Nitrogen Metabolism: Definition, Abiological Nitrogen Fixation, Symbiotic Nitrogen Fixation, Ammonification, Denitrification, Nitrate Assimilation, and, Formulation of Amino Acids

Nitrogen Metabolism: Definition, Abiological Nitrogen Fixation, Symbiotic Nitrogen Fixation, Ammonification, Denitrification, Nitrate Assimilation, and, Formulation of Amino Acids


The Nitrogen cycle keeps the plants’ regular supply of nitrogen up.

Abiological Nitrogen Fixation

There are two types of abiological nitrogen fixation:

Fixation of Nitrogen in industry

Under pressure and temperature, nitrogen and hydrogen combine to produce ammonia in industry.

Natural Abiological Nitrogen fixation

– Nitric oxide (NO) is produced when the air’s N2 and O2 react as a result of clouds lighting and thundering

– Nitric oxide undergoes further oxidation with the help of oxygen to produce nitrogen peroxide (NO2)

– Nitric acid (NHO3) and nitrous acid (HNO2) are produced when NO2 reacts with water

– The acid and rainwater fall together.

It now reacts with alkaline radicals to produce water-soluble NO3 (nitrates) and NO2 ( nitrides)

2NO ( in presence of lightening)

N2 + O2———–   2NO2

2NO2 + H2O———- HNO2 + HNO3

Biological Nitrogen Fixation

Some blue-green algal (Anabaena, Nostoc) and free-living (Azotobacter) bacteria take in atmospheric nitrogen, convert it to ammonia, and mix it with organic acids to form amino acids.

Free-Living Nitrogen-Fixing Bacteria

– The saprotrophic bacteria that fix nitrogen are Azotobacter, Beijerinckai and Bacillus, Clostridium.

Desulphovibrio is a chemotrophic nitrogen-fixing bacteria.

– The anaerobic photoautotrophic bacteria Rhodopsludomonas, Rhodospirillum, and Chromatium fix nitrogen.

Free-Living Nitrogen-Fixing Cyanobacteria

– Nitrogen is fixed by several blue-green algae that are free-living. Anabaena, Nostoc, Calothirs, Aulosira, and trichodesmium are a few examples.

While cylinderospermum is active in sugarcane and maize fields, aulosira ferilissima is the most active nitrogen-fixing rice field.

Symbiotic Nitrogen-Fixing Cyanobacteria

– Lichens, Anthoceros, Azolla, and cycad roots frequently contain Anabaena and Nostoc species as symbionts.

– The water fern Azolla pinnata has azolla and anabaena in its fronds. For nitrogen fixation, it is frequently injected into rice fields.

Symbiotic Nitrogen-Fixing Bacteria

Rhizobium and Aerorhizobium are found in Sesbania rostrata’s stem and root nodules, respectively.

– Several non-legume plants, including casuarinas (Australian pine), Myrica, and Alnus, have Frankia as a symbiont in their root nodules

( Alder)

– Several species of Rubiaceae and Myrsinaceae form symbiotic associations with Xanthomonas and Mycobacterium.

Rhizobium is the most significant bacterium for cropland since it is connected to pulses and other Fabaceae-family legumes.

Symbiotic Nitrogen Fixation 

Leguminosae family members’ root nodules contain bacteria that live in symbiotic relationships with one another. Rhizobium leguminosaeum is the most well-known symbiotic bacteria that fix nitrogen.

Leguminosae plants, including beans, peas, grains, soybeans, and others, have tiny nodules that resemble swellings on their major roots. Through the infection thread, Rhizobium enters the cortex of the root. simulating more vigorous division to create nodules on the root.

Legumes produce chemical attractants in their roots. When bacteria build up on root hairs, they release a nod factor that causes the root hairs to curl up around the bacteria. The bacterial population increases along with the infection thread. It divides, and the ends of its branches meet at the protoxylem points of the vascular strand. Swellings or nodules are produced by the infection. Auxin produced by cortical cells and cytokinin released by invading bacteria both promote nodule growth. the cytokinin released by invasive germs and the infected cells. Cells that are infected expand. Bacteriods are the sporangiated polyhedral structures formed when bacteria stop dividing. Some bacteria, however, split, maintain their regular structures, and spread to other places.

When a section of root nodules is examined, the presence of a pigment, leghaemoglobin, is found to lend a pinkish colour to it. This is because infected cells often have bacteriophages that occur in groups and are encircled by the host membrane. This haemoglobin-related pigment aids in fostering the best conditions possible for nitrogen fixation. Leghaemoglobin is an oxygen-scavenger similar to haemoglobin. With the aid of the nitrogenase enzyme, which operates in anaerobic environments, nitrogen is fixed. Leghemoglobin defends nitrogenase when it mixes with oxygen.

