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THE NITROGEN CYCLE @ Described i n 1889 by the Polish scientist Sergei Winogradsky @ It consists of three phases:

THE NITROGEN CYCLE @ Described i n 1889 by the Polish scientist Sergei Winogradsky @ It consists of three phases: # Nitrogen fixation # Nitrogen assimilation # Nitrogen recovery. Nitrogen Fixation: @ This is reduction (adding H) of atmospheric

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THE NITROGEN CYCLE @ Described i n 1889 by the Polish scientist Sergei Winogradsky @ It consists of three phases:

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  1. THE NITROGEN CYCLE @ Described in 1889 by the Polish scientist Sergei Winogradsky @ It consists of three phases: # Nitrogen fixation # Nitrogen assimilation # Nitrogen recovery

  2. Nitrogen Fixation: @ This is reduction (adding H) of atmospheric nitrogen to ammonia, nitrites and nitrates. @ Nitrogen fixation can occur in two ways: # By electrical (lightening) reactions in the atmosphere. # By microorganisms, e.g. fungi, cyanobacteria, and free-living bacteria.

  3. @ These organisms fix nitrogen symbiotically or non-symbiotically & form ammonia inside their cells.(تعايش) @ They are grouped into three categories: # Free-living nitrogen-fixing bacteria: * Obligate aerobes: Azotobacter * Facultative anaerobes: Klebsiella * Obligate anaerobes: Clostridium # Cyanobacteria(Blue-green bacteria) # Symbiotic bacteria of legume plants (البقوليات)

  4. Non-symbiotic nitrogen fixation: @ Free-living bacteria existing in soil make non-symbiotic nitrogen fixation by secreting nitrogenase enzyme. @ Nitrogenase enzyme binds with atmospheric nitrogen and an electron carrier, in presence of ATP. Then picks up hydrogen to form ammonia, ADP, & inorganic phosphate. Nitrogenase N2 + 6H+ + 6e- + 2ATP-------→2NH3 + 12ADP + 12Pi

  5. Symbiotic Nitrogen Fixation: @ This is done by soil bacteria called Rhizobium that infect roots of the legume plants @ The two partners (bacteria & plant) come in contact by their cell surfaces. @ The bacteria establish themselves in roots of plants & formroot nodules @ Nitrogen in soil is picked up by these nodules & converted into ammonia by a process called ammonification.

  6. Nitrogen Assimilation: @ The ammonia is used by the plant nitrogen- fixing organisms to form plant protein @ Animals eat plants, and plant proteins are converted to animal proteins. @ Nitrogen is used by organisms in two ways: # Deamination to ammonia, and then used for making proteins. # Transamination to ammonium, which is converted to nitrites, and later to nitrates.

  7. @Most organisms use ammonia as nitrogen source @ Others use nitrate as a nitrogen source, and reduces it to ammonia inside cell. @ Ammonia is converted to nitrates in two steps: # Soil nitrite bacteria) Nitrosomonas)oxidize ammonia to nitrites. # Then soil nitrate bacteria) Nitrobacter ) oxidize these nitrites to nitrates.

  8. Nitrogen Recovery: @ The proteins produced by plants are consumed by them & then pass to the soil as decaying products. @ Animal carcasses also pass to the soil as waste products @ Soil bacteria decompose these decaying & waste products into ammonium, nitrites, and finally into nitrates. @ Soil denitrifying bacteria complete the nitrogen cycle by converting nitrates into nitrogen gas which passes to the atmosphere.

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