Question

Briefly describe the mechanism of biological nitrogen fixation. 

OR 

What is meant by biological nitrogen fixation ? Elaborate your answer with the example 

OR 

Explain symbiotic nitrogen fixation in leguminous plants.

Answer 1

Biological nitrogen fixation is the process in which certain living organisms convert the free nitrogen into those compounds which can be used by plants. 

(i) The organisms may be free-living bacteria (e.g., Azotobacter), symbiotic bacteria like Rhizobium or Cyanobacteria. 

(ii) The enzyme involved in the process is nitrogenase, which is a Mo-Fe protein. 

(iii) During this process, the atmospheric nitrogen is reduced by the addition of hydrogen. 

(iv) The three bonds between the two nitrogen atoms (N ≡ N) are broken and hydrogen is progressively added to form ammonia. 

(v) This fixation requires three components: 

(a) ATP, to supply energy. 

(b) A strong reducing agent, to transfer hydrogen atoms. 

(c) The enzyme system (nitrogenase) 

(vi) The overall reaction is as follows : 

N₂ + 8e- + 8H⁺ + 16 ATP-→ 2NH₃ + H₂ + 16 ADP +16 Pi