Biological Nitrogen Fixation Questions in English

(B) The enzyme nitrogenase is highly sensitive to molecular oxygen. It requires anaerobic conditions to function effectively. In nitrogen-fixing organisms like $Rhizobium$,the enzyme is protected from oxygen by the presence of leghemoglobin,which creates an anaerobic environment within the root nodules.

(B) Nitrogen fixation is a highly energy-intensive process. The enzyme nitrogenase requires a significant amount of $ATP$ to break the strong triple covalent bond between nitrogen atoms $(N \equiv N)$. This energy is derived from the respiration of the host plant,which provides the necessary $ATP$ and reducing power (electrons) to the nitrogen-fixing bacteria (such as $Rhizobium$) residing in the root nodules. Therefore,the energy is obtained from the plant.

(A) The given reaction represents the biological nitrogen fixation process,which is catalyzed by the enzyme nitrogenase.
In this process,atmospheric nitrogen $(N_2)$ is reduced to ammonia $(NH_3)$.
This reaction requires a significant amount of energy in the form of $16$ molecules of $ATP$ for each molecule of $N_2$ reduced.
The enzyme nitrogenase is highly sensitive to oxygen and is typically found in nitrogen-fixing microorganisms like Rhizobium.

(D) $1$. $Rhizobium$ is a symbiotic nitrogen-fixing bacterium that lives in the root nodules of leguminous plants.
$2$. $Azospirillum$ and $Azotobacter$ are free-living nitrogen-fixing bacteria found in the soil.
$3$. They enrich the nitrogen content of the soil by fixing atmospheric nitrogen.
$4$. Therefore,both $Azospirillum$ and $Azotobacter$ are free-living nitrogen-fixing microbes.

(B) $Azotobacter$ and $Beijerinckia$ are well-known examples of free-living,aerobic,nitrogen-fixing bacteria found in the soil.
$Rhizobium$ and $Frankia$ are symbiotic nitrogen fixers.
$Anabaena$ is a cyanobacterium (often symbiotic or free-living,but not strictly aerobic in the same context as $Azotobacter$ for nitrogen fixation).
$Rhodospirillum$ is an anaerobic nitrogen fixer.
$Pseudomonas$ and $Thiobacillus$ are involved in denitrification,not nitrogen fixation.

(D) diazotroph is a microorganism that is able to fix atmospheric nitrogen into ammonia.
$Rhizobium$, $Azotobacter$, and $Beijerinckia$ are well-known nitrogen-fixing bacteria (diazotrophs).
$Bacillus \text{ } polymyxa$ is primarily known as a phosphate-solubilizing bacterium $(PSB)$ that converts insoluble inorganic phosphate compounds into soluble forms, rather than being a primary nitrogen fixer.

(C) $Azospirillum$ and $Azotobacter$ are well-known examples of free-living nitrogen-fixing bacteria.
$Rhizobium$ is a symbiotic nitrogen-fixing bacterium found in the root nodules of leguminous plants.
$Bacillus thuringiensis$ is used as a bio-insecticide.
$Staphylococci$ are pathogenic bacteria.
Therefore,the correct option is $C$.

(D) The correct answer is $D$.
$Azospirillum$ and $Azotobacter$ are well-known examples of free-living nitrogen-fixing bacteria found in the soil.
$Rhizobium$ is a symbiotic nitrogen-fixing bacterium.
$Nostoc$ and $Anabaena$ are cyanobacteria that can fix nitrogen,but they are often associated with symbiotic relationships (e.g.,in $Azolla$ or $Cycas$ roots) or exist in colonies,whereas $Azospirillum$ and $Azotobacter$ are specifically classified as free-living soil bacteria.

(A) $Azospirillum$ is a well-known nitrogen-fixing bacterium that lives freely in the soil.
Unlike $Rhizobium$,which forms a symbiotic relationship with the root nodules of leguminous plants,$Azospirillum$ can fix atmospheric nitrogen while living independently in the soil.