Nitrogen cycle


 Nitrogen Cycle

Atmospheric  nitrogen gas ( N2) is unavailable  to plants. Plants, therefore,  depend on various  types of nitrogen_ fixing  bacteria  to take up nitrogen gas make it available  to them  as some form of organic nitrogen. Nitrogen fixation  occurs when nitrogen gas is chemically  reduced  and nitrogen is added to organic compounds. Atmospheric  nitrogen  is converted to ammonium  ( NH4+) by some cyanobacteria in aquatic ecosystems and by nitrogen _ fixing  bacteria  in the nodules on roots of legume ( beans, peas, clover, etc.) Plants in terrestrial  ecosystems. 


Apart from  carbon, hydrogen and oxygen, nitrogen is the most prevalent element in living organisms. Nitrogen is a constituent of amino acidsproteins, hormones,  chlorophylls and many of the vitamins. Plants complete with microbes for the limited nitrogen  that is available  in soil. Nitrogen  is a limiting  nutrient for both natural and agricultural  eco_ systems.  Nitrogen  exists as two nitrogen atoms joined  by a very strong triple covalent  bond ( N=N). The process of conversion  of nitrogen ( N2) to ammonia  is termed as nitrogen_ fixation.  In nature, lightning  and ultraviolet radiation  provide enough  energy to convert nitrogen to nitrogen  oxides ( NO, NO2, N2O). Indistrial combustions , forest fires, automobiles  exhausts and power _ generating  stations are also sources of atmospheric nitrogen oxides. Decomposition of organic nitrogen of dead plants and animals into ammonia is ed ammonofication. Some of this ammonia volatilises  and re_ enters the atmosphere but most of it is converted into nitrate by soil bacteria in the following  steps: 

Ammonia is first oxidation  to nutrite by the bacteria . Nitrosomonas and / or Nitrococcus. The nitrite is further oxidised to nitrate  with the help of the bacterium. Nitrobacter. These steps are called nitrification. These nitrifying bacteria  are chemoautotrophs. 


 The nitrate thus formed is absorbed by plants and is transported to  the leaves. In leaves, it is reduced to form ammonia that finally  forms  the amine groups  of amino acids. Nitrate present in the soil is also reduced to nitrogen by the process of denitrification.  Denitrification  is carried by bacteria  Pseudomonas and Thiobacillus. 

Biological  Nitrogen Fixation: 

Very few living organisms  can utilise the nitrogen  in the form N2, available  abundantly  in the air. Only certain prokaryotic species are capable   of fixing  nitrogen.  Reduction of nitrogen to ammonia  by living  organisms is called biological  nitrogen fixation . The enzyme,  nitrogenase  which  is capable  of nitrogen reduction  is present exclusively  in prokaryotes . Such microbes are called N2_ fixers. 

        N= N➡️➡️➡️➡️➡️➡️➡️ NH3

The nitrogen _ fixing microbes could be free_ living or symbiotic. Examples  of free_ living  nitrogen_ fixing aerobic microbes are Azotobacter and Beijernickia while, Rhodospirillum is anaerobic  and Bacillus  free _ living. In addition , a number of cyanobacteria  such as Anabaena and Nostoc are also free _ living  nitrogen _ fixaters. 

Symbiotic  biological  nitrogen fixation 

Several types of symbiotic biological  nitrogen fixing association are known. The most prominent  among them is the legume_ bacteria  relationship. Species  of rod_ shaped Rhizobium has  such relationship with the roots of several  legumes such as alfalfa, sweet clover, sweet pea, lentils, garden pea, broad bean, clover beans, etc. The most common  association  on roots is as nodules. These nodules are small outgrowths on the roots. The microbe, Frankia, also produces nitrogen_ fixing  nodules  on the roots of non_ leguminous  plants ( e.g., Alnus).  Both Rhizobium  and Frankia are free _ living  in soil, but as symbionts,  can fix atmospheric  nitrogen. 

