Nitrogen cycle :
- Nitrogen fixation: conversion of molecular nitrogen into ammonia.
- Biological nitrogen fixation: Conversion of atmospheric into organic compounds by living organisms.
- Ammonification: decomposition of organic nitrogen of dead plants and animals into ammonia is called Ammonification. (Nitromonasbacteria)
- Nitrification. Ammonia oxidized into nitrite by Nitrosomonasand Nitrococcus bacteria. The nitrite further oxidized to nitrate with the help of Nitrobacter. These steps are called nitrification.
- Assimilation:
- Nitrates absorbed by plant from soil and transported to the leaves.
- In the leaves nitrates reduced to form ammonia that finally forms the amine group of amino acids.
- Denitrification: Nitrate in the soil is also reduced to molecular nitrogen. This process is carried by bacteria like Pseudomonas and Thiobacillus.
Biological nitrogen fixation :
- Reduction of nitrogen to ammonia by living organisms is called biological nitrogen fixation.
- The enzyme nitrogenase which catalyses the process are present in prokaryotes, called nitrogen fixer.
- Nitrogen fixing microbes could be free-living or symbiotic.
- Free-living nitrogen fixing aerobic microbes are Azotobacter and Beijernickia.
- Free-living nitrogen fixing anaerobic microbes are Rhodospirilium.
- A number of cyanobacteria like Anabaena and Nostoc are free-living nitrogen fixer.
Symbiotic nitrogen fixation :
- Best example of symbiotic nitrogen fixation is observed in legume-Rhizobium bacteria.
- Rhizobium form root nodules in leguminous plants.
- 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 symbiont, can fix atmospheric nitrogen.
- The root nodules contain pink coloured pigment contains a protein called leg-haemoglobin.
Nodule formation :
- Nodule formation involves a sequence of multiple interactions between Rhizobium and roots of the host plant.
- Rhizobia multiply and colonize the surroundings of roots and get attached to the epidermal and root hair cells.
- An infection thread is produced carrying the bacteria into the cortex of root.
- Bacteria released from the thread into the cells which differentiated into special nitrogen fixing cells.
- Nodule develops vascular connection for exchange of nutrients.
- The nodule contains an enzyme called nitrogenase.
- Nitrogenase is a Mo-Fe protein and catalyses the conversion of atmospheric nitrogen to ammonia.
- Nitrogenase is highly sensitive to molecular oxygen; it requires anaerobic condition.
- Nodule contains a special protein called leg-haemoglobin.
- Leg-haemoglobin acts as oxygen scavenger and provides anaerobic condition to the bacteria inside the nodules; protect the enzyme nitrogenase from oxidation.
- Ammonia synthesis by nitrogenase is energetically expensive process; 8 ATP required synthesizing each molecule of NH3.
Fate of ammonia :
- At physiological pH, the ammonia is protonated to form NH4+.
- Most of plant assimilated nitrate and ammonium ions.
- Reductive amination: the ammonia reacts with α-ketoglutaric acid and forms Glutamic acid.
- Transamination: 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 by the process of transamination other amino acids are synthesized.
- Two important amides – asparagines and glutamine found in the protein of plant.
- They are formed from two amino acids namely aspartic acid and Glutamic acid respectively.
