Biological Nitrogen Fixation Questions in English

(B) The enzyme $nitrogenase$ is highly sensitive to molecular oxygen and requires an anaerobic environment to function effectively. In the root nodules of leguminous plants,$leghemoglobin$ acts as an oxygen scavenger. It binds to oxygen and keeps the concentration of free oxygen very low,thereby protecting the $nitrogenase$ enzyme from oxidative damage and ensuring the nitrogen fixation process continues efficiently.

(B) Heterocysts are specialized,thick-walled cells found in certain filamentous cyanobacteria (blue-green algae) like $Nostoc$ and $Anabaena$.
These cells are the sites of nitrogen fixation.
The enzyme $Nitrogenase$ is present in heterocysts,which is responsible for the conversion of atmospheric nitrogen $(N_2)$ into ammonia $(NH_3)$.
$Nitrogenase$ is highly sensitive to oxygen,and heterocysts provide an anaerobic environment to protect this enzyme,allowing it to function effectively.

(A) Biological nitrogen fixation is the process by which atmospheric nitrogen $(N_2)$ is converted into ammonia $(NH_3)$.
This process is carried out by nitrogen-fixing bacteria such as $Rhizobium$,$Azotobacter$,and $Clostridium$ using the enzyme nitrogenase.
Therefore,the correct conversion is $N_2 \rightarrow NH_3$.

(A) Soil fertility is primarily enhanced by the addition of nitrogenous compounds to the soil. Nitrogen-fixing bacteria,such as $Rhizobium$,$Azotobacter$,and $Anabaena$,convert atmospheric nitrogen into ammonia,which plants can absorb and utilize for growth. This process increases the nitrogen content of the soil,thereby improving its fertility. Denitrifying bacteria,on the other hand,convert nitrates back into atmospheric nitrogen,which reduces soil fertility. Plasmalemma and cell membrane are cellular structures and do not contribute to soil fertility.

(D) Nitrogen fixation by $Rhizobium$ in the root nodules of leguminous plants is a complex biochemical process.
This process is highly sensitive to the presence of oxygen.
$Rhizobium$ produces an oxygen-scavenging pigment called $leghemoglobin$ (often referred to as the 'oxygen buffer').
$Leghemoglobin$ maintains a low oxygen concentration in the nodules,which is essential for the activity of the enzyme $nitrogenase$.
Since none of the options (Potassium,Nitrogen,Phosphorus) directly stimulate the fixation process in the context of the question's focus on essential environmental factors,the correct answer is $D$.

(D) $Sesbania \text{ } rostrata$ is a leguminous plant that is highly valued as green manure. The unique characteristic of this plant is that it forms nitrogen-fixing nodules not only on its roots but also on its stem. This dual nodulation allows it to fix atmospheric nitrogen more efficiently than other species, making it an excellent source of organic nitrogen for soil enrichment.

(D) $Sesbania$ $rostrata$ is a leguminous plant that is highly valued as a green manure crop. The primary reason for its preference over other species is its unique ability to form nitrogen-fixing nodules on both its roots and its stems. This dual nodulation allows the plant to fix atmospheric nitrogen more efficiently,thereby significantly increasing the nitrogen content of the soil when it is used as green manure.

(C) $Rhizobium$ is a symbiotic nitrogen-fixing bacterium that typically forms root nodules in leguminous plants (like peas,beans,etc.).
Wheat is a cereal crop and does not form a symbiotic relationship with $Rhizobium$.
Therefore,introducing $Rhizobium$ into a wheat field will not lead to significant nitrogen fixation,as these bacteria require specific host plant signals to establish infection and fix atmospheric nitrogen.
Consequently,the soil nitrogen content will not be significantly enriched by the addition of these bacteria alone.

