Nitrogen Fixation

 Introduction :-

•                                        Nitrogen gas 78.084% is atmosphere.
•                                        In soil it is found in the form of N2.
•                                        Nitrogen less active not stable.

  Definition :-

                                  Nitrogen fixation is a process by which nitrogen of earth is converted into ammonia and other molecules available for living organism.

  Importance :-

•                           Major source in plant next to water. 
•                           Building block of plants
•                           Consistently of :-
•                           Chlorophyll cytochrome many vitamins

  Important role in :-

•   Metabolism 
•   Reproduction
•   Hereditary

Types of fixation :-

   Physical               Chemical              Biological

    N2 + O2                    N2 + 3H2                   Asymbiotic

  Nitric Oxide                  2NH3                      Symbiotic

Biological  Nitrogen Fixation :-

                                                  Nitrogen is in abundance is atmosphere but cannot be taken up by plant because it is in highly from and to show some Eukaryotic tricks and two so plant can use it.

                                                                  Symbiotic Nitrogen

               Symbiotic nitrogen fixation in legumes. This is the  best and most important type of symbiotic nitrogen fixation.Legumes (about 1700 species) belong to the Fabaceae . They form root or in a few cases stem, nodules containing nitrogen fixing rhizobia most of which belong a proteobacteria. These symbonts are related to Agrabacterium an organism that can invade plant , tissue and from tumerous growth. This might explain how the rhizobiumlegume relationship originally involved. However , a small number of Beta- proteobacteria has also  been documented as tropical legume  symbionts so the relationship with Agrobacterium is uncertain among non-legumes only a tree belonging to the elm family (genus parasaponia)  is know to form rhizobia nodules that contain Bradyrhizobium or Rhizobium symbionts . 

          Rhizobia  occur as free living bacteria in soils. They are relatively rare in which legume have not been grown  over a period of many years, but are especially e numerous  in the rhizophere ( the soil surrounding roots) of leggings perhaps they are stimulated by roots exudates. Under microaeobic  conditions  they can be induced to fix nitrogen to a variable degree. The extent to which they fix nitrogen in soils  is not know, but it would seem likely that this property is adaptive under some circumstances . Rhizobia multiply around germinating Legumes.  Infection and subsequent nodules  formation requires adhesion  to root hairs and whether this takes place depends on the species of legum and the symbiote strain some strains( cross inoculation groups) can infect  several species of legumes and some legumes can nodules  with different rhizobia strance.  Adhesions depends on specific lectins  produced by the host plant and on specific polysaccharide cell coatings produced by the bacteria. Leghaemoglobin represents one of the more notable features of the rhizobialegume symbiosis. It is a true haemoglobin the synthesis of which depend on symbionts genes for the protein. Leghaemoglobin which has a high  affinity for oxygen maintains  low oxygen tension with in nodules this protecting the highly oxygen sensitivity nitrogenase while at the same time supply the symbionts with enough oxygen to maintain a high rate of aerobic metabolism. This is essential for producing the large supply of ATP needed nitrogen fixation.

Asymbiotic Nitrogen Fixation 

 Some of the bacteria and most of the cyanobacteria comprise this class of microorganisms. They are also called free living dizotrophs.  Among cyanobacteria unicellular, filaments non - hitrcyctous fix nitrogen independently. Both aerobic and anaerobic bacteria are free        living diazotrophs. Water, oxygen , nutrients are required in optimum amount so that the microorganism can grow cyanobacteria group mainly in the crop fields. The site of nitrogen fixation in the cyanobacteria is heterocyst  because the enzyme( nitrogenase) required for nitrogen fixation acts  under anaerobic conditions.