Nitrogen Fixation explained in a simple manner with an example. NEET Botany XI Mineral Nutrition
AIM
Nitrogen metabolism
Nitrogen Fixation:
The conversion of inert gaseous molecular dinitrogen into salts of nitrogen that can be assimilated by plants is called nitrogen fixation. It is of two types, Abiological (natural and industrial) and biological.
Biological Nitrogen Fixation:
It constitutes 60% of total nitrogen fixation. It was first studied by Winogradsky. Nitrogen fixers are of two types, free living and symbiotic.
1. Free Living Nitrogen Fixing Bacteria:
(a) Aerobic: Azotobacter, Azospirillum
(b) Anaerobic: Clostridium, Methanococcus
2. Free Living Nitrogen Fixing Blue-green Algae:
Anabaena, Nostoc, Aulosira, Cylindrospermum
3. Symbiotic Nitrogen Fixing Bacteria:
Rhizobium in root nodules of legumes, Aerorhizobium in stem of Sesbania, Bradyrhizobium japonicum in Soyabean, Frankia in Alnus, Xanthomonas and Mycobacterium form leaf nodules in Ardisia and several members of family Rubiaceae.
Nodule Formation:
Rhizobium lives saprophytically in soil. Roots of legumes excrete sugars or specific amino acids which attract the bacteria. Rhizobia produce ‘Nod factor’ which help the bacteria to get attached to root hair. Nod factor induce curling of root hair. At the tip of root hair cell wall degenerate and infection thread is formed in root hair. Infection thread reaches cortical cells and branches repeatedly. Bacteria enter into cortical cells of the host through the infection thread. Bacteria enlarge and get differentiated into bacteroids. Peribacterial membrane surrounds the group of bacteroids. Outside bacteroid membrane a pink pigment, leg haemoglobin is present. It acts as oxygen scavenger and gives pink colour to nodules. Nodule acts as a site for N_2 fixation. Enzyme nitrogenase is required for N_2 fixation. Enzyme is Mo-Fe protein and catalyses the conversion of atmospheric N_2 to NH_3.
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