Question

Which complex is responsible for the reduction of molecular nitrogen to the level of $NH_3$ the root nodule of legume?
A. Aminase
B. Nitrogenase
C. Nitrate reductase
D. Nitrite reductase

Answer 1

The molecular nitrogen is converted to ammonia in the process of nitrogen fixation. In the atmosphere, nitrogen is present as molecular di nitrogen which is converted to ammonia by several means.

Complete step by step answer:
If the nitrogen fixation is done by living organisms, then it is known as biological nitrogen fixation. Nitrogen fixing bacteria or cyanobacteria are either free living or symbiotic in nature.
In leguminous plants, a special structure is formed in the roots for nitrogen fixation. It is called a root nodule. It is formed by the curling of root hair when the plant gets in contact with bacteria Rhizobium. Infection thread is formed and the bacteria invades the root hair. Some bacterias among these are modified into rod shaped structures called bacteroides. In the final step, a vascular connection is also enabled between the nodule and the host.
Root nodules primarily contain the enzyme Nitrogenase. It catalyses the conversion of nitrogen to ammonia. This ammonia produced is used for synthesis of amino acids and finally into proteins by joining several amino acids.
Root nodules also contain a special molecule called leg hemoglobin. Know that enzyme nitrogenase is highly sensitive for oxygen. It needs anaerobic condition for its function. Leg hemoglobin acts as an oxygen scavenger and protects the enzyme by providing an anaerobic environment.

Hence the correct option is B.

Note:
The gene responsible for nodule formation in legumes are known as plant nod genes. Nitrogen fixation occurs under control of plant nod genes, bacterial nod genes, nif genes and fix genes. Note that three bonds between nitrogen atoms are broken and ammonia is formed in this process.