Nitrogen assimilation is the process by which inorganic nitrogen compounds are used to form organic nitrogen compounds such as amino acids, amides, etc. Plants and other organisms, which cannot utilise nitrogen molecules directly, depend on the absorption of nitrogen as nitrates or ammonia. Animals depend on organic nitrogen available in their food.
Nitrogen is one of the most important elements along with carbon, hydrogen and oxygen. It is part of biomolecules such as nucleic acids (DNA, RNA), proteins (amino acids), hormones, vitamins, chlorophylls, alkaloids, enzymes and many other cellular components. It is necessary for sustaining life.
Nitrogen is present abundantly in the atmosphere (78%) but only a few organisms can utilise the atmospheric nitrogen directly. Plants and microorganisms compete for the little nitrogen available in the soil in the usable form as ammonia, nitrite, nitrates, etc. Nitrogen acts as a limiting agent naturally as well as for agricultural purposes.
Nitrogen occurs as dinitrogen (N2) in the atmosphere, wherein the two Nitrogen are bonded together by triple covalent bonds. Only some organisms like bacteria, blue-green algae (cyanobacteria) are capable of utilising and fixing atmospheric nitrogen. So it is important to convert nitrogen into other forms to make it available for absorption by plants and other organisms.
The nitrogen cycle comprises nitrogen fixation, ammonification, nitrification, denitrification and assimilation.
- Nitrogen fixation – It is the process of converting atmospheric nitrogen to ammonia. Nitrogen fixation takes place naturally by lightning, UV radiation. It also occurs through industrial processes and by living organisms. The reduction of nitrogen to ammonia by living organisms is called biological nitrogen fixation.
- Ammonification – It is the process by which decomposition of dead and decaying organic matter leads to the formation of ammonia.
- Nitrification – The conversion of ammonia to nitrite and nitrate in the soil by bacteria is termed nitrification. Ammonia is first converted to nitrite by the bacteria Nitrosomonas and Nitrococcus and then it is further oxidised to nitrate by the bacterium Nitrobacter.
- Denitrification – Nitrate in the soil also gets converted back to nitrogen by the process of denitrification. Bacteria like Pseudomonas and Thiobacillus carry out denitrification.
- Assimilation – It is the process by which nitrate and ammonia are utilised to form organic compounds.
Nitrogen Assimilation in Plants
Plants absorb nitrogen from the soil in the form of nitrates and ammonium ions. Ammonium ions and nitrates are absorbed by the plants via their respective transporters.
Nitrate after absorption is transported to the leaves and gets reduced to ammonia. Further, ammonia is converted into amine groups of various amino acids.
Nitrate reduction takes place in two steps. First, nitrate is reduced to nitrite in the cytoplasm by nitrate reductase and then nitrite is reduced to ammonia by nitrite reductase in the chloroplasts. It also occurs in plastids in the roots.
This ammonia gets incorporated into glutamate to form glutamine. The reaction is catalysed by glutamine synthetase. By transamination, other amino acids are formed like asparagine. This is called the glutamine synthetase-glutamate synthase (GS-GOGAT) pathway.
Ammonia gets protonated at the physiological pH and is converted into ammonium ions. Plants cannot accumulate ammonium ions as it is fairly toxic. It gets converted to amino acids by various mechanisms.
Synthesis of amino acids from ammonium ion (NH4+) in plants
The synthesis of amino acids from NH4+ takes place by two main pathways.
- Reductive Amination: Glutamate is formed as a result of the reaction of ammonium with 𝛂-ketoglutaric acid. The reaction is catalysed by glutamate dehydrogenase (GDH pathway).
- Transamination: In this process, the amino group is transferred from one amino acid to the other keto acid, leading to the formation of another amino acid. Glutamic acid is the main amino acid from where the transfer takes place. The enzyme transaminase catalyses this reaction.
𝛂-ketoglutaric acid + NH4+ + NADPH → glutamate + H2O + NADP
Asparagine and Glutamine are formed by the addition of another amino group to aspartic acid and glutamic acid, respectively. These amides are transported through xylem vessels to the other parts of plants.
Some plants like soybean, transport fixed nitrogen from nodules as ureides. It occurs along with the ascent of sap due to transpiration.
This was in detail about Nitrogen assimilation. Check the notes on Nodule Formation to learn in detail about symbiotic nitrogen fixation.
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