Q.What do you understand by symbiotic nitrogen fixation?

Symbiotic Nitrogen Fixation

Think about what a plant needs to grow. Nitrogen is one of the most important nutrients — it's a core building block of proteins and DNA. But here's the problem: plants cannot use the nitrogen gas ($N_2$) that makes up 78% of the air around us. That nitrogen is locked in a triple bond between two atoms, incredibly stable and unreactive. Plants can only absorb nitrogen in fixed forms like ammonia ($NH_3$) or nitrate ($NO_3^-$).

So where does fixed nitrogen come from? Lightning strikes fix a tiny amount. Industrial fertilisers fix a huge amount. But long before factories, nature had its own solution: certain bacteria learned to break that triple bond and convert $N_2$ into ammonia. This process is called nitrogen fixation.

Now, fixing nitrogen is energetically expensive. The enzyme that does it, nitrogenase, is destroyed by oxygen. Most bacteria that fix nitrogen live freely in the soil, but they fix only small amounts. Some bacteria found a better strategy: partner with a plant.

The Mutualistic Partnership

Enter the Rhizobium bacteria and leguminous plants (peas, beans, groundnuts, clover, etc.). The plant provides the bacteria with a protected, low-oxygen home inside its roots, plus a steady supply of carbohydrates (energy). In return, the bacteria fix atmospheric nitrogen into ammonia and supply it directly to the plant. Both partners benefit — that's what mutualism means.

The plant creates special structures called root nodules to house the bacteria. These nodules are like custom-built apartments. The plant controls oxygen levels inside using a protein called leghemoglobin (a pinkish molecule related to our own haemoglobin) that binds oxygen tightly, keeping the environment just right for nitrogenase to work.

The Precise Statement

Symbiotic nitrogen fixation is the biological process in which the bacterium Rhizobium (or related genera like Bradyrhizobium, Sinorhizobium) lives inside the root nodules of leguminous plants and converts atmospheric dinitrogen ($N_2$) into ammonia ($NH_3$) using the enzyme nitrogenase, in a mutually beneficial (mutualistic) relationship.

The overall chemical reaction is:

$N_2 + 8H^+ + 8e^- + 16 ATP \rightarrow 2NH_3 + H_2 + 16 ADP + 16 P_i$ …