(C) The organisms $Anabaena$,$Nostoc$,and $Oscillatoria$ are all $Cyanobacteria$ (blue-green algae) which are photoautotrophic,meaning they perform photosynthesis to produce their own food.
$Azospirillum$ is a nitrogen-fixing bacterium that is chemoheterotrophic; it relies on organic compounds for energy and carbon,often living in association with the roots of grasses and cereals.
Therefore,$Azospirillum$ is the odd one out as it is not a photoautotroph.

(B) Bacteria that fix atmospheric nitrogen while living freely in the soil are known as free-living nitrogen-fixing bacteria.
$Azospirillum$ and $Azotobacter$ are well-known examples of free-living nitrogen-fixing bacteria.
$Pseudomonas$ is generally involved in denitrification,and $Nostoc$ is a cyanobacterium that can be free-living or symbiotic,but in the context of standard textbook classifications for free-living nitrogen fixers,$Azospirillum$ and $Azotobacter$ are the primary examples provided.
Therefore,the correct pair is $(ii)$ and $(iii)$.

(C) $Rhizobium$ is a symbiotic bacterium that lives in the root nodules of leguminous plants.
It possesses the enzyme nitrogenase,which allows it to convert atmospheric nitrogen $(N_2)$ into ammonia $(NH_3)$,a form that plants can readily absorb and utilize for growth.
This process is known as biological nitrogen fixation.

(D) Nitrogen-fixing bacteria can be symbiotic or free-living.
$Rhizobium$ is a symbiotic nitrogen-fixing bacterium that lives in the root nodules of leguminous plants.
$Azospirillum$ and $Azotobacter$ are examples of free-living nitrogen-fixing bacteria found in the soil.
Therefore,both $Azospirillum$ and $Azotobacter$ are free-living nitrogen fixers.

(D) Nitrogen fixation is the process of converting atmospheric nitrogen into ammonia,which can be utilized by plants.
Many bacteria are capable of fixing atmospheric nitrogen.
$Azospirillum$ and $Azotobacter$ are well-known examples of free-living nitrogen-fixing bacteria found in the soil.
$Nostoc$ is a cyanobacterium that can fix nitrogen,but it is often found in symbiotic associations (e.g.,in $Anthoceros$ or $Cycas$ roots) or in colonies,though it can also be free-living.
However,since both $Azospirillum$ and $Azotobacter$ are classic examples of free-living nitrogen fixers,the most appropriate answer is that both $(A)$ and $(B)$ are correct.

(A) The correct answer is $A$. $Azospirillum$ is a free-living bacterium that lives in the soil and fixes atmospheric nitrogen.
$Rhizobium$ is a symbiotic bacterium that lives in the root nodules of leguminous plants.
$Lactobacillus$ is used in the production of curd from milk.
$Pseudomonas$ is generally a soil bacterium involved in decomposition and denitrification,not primarily known as a free-living nitrogen fixer in the context of biofertilizers.

(B) Azospirillum is a genus of Gram-negative,nitrogen-fixing bacteria that live in close association with the roots of various plants,including grasses and cereals. It acts as a biofertilizer by fixing atmospheric nitrogen $(N_2)$ into the soil,converting it into ammonia $(NH_3)$,which can be easily absorbed by plants. This process helps in enhancing the growth and yield of crops.

(C) Nitrogen fixation is the process of converting atmospheric nitrogen $(N_2)$ into ammonia $(NH_3)$.
This process is catalyzed by the enzyme nitrogenase.
Nitrogenase is a complex metalloprotein that requires Molybdenum $(Mo)$ and Iron $(Fe)$ as essential cofactors for its catalytic activity.
Therefore,Molybdenum $(Mo)$ is a critical micronutrient for nitrogen-fixing organisms like Rhizobium.

(D) Nitrogen fixation is the process of converting atmospheric nitrogen $(N_2)$ into ammonia $(NH_3)$.
This process is catalyzed by the enzyme nitrogenase.
$Molybdenum$ $(Mo)$ is a critical component of the nitrogenase enzyme complex,which is essential for the biological nitrogen fixation process in plants,particularly in legumes.
Therefore,$Molybdenum$ is the correct mineral element involved in nitrogen fixation.

