Explain the process/mechanism of biological nitrogen fixation.

(N/A) $ \Rightarrow $ The root nodule contains all the necessary biochemical components such as the enzyme nitrogenase and leg-haemoglobin.
$ \Rightarrow $ The nitrogenase enzyme is a $Mo-Fe$ protein and catalyzes the conversion of atmospheric nitrogen to ammonia.
$ \Rightarrow $ Ammonia is the first stable product of nitrogen fixation.
$ \Rightarrow $ The steps of conversion of atmospheric nitrogen to ammonia by the nitrogenase enzyme complex found in nitrogen-fixing bacteria are shown in the figure.
$ \Rightarrow $ The overall chemical reaction is as follows: $N_{2} + 8e^{-} + 8H^{+} + 16ATP \rightarrow 2NH_{3} + H_{2} + 16ADP + 16Pi$
$ \Rightarrow $ The enzyme nitrogenase is highly sensitive to molecular oxygen. It requires anaerobic conditions to function.
$ \Rightarrow $ To protect these enzymes from oxygen, the nodule contains an oxygen scavenger called leg-haemoglobin.
$ \Rightarrow $ 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).
$ \Rightarrow $ As per the equation, the ammonia synthesis by nitrogenase requires a very high input of energy ($8ATP$ for each $NH_{3}$ produced).
$ \Rightarrow $ The energy required is obtained from the respiration of the host cells.