrogen - higher only

Plants and animals need nitrogen to make proteinsproteins: organic compounds made up of amino acid molecules. One of the three main food groups, proteins are needed by the body for cell growth and repair.. But they cannot get nitrogen directly from the air because, as a gas, nitrogen is fairly unreactive. Plants are able to take up nitrogen compounds [compound: A compound is a substance formed by the chemical union (involving bond formation) of two or more elements. ] such as nitrates and ammonium salts from the soil.

Nitrogen fixation

Making nitrogen compounds from nitrogen in the air is called nitrogen fixation.

Nitrogen fixation happens in three ways:

  • The energy in lightning splits nitrogen molecules into individual nitrogen atoms. These react with oxygen to form nitrogen oxides. Nitrogen oxides are washed to the ground by rain, where they form nitrates in the soil.
  • Nitrogen-fixing bacteria found in the soil and in the root nodules of leguminous plants, such as peas, beans and clover, fix nitrogen gas into nitrogen compounds.
  • The Haber process is used by industry to produce ammonia from nitrogen and hydrogen. Ammonia is used to make nitrogen compounds that are used as fertiliser by farmers.

Nitrogen compounds

Nitrogen compounds in living things are returned to the soil through:

  • excretion and egestion by animals
  • the decay of dead plants and animals

Denitrifying bacteria present in soil break down nitrogen compounds and release nitrogen gas into the air.

The Nitrogen Cycle

As a result of these processes, nitrogen is cycled continually through the air, soil and living things. This is called ‘The Nitrogen Cycle’.

This animation summarises the main features of the nitrogen cycle.

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