(D) Nitrogenase is a complex enzyme responsible for biological nitrogen fixation. It consists of two protein components: the $Fe$ protein and the $MoFe$ protein. Both components contain iron $(Fe)$ as a critical cofactor for electron transfer and catalytic activity. Therefore,iron is essential for the activity of nitrogenase.

(D) The nitrogenase enzyme complex is a $Mo-Fe$ protein. It consists of two components: the $Fe$ protein (dinitrogenase reductase) and the $Mo-Fe$ protein (dinitrogenase). Both iron $(Fe)$ and molybdenum $(Mo)$ are essential for its catalytic activity. Among the given options,Iron $(Fe)$ is the correct mineral required for the structure and function of the nitrogenase enzyme complex.

(A) The nitrogenase enzyme is a complex metalloprotein responsible for biological nitrogen fixation.
It requires specific metal cofactors for its catalytic activity.
The enzyme contains a molybdenum-iron $(MoFe)$ protein and an iron $(Fe)$ protein.
Molybdenum is an essential component of the nitrogenase enzyme complex,specifically within the $Fe-Mo$ cofactor,which is critical for the reduction of atmospheric nitrogen $(N_2)$ to ammonia $(NH_3)$.

(B) The correct answer is $B$.
Many free-living bacteria and blue-green algae are capable of fixing atmospheric nitrogen.
$Rhodospirillum$ is a free-living,photosynthetic,anaerobic,nitrogen-fixing,non-sulphur bacterium.
It is capable of synthesizing its organic food in the presence of light and in the absence of $O_2$ through a process known as bacterial photosynthesis.
$Beijernickia$ and $Azotobacter$ are free-living but aerobic nitrogen-fixing bacteria.
$Rhizobium$ is a symbiotic nitrogen-fixing bacterium.

(D) $Casuarinaceae$ is the family of dicotyledonous flowering plants placed in the order $Fagales$.
$Casuarina$ is a member of this family,characterized by drooping equisetoid twigs,evergreen nature,and being monoecious or dioecious.
The roots of $Casuarina$ possess nitrogen-fixing nodules that contain the soil actinomycetes called $Frankia$,which is a filamentous bacterium.

(C) : Leghaemoglobin is a pinkish pigment present in the root nodules of leguminous plants. It acts as an oxygen scavenger and protects the nitrogenase enzyme from oxygen poisoning,as nitrogenase is highly sensitive to molecular oxygen.

(D) The correct answer is $(d)$. The enzyme nitrogenase is a $Mo-Fe$ protein that catalyzes the conversion of atmospheric nitrogen $(N_2)$ into ammonia $(NH_3)$,which is the first stable product of biological nitrogen fixation.
Nitrogen fixation is the process of converting inert atmospheric dinitrogen $(N_2)$ into usable nitrogenous compounds like ammonia,amino acids,etc.
Biological nitrogen fixation is carried out by both free-living and symbiotic microorganisms.
In leguminous plants,symbiotic nitrogen-fixing bacteria (like $Rhizobium$) reside in root nodules.
These nodules provide the necessary environment,including the enzyme nitrogenase and the oxygen-scavenging pigment leghaemoglobin,to facilitate the reduction of $N_2$ to ammonia.

(A) : The enzyme $Nitrogenase$ is found in prokaryotic nitrogen-fixing organisms. The process of biological nitrogen fixation is highly energy-intensive and requires a significant amount of $ATP$ to reduce $N_2$ to $NH_3$. This is represented by the following equation:
$N_2 + 8e^- + 8H^+ + 16ATP \xrightarrow{Nitrogenase} 2NH_3 + H_2 + 16ADP + 16P_i$

(B) The root nodules of legumes contain the enzyme nitrogenase and the protein leghaemoglobin.
Nitrogenase catalyzes the conversion of atmospheric nitrogen into ammonia.
This enzyme is highly sensitive to molecular oxygen and requires anaerobic conditions to function effectively.
The nodules have specific adaptations to ensure that the enzyme is protected from oxygen.
Leghaemoglobin acts as an oxygen scavenger,binding to oxygen and maintaining a low-oxygen environment within the nodule to protect the nitrogenase enzyme from inactivation.

