The Nitrogen Cycle

Details of all the processes in the nitrogen cycle including examples of bacteria

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Nitrogen-fixing bacteria

  • Rhizobium
  • Live freely in soil and in roots of leguminous plants (mutualism)
  • Able to fix N2 thanks to an enzyme called Nitrogenase which catalyses conversion of nitrogen (g) to Ammonium ions

1. Leguminous plant germinates, Roots produce proteins called lectins.

2. Lectins bind to polysaccharides on the cell surface of the bacteria.

3. Bacteria invade roots, spreading along the root hairs.

4. Some cells in the root are stimulated to divide and produce nodules, which the bacteria use to build up colonies inside.

  • Conditions required by bacteria:
  • Supply of Hydrogen (from NADPH in plant)
  • Supply of ATP (from metabolism of sucrose in plant)
  • Anaerobic conditions
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Taking Nitrogen out of the air

Lightning can cause Nitrogen molecules to react with Oxygen molecules to form Nitrogen Oxides, which dissolve in rain water and are carried to the ground

The Haber Process reacts Nitrogen and Hydrogen from the air with each other to form Ammonia. The Ammonia is then used to make Ammonium Nitrate (a fertiliser) which is spread on the ground.

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The Role of Animals, nitrifying bacteria and denit

  • Animals can only use Nitrogen that is already part of an organic molecule (cannot take it straight out the air)
  • Most Nitrogen obtained from proteins in our diet, and also from nucleic acids
  • Death = Proteins broken down into amino acids by decomposers. Some Amino acids are broken down into Ammonia in ammonification.

Nitrifying bacteria:

  • eg. Nitrosomonas and Nitrobacter
  • Only occurs in well-aerated soil (bogs deficient in Nitrates)
  • Ammonia to Nitrites to Nitrates

Denitrifying bacteria:

  • Reverse nitrogen fixation
  • Nitrates converted to Nitrogen (g)
  • Common in sewage treatment works, compost heaps and wet soils.
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Hermione Granger


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