 Uproot any one plant of a common pulse, just before flowering .You will see near_ spherical  outgrowths on the roots. These are nodules. If  you cut through  them you  will notice that  the  central portion  is red or  pink. What makes the nodules  pink? This is due to the presence  of leguminous  haemoglobin  or  leg_ haemoglobin. 

Nodule Formation 

Nodule formation  involves a sequence  of multiple interactions between  Rhizobium  and roots of the host plant. Principle  stages in the nodule formation are summarised as follows: 

Rhizobia multiply and colonise the surrounding  of roots and get attached to epidermal  and root hair cells. The root_ hairs curl and the bacteria invade  the root_ hair. An infection thread is produced carrying  the bacteria  into the cortex of the root, where they initiate the nodule formation  in the cortex of the root. Then the bacteria  are released from the thread into the cortex of the root. Then the bacteria  are released  from the  thread into the cells which leads to the differentiation of specialised nitrogen fixing  cells. The  nodule thus formed, establishes a direct vascular  connection  with the host for exchange of nutrients. These events are depicted .

 The nodule contains  all the necessary  biochemical  components,   such as the enzyme nitrogenase  and leghaemoglobin. The enzyme  nitrogenase is a Mo_ Fe protein  and catalyses the conversion of atmospheric  nitrogen to ammonia, the first stable product  of nitrogen fixation. 

  Development of root nodules in soyabean: 

(a) Rhizobium  bacteria  contact a susceptible  root hair, divide near  it, 

(b) Successful infection  of the root hair causes it to curl.

(c) Infected thread  carries the bacteria  to the inner cortex. The bacteria  get modified  into rod_ shaped bacteroids  and cause inner cortical  and pericycle  cells to divide. Division  and growth of cortical and pericycle cells lead  to nodule formation  

(d) A mature nodule is complete with vascular  tissues  continuous  with those of the root. 

 The reaction  is as follows:

The enzyme  nitrogenase  is highly sensitive to the molecules  oxygen; it requires anaerobic  conditions. The nodules have adaptations that ensure that the enzyme  is protected  from oxygen. To protect  these enzymes,  the nodule contains  an oxygen scavenger celled leg_ haemoglobin. It is interesting  to note that these microbes live as aerobic  under free_ living conditions  ( where nitrogenase is not operational), but during nitrogen _ fixing events, they  become anaerobic  ( thus protecting  the nitrogen  enzyme). You must  have noticed in the above reaction  that the ammonia synthesis  by nitrogenase requires  a very high input of energy  ( 8 ATP for each NH3 produced). The energy required, thus, is obtained  from the respiration  of the host cells. 

Fats of ammonia: At physiological  pH, the  ammonia  is protonated to form NH+4 ( ammonium) ion. While most  of the plants can assimilate  nitrate as well as ammonium ions, the latter is quite toxic to plants and hence cannot accumulate in them. Now see how  the NH+4 is used to  synthesise amino acids in plants. There are two main ways in which this can take place: 

(i)  Reductive animation: In these processes, ammonia  reacts with æ_ ketoglutaric acid and forms glutamic acid as indicated  in the equation  given below: 

(ii) Teansamination : It involves the transfer of amino group from  one amino acid to the keto group of a keto acid. Glutamic acid is the main amino acid  from which the transfer of NH2, the amino group takes place and other amino acids are formed through transaminstion. The enzyme transaminase catalysts  all such reactions. For example,  

The two most important  amides_ asparagine  and glutamine _ formed in plants are a structural part of proteins. They are formed from  two amino acids, namely aspartic acid and glutamic acid, respectively,  by addition  of another amino group to each. The hydroxyl part of the  acid is replaced by another NH-2 radicle. Since amides contain more nitrogen than  the amino acids, they are transported to other parts of the plant via xylem vessels. In addition , along with the transportation  stream the nodules of some plants (e.g., soyabean) export the fixed nitrogen as ureides. These compounds also have a particularly  high nitrogen to carbon ratio. 


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