Question

Biological nitrogen fixers
a) Able to fix nitrogen into salts of nitrogen
b) Able to fix nitrogen into usable forms
c) Able to convert nitrogen into nitrates
d) Able to convert nitrogen into nitrates only
A. a, b, c
B. a, c
C. b, c
D. a, b

Answer 1

Hint:- Nitrogen is an essential element as it forms an integral component of major biomolecules necessary for life. However, atmospheric nitrogen though abundant is not readily available for use by living organisms. The method by which elemental nitrogen is converted into nitrogenous compounds is called nitrogen fixation. It occurs either through physiological or biological agents.

Complete step-by-step solution:- Biological nitrogen fixation refers to the conversion of atmospheric or elemental nitrogen into usable organic form with the help of living agents. The organic salts of nitrogen in soil get dissolved in water and plants readily absorb these usable nitrogen salts from the soil. Biological nitrogen fixation takes place by two methods, symbiotic nitrogen fixation and non-symbiotic fixation.
Some prokaryotes like bacteria partner with a host plant and fix atmospheric nitrogen. This type of nitrogen fixation is called symbiotic nitrogen fixation and organisms making such associations are called symbionts. The plant provides sugars to the bacteria to fix nitrogen whereas, the bacteria provide fixed nitrogen to the host plant for its growth. A classic example of symbiotic nitrogen fixing association is Legumes and Rhizobium. Other examples include Anabaena with Azolla leaves, Blue green algae in lichen thallus, Nostoc in Cycas roots, etc.
Atmospheric nitrogen is also fixed by several free living microbes like aerobic and anaerobic bacteria, cyanobacteria and few fungi. These microbes can be found in soil or water. Some examples of free living or non-symbiotic nitrogen fixers are Azotobacter (aerobic bacterium), Clostridium (anaerobic bacterium) Rhodospirillum (photosynthetic bacterium), Yeast (fungus), and Calothrix (cyanobacteria).
Since, biological nitrogen fixers are capable of fixing elemental nitrogen into salts of nitrogen like nitrate, nitrite and finally ammonia which being toxic is converted into usable form like amino acid. So, the correct option is (D), a, b,

Note:-
Two important biomolecules required for nitrogen fixation are Nitrogenase and Leghaemoglobin. Enzyme nitrogenase is a Molybdoferedoxin or Mo-Fe protein complex. It acts as a catalyst during the reduction of atmospheric nitrogen into ammonia and is highly sensitive to oxygen. Leghaemoglobin is an oxygen scavenger which protects nitrogenase during nitrogen fixation.