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Nitrogen Cycle

Nitrogen is found in essential compounds such as proteins, nucleic acids, vitamins and in growth regulators. Nitrogen cycle helps in the continuous supply of nitrogen to the living organisms. During this process, the atmospheric nitrogen is fixed into the organic combinations, such as amino acids, proteins and nucleic acids, etc., in living organisms via inorganic forms such as NH4+. As living organisms die and decay inorganic nitrogen is liberated. After their death the animals and plants are decomposed by the microbial activities to produce ammonia. The ammonia is first converted into nitrites and then to nitrate. This process is referred to as nitrification. The conversion of ammonia into nitrite is carried out by the bacteria, Nitrosomonas, and that of nitrite into nitrate is by Nitrobacter.
2NH3 + 3O2 2NO2- + 2H2O
2NO2- + O22NO3-

Nitrate is either available to the plant or converted into nitrogen gas in the process of denitrification by other microorganisms, e.g. Pseudomonas. The nitrogen gas may be again fixed in the form of NH4+ through the process of biological nitrogen fixation.

Biological Nitrogen Fixation

Dinitrogen gas (N2) is useful to plants and for their nitrogen requirements they have to depend on a few species of monera which are capable of converting N2 into more generally usable forms such as ammonia, nitrites or nitrates. These bacteria are called nitrogen fixers, and the process is called as nitrogen fixation. The best known of the nitrogen fixers belong to the genus Rhizobium. Species of Rhizobium are found free living in the soil, where they do not fix nitrogen. They are also found in the nodules of the roots of plants from the family Leguminosae which includes plants such as Pisum, soyabean, alfafa and a large number of shrubs and trees. The root nodules of these plants contain Rhizobium which fixes nitrogen.

Root Nodules on Ground Nut Root

In the formation of the symbiotic relationship between the bacteria and the plant in the root nodule, the bacteria are attached to a root hair first and make contact with it. A small tube called infection thread is formed in the root hair and the thread grows into the cortex of the root. The bacteria undergo division and move within the thread as it penetrates. When the tip of an infection thread enters a tetraploid cell, it bursts, releasing the bacteria into the cytoplasm. The infected plant cells divide forming root nodules, and the cells of Rhizobium enlarge and develop into nitrogen fixing bacterioids within vacuoles.

Neither the free-living Rhizobium species nor an uninfected legume can fix nitrogen, only when the two are in close association as found in root nodules does the reaction take place. This sort of symbiotic relationship between two organisms, where both the organisms benefit from the relationship, is called mutualism.

Rhizobium in a Legume

Nitrogen Fixation

In the process of nitrogen fixation, the dinitrogen molecule is progressively reduced by the addition of pairs of hydrogen ions;

Each of these reactions occurs while the reactants (dinitrogen and hydrogen) are firmly bound to the surface of a single enzyme called nitrogenase. These reactions are diagrammatically represented. Under anaerobic conditions the enzyme nitrogenase binds a dinitrogen molecule, while the dinitrogen is reduced by the addition of three successive pairs of hydrogen atoms. The final products are two molecules of ammonia which are released so that the enzyme nitrogenase is free to bind another dinitrogen molecule. The nitrogenase molecule contains molybdenum and several iron atoms.

The process of nitrogen fixation requires a strong reducing agent for the transfer of hydrogen atoms to the nitrogen molecule initially and to the intermediate products, subsequently. The process also requires a great deal of ATP. Depending on the species of nitrogen fixer, either respiration or photosynthetic metabolism provides the necessary ATP molecules and the reducing agent.


The nitrogen thus fixed in the form of ammonia is toxic to plants. Plants generally absorb ammonia in the form of ammonium ions (NH4-) which can be taken up safely. However, most plants absorb nitrogen in the form of nitrates (NO3-). The oxidation of ammonia to nitrate is accomplished by the activities of three bacterial genera in the soil. This process is called nitrification. In nitrification the initial step is the conversion of ammonia to nitrites. Two bacterial genera namely Nitrosomonas and Nitrosococcus are capable of doing this. Then, the Nitrobactor bacteria bring about the conversion of nitrites to nitrates.

Nitrate Reduction: In plants the nitrogen metabolism is a complex process. Plants reduce the nitrates all the way to ammonia and finally the nitrogen is incorporated into amino acids. These reactions are all carried out with the help of the plant's own enzymes and the latter steps of nitrate reduction occur in chloroplasts.

Nitrogen Fixation by Nitrogenease

Synthesis of Amino Acids

Amino acids are generally the initial products of nitrogen assimilation. Each amino acid contains at least one carboxyl (-COOH) group and one or several amino )-NH2) groups. There are two main processes by which majority of amino acids in plants are synthesized.
  1. Reductive amination
    In this process, ammonia reacts with α -ketoglutaric acid and forms Glutamic acid as indicated below
  2. Transamination
    It involves the transfer of amino group from one amino acid to the keto group of keto acid. Glutamic acid is the main amino acid from which other 17 amino acids are formed through transmination. The enzyme responsible for such reaction is termed as transaminase.


The two most important amides found in plants are asparagines and glutamine. These are formed from two amino acids, namely glutamic acid and aspartic acid. In this process, hydroxyl part of the acid is replaced by another NH2 radicle. The reaction takes place in the presence of the enzymes glutamine synthetase or asparagines synthetase. Amides contain more nitrogen than amino acids and are structural part of most proteins.

Protein Synthesis

Proteins consist of one or more chains called polypeptide chains, each of which consists of hundreds of amino acids. The number of amino acids varies greatly among proteins and therefore, the molecular weight of proteins also varies. The linkage of amino acids and amides in the polypeptide chain occurs through peptide bond, involving the carboxyl group one amino acid and the amino group of the next. The proteins are highly specific due to sequence in which the amino acids are present in the protein.

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