Page 4 - LN-CH-12

• The process of conversion of nitrogen to ammonia is called nitrogen fixation. In
nature lightening and ultraviolet radiation provide energy to convert atmospheric

nitrogen into nitrogen oxide ( No, N02 and N20).

• Industrial combustion, forest fire and automobiles along with thermal power plants
produce nitrogen oxides.

• The decomposition of organic nitrogen of dead plants and animals into ammonia is
called ammonification.

• Ammonia is first oxidized to nitrite by bacteria Nitrosomonas or Nitrococcus which is

further oxidized to nitrate with help of bacteria Nitrobactor. These processes are
called nitrification.

• Nitrates formed is absorbed by plants and transported to leaves. Nitrates is converted

into free nitrogen by the process called denitrificaion by bacteria Pseudomonas and
Thiobacillus.

• Reduction of nitrogen to ammonia by living organism is called Biological Nitrogen
Fixation. The enzyme nitrogenase is present in prokaryotic organism called nitrogen

fixer.

• Nitrogen fixing microbes may be symbiotic (Rhizobium) or free living (Nostoc,
Azotobactor, Anabaena).

• Symbiotic biological nitrogen fixation includes legume-bacteria relationship in which

rod shaped Rhizobium lives with syrnbiotic relation with nodules of Leguminous
plants.

• Central portion of nodule is pink or red due to presence of leguminous haemoglobin
or leg-haemoglobin.

Nodule formation involves sequence of interaction between root and Rhizobium as follows-

• Rhizobia increase in number and attach with epidermis of roots. Root hairs curls and
bacteria invade it. An infection thread is formed that carries the bacteria into cortex

of root.

• Nodule formation starts in cortex of root. Bacteria is released from thread to cells
which leads to formation of specialized nitrogen fixing cells.

• Nodules establish direct vascular connection with host for exchange of nutrients.
• Nodule contains all necessary biochemical components like enzyme nitrogenase and

leg-haemoglobin.