MINERAL NUTRITION

 

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.

 

CBSE Biology (Chapter Wise) Class XI ( By Mr. Hare Krushna Giri ) 
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