Biology XI: 12 Mineral Nutrition(5 Biological Nitrogen Fixation)

Biological Nitrogen Fixation Reduction of nitrogen to ammonia by living organisms is called biological nitrogen fixation. Certain prokaryotes are able to fix nitrogen because the enzyme nitrogenase is present exclusively in them.N ≡ N NH3 Nitrogen-fixing microbes can be classified as follows: Nodule Formation Nodules are the small outgrowths on roots. Involves interaction between the roots of the host plant and Rhizobia that multiply and colonise the roots, attaching themselves to the root hair cells and the epidermal cells Root hair curls and bacteria infest there, and finally, reach the cortex, where it starts nodule formation. An infection thread carries the bacteria to the cortex. In the next step, the bacteria get released from the thread and invade the cortex cells. Specialised nitrogen-fixing cells differentiate and form the nodules. Nodules establish a direct vascular connection with the host for exchange of nutrients. Nodules also carry the enzyme nitrogenase and the pink coloured leg haemoglobin. Nitrogenase: A Mo−Fe protein; catalyses the conversion of N2 to ammonia; strictly anaerobe Leg haemoglobin: Oxygen scavenger, which protects the oxygen-sensitive enzyme nitrogenase N2 + 8e− + 8H+ + 16ATP → 2NH3 + H2 + 16ADP + 16Pi The energy required in this process comes from cellular respiration of root cells. Ammonia, so formed, can either form nitrate (which plants can assimilate), or form NH4+ (which is quite toxic, but can undergo further reactions to produce amino acids). NH4+ (ammonium ion) can undergo two types of reactions to produce amino acids: Transamination − transfer of the amino group from one amino acid to the keto group of the keto acid catalysed by the enzyme transaminaseE.g., formation of amides, asparagine and glutamine from amino acids, aspartic acid and glutamic acid, by the addition of the amino groupFixed nitrogen, as in soyabean, is sometimes transported as ureides along with transpiration. Reductive amination − glutamate is formed from NH4+ by the reaction with -ketoglutaric acid. -ketoglutaric acid + NH4+ + NADPHGlutamate + H2O + NADP