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The Nitrogen Cycle
All living organisms depend upon the availability of
nitrogen compounds for their growth and development
Large amounts of nitrogen are required for the formation of
proteins, nucleic acids and other cellular components
Four processes participate in the cycling of nitrogen
through the biosphere:


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A relatively small amount of
nitrogen fixation occurs
during lightning

The enormous amounts of
energy released by lightning
break nitrogen molecules
apart, allowing their atoms to
combine with oxygen and
form nitrogen oxides

These oxides dissolve in rain
and form nitrates that are
carried to the earth's surface

Nitrates formed…

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Biological Nitrogen Fixation

Nitrogen-fixing bacteria possess
the enzyme nitrogenase that
enables them to reduce nitrogen
gas into ammonia

Nitrogen + hydrogen



N2 + 3H2 2NH3

The Nitrogen Cycle
Most plants absorb nitrogen through their
roots in the form of nitrate (NO3-) or
ammonium ions (NH4+)
Nitrate ions…

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Nitrogen-containing organic compounds, such as
proteins, pass along food chains

Nitrogen-containing organic compounds are made available
to the decomposers when animals excrete nitrogenous waste
products, and when animals and plants die

Saprophytic fungi and bacteria are the principal decomposers
responsible for converting organic nitrogen compounds into
AMMONIA (NH3) ­ this…

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Chemosynthetic bacteria play an essential role in
the nitrogen cycle

These bacteria are autotrophs and obtain their
energy by oxidising simple inorganic compounds

In both terrestrial and aquatic environments, the
ammonia (NH3) released by the decomposers dissolves
in water to form ammonium ions (NH4+)

Species of…

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The nitrogen cycle is closed when
a group of bacteria known as the
denitrifying bacteria reduce
nitrates to nitrogen gas
Denitrifying bacteria live deep in
the soil and in aquatic sediments
where oxygen levels are low
These bacteria reverse the
nitrifying process and convert
nitrates to nitrites, and nitrites…


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