Answer:

Explanation:

• Nitrogen EXISTS as two nitrogen atoms joined by a very strong triple covalent bond (N ≡ N). The process of conversion of nitrogen (N₂) to ammonia is termed as nitrogen-fixation.
• In nature, lightning and ultraviolet radiation provide enough energy to convert nitrogen to nitrogen oxides (NO, NO₂, N₂O).
• Industrial combustions, forest fires, automobile exhausts and power-generating stations are also sources of atmospheric nitrogen oxides.
• Decomposition of organic nitrogen of dead plants and ANIMALS into ammonia is called ammonification.
• Some of this ammonia volatilises and re-enters the atmosphere but most of it is converted into nitrate by soil bacteria.
• Ammonia is first oxidised to nitrite by the bacteria Nitrosomonas and/or Nitrococcus.
• The nitrite is further oxidised to nitrate with the help of the bacterium Nitrobacter. These steps are called nitrification. These nitrifying bacteria are chemoautotrophs.
• The nitrate thus formed is absorbed by plants and is transported to the leaves.
• In leaves, it is reduced to form ammonia that finally forms the amine group of amino acids.
• Nitrate present in the soil is also reduced to nitrogen by the process of denitrification. Denitrification is carried by bacteria Pseudomonas and Thiobacillus.

2NH₃ + 3O₂ ⇒ 2NO₂⁻ + 2H⁺ + 2H₂O

2NO₂⁻ + O₂  ⇒2NO₃⁻

AMMONIFICATION

    N ≡ N ⇒ NH₃ by nitrogenase

N₂ + 8e⁻ + 8H⁺ +16ATP ⇒ 2NH₃ + H₂ + 16ADP + 16Pi

• Nodule formation involves a sequence of MULTIPLE interactions between Rhizobium and roots of the host plant.
• The nodule contains all the necessary biochemical components, such as the enzyme nitrogenase and leghaemoglobin.
• The enzyme nitrogenase is a Mo-Fe protein and catalyses the conversion of atmospheric nitrogen to ammonia.
• The enzyme nitrogenase is highly sensitive to the molecular oxygen; it requires anaerobic conditions.
• The nodules have adaptations that ensure that the enzyme is protected from oxygen.
• To protect these enzymes, the nodule contains an oxygen scavenger called leg-haemoglobin. It is interesting to note that these microbes live as aerobes under free-living conditions (where nitrogenase is not operational), but during nitrogen-fixing events, they become anaerobic (thus protecting the nitrogenase enzyme).
• You must have NOTICED in the above reaction that the ammonia synthesis by nitrogenease requires a very HIGH input of energy (8 ATP for each NH3­ produced). The energy required, thus, is obtained from the respiration of the host cells.