On observing the section of root nodule
(i) The pink colour of root nodule is due to the presence of pigment leghaemoglobin.
(ii) The pigment prevents enzyme nitrogenase from oxygen as this enzyme is highly sensitive to oxygen and functions, only in anaerobic conditions (absence of oxygen).
(iii) During the process of biological nitrogen fixation the molecular nitrogen present in the air is converted into inorganic nitrogenous compound (such as nitrite, nitrate and ammonia) through bacteria, cyanobacteria (blue-green algae), etc.
The organism reduces nitrogen by the help of an enzyme nitrogenase (present in only certain prokaryotes).