It is thought that throughout the nitrogen fixation process, the free, atmospheric nitrogen is first attached to the surface of the enzyme and is not released until it has been reduced to ammonia. Nitrogen fixation requires the enzyme nitrogenase, molecules for trapping ammonia generated by reduction of dinitrogen and reducing power like NADPH, as well as a source of energy like ATP. The enzyme nitrogenase also has iron and molybdenum. They both participate in the attachment of a nitrogen molecule. The two nitrogen atoms’ connection to the metallic components weakens their bonds with one another. Hydrogen from a reduced coenzyme interacts with the nitrogen molecule, which is already weak. Diamide (N2H2), hydrazine (N2H4), and subsequently ammonia (2NH3) are all produced. Ammonia is not released. It is poisonous in even minute amounts. The nitrogen fixers provide organic acids as a defence against it.

Nitrogenase in bacteroids must have access to ATP as well as a reduced substrate that can give nitrogen hydrogen atoms to reduce it into ammonia. The reduced substrate is taken from host cells and is used to produce ATP in the bacteroid respiratory chain system. Ferredoxin and reduced NADP serve as electron carriers in the process, with glucose-6-phosphate being thought of as the reduced substrate. The non-heme iron (NH1) protein component of nitrogenase binds with ATP to alter its shape and make it a potent oxidant. To convert N2 into NH3, this potent reductant can transfer electrons.

For each pair of electrons transported to nitrogen, it was discovered through in vitro studies of the process that at least four ATP molecules are hydrolyzed. As a result, twelve molecules of ATP are needed to reduce one molecule of nitrogen into a molecule of ammonia since each molecule of nitrogen requires the reduction of six electrons.

Ammonification – Organisms that cause decay carry out this process. They affect the proteins in the nitrogenous excretions and the dead bodies of living things. Like B. Vulgaris, Actinomyces, and mesentericus Microorganisms utilise organic acids produced by their metabolism to break down proteins first into amino acids.

Nitrification – It is a phenomenon when ammonium nitrogen turns into nitrate oxygen. Nitrite and nitrate formation are the two phases in the process. Aspergillus flavus is capable of carrying out both procedures. Ammonium ions are initially converted to nitrites by Nitrosococcus and Nitrosomonas. In the second phase, nitrocystis and Nitrobacter convert nitrates into nitrates.

Denitrification – Some microorganisms use nitrate and other oxidised ions as sources of oxygen when they are in anaerobic circumstances. Nitrates are converted into gaseous nitrogen compounds during the process. Later ones emerge from the ground. Pseudomonas denitrificans, Thiobacillus denitrificans, and Micrococcus denitrificans are common bacteria that denitrify soil.

Nitrate Assimilation 

Higher plants absorb and assimilate the nitrates (NO3 -), which are produced by nitrification, in a process known as nitrate assimilation.

Due to integrating amino acids, amides, proteins, and other macromolecules, plant roots that have received nitrates first transform them into nitrates.

Nitrogen assimilation is the conversion of nitrate to ammonia.

The following is the overall summary equation for nitrate to ammonia conversion.

NO3 – + 8 electrons+ 10H+ ——-NH4 – + 3H2O

Nitrate to nitrite conversion

The reaction is carried out in the cytosol outside of any organelle and is catalysed by the enzyme nitrate reductase. NADH is required as an electron donor, FAD as a prosthetic group, cytochrome b557 as an electron carrier, and molybdenum (Mo) as an enzyme activator.

Reduction in Nitrite

The nitrite reductase enzyme catalyses the process, and reduced ferredoxin appears to be the reaction’s most likely electron supplier.

Ammonia is the end product of nitrite reduction, which necessitates the use of power.

Formulation of Amino Acids

Reductive Ammination: In this process, glutamic acid is produced when ammonia interacts with -ketoglutaric acid.

Catalytic Amidation: Ammonia and the amino acid glutamate combine to create amide glutamine in the presence of the enzyme synthestase and ATP. In the presence of decreased coenzyme, glutamine and -ketoglutarate combine to generate two molecules of glutamate.

Transamination is when an amino group from one amino acid is transferred to the keto group of an organic acid. The primary amino acid from which the other 17 amino acids are generated through transamination is glutamic. The transaminase enzyme is in charge of such a process.

Frequently Asked Questions

Question: What is meant by nitrogen metabolism?

Ans: The process by which inorganic nitrogen is incorporated into organic substances to enable the creation of proteins and protoplasm

Question: What are the steps of nitrogen metabolism?

Ans: Nitrogen fixation, Nitrification, Assimilation, Ammonification, and Denitrification

Question: What is the role of nitrogen in metabolism?

Ans: Because the entire life process depends on these molecules containing nitrogen,

Question: Which is essential for nitrogen metabolism?

Ans: ‘Molybdenum’

Question: What is the end product of nitrogen metabolism?

Ans: In mammals is urea

Question: What are the 3 types of nitrogenous waste?

Ans:  (a) ammonia, (b) urea, and (c) uric acid.

Question: Which amino acid plays role in nitrogen metabolism?

Ans: Amino acid Glutamine

Question: How is nitrogen excreted?

Ans: Through the urea cycle.

Question: What is the name of the process of the nitrogen cycle?

Ans: The major transformations of nitrogen are nitrogen fixation, nitrification, denitrification, anammox, and ammonification

Question: Where do plants store nitrogen?

Ans: Roots

Question: What is the largest source of nitrogen?

Ans: Atmosphere

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