(A) The correct answer is $Azotobacter$.
$Azotobacter$ is a genus of free-living diazotrophic bacteria which are found in the soil and are capable of fixing atmospheric nitrogen independently.
$Rhizobium$ is a symbiotic nitrogen-fixing bacterium that lives in the root nodules of leguminous plants.
$Cyanobacteria$ (like $Anabaena$ and $Nostoc$) can fix nitrogen,but in the context of soil-dwelling free-living bacteria,$Azotobacter$ is the classic example provided in the $NCERT$ textbook.

(A) Nitrogen fixation is the process of converting atmospheric nitrogen into ammonia.
Free-living nitrogen-fixing microbes can be aerobic or anaerobic.
$Azotobacter$ and $Beijerinckia$ are well-known examples of free-living,aerobic nitrogen-fixing bacteria.
$Rhodospirillum$ is an anaerobic nitrogen-fixing bacterium.
$Anabaena$ and $Nostoc$ are free-living or symbiotic cyanobacteria that fix nitrogen,but they are not strictly classified as aerobic in the same context as $Azotobacter$ for this specific question.
Therefore,the correct option is $A$.

(D) The biological nitrogen fixation equation is given by: $N_2 + 8e^- + 8H^+ + 16 \, ATP \rightarrow 2NH_3 + H_2 + 16 \, ADP + 16 \, Pi$.
Comparing this with the given equation: $a + 8e^- + b + 16 \, ATP \rightarrow c + d + 16 \, ADP + 16 \, Pi$.
We find that $a = N_2$,$b = 8H^+$,$c = 2NH_3$,and $d = H_2$.
Therefore,option $D$ is the correct representation.

(B) $Azotobacter$ is a free-living,aerobic,and non-photosynthetic bacterium that fixes atmospheric nitrogen.
$Rhizobium$ is a symbiotic nitrogen-fixer.
$Rhodospirillum$ is a photosynthetic bacterium.
$Nostoc$ is a photosynthetic cyanobacterium that can fix nitrogen.

(B) Biological nitrogen fixation is performed by various prokaryotes.
$Azotobacter$ and $Beijerinckia$ are examples of free-living,aerobic nitrogen-fixing bacteria.
$Rhodospirillum$ is an example of a free-living,anaerobic nitrogen-fixing bacterium.
$Anabaena$ and $Nostoc$ are free-living,cyanobacteria that fix nitrogen in specialized cells called heterocysts.
Therefore,the correct option is $B$.

(A) The biological nitrogen fixation process is catalyzed by the enzyme nitrogenase. The overall equation for the reduction of nitrogen to ammonia is given by:
$N_2 + 8e^- + 8H^+ + 16 ATP \rightarrow 2NH_3 + H_2 + 16 ADP + 16 Pi$.
Comparing this with the given equation $N_2 + 8e^- + X + 16 ATP \rightleftharpoons 2NH_3 + Y + 16 ADP + 16 Pi$,we can identify that $X = 8H^+$ and $Y = H_2$.
Therefore,the correct option is $A$.

(C) The red-coloured hemoprotein mentioned is $Leghemoglobin$.
It is found in the root nodules of leguminous plants.
$Leghemoglobin$ acts as an oxygen scavenger,which means it has a very high affinity for oxygen.
It protects the enzyme $Nitrogenase$ from the inhibitory effects of oxygen,as $Nitrogenase$ is highly sensitive to oxygen and requires anaerobic conditions to function efficiently during biological nitrogen fixation.

(A) The enzyme nitrogenase,which is responsible for biological nitrogen fixation,is highly sensitive to molecular oxygen $(O_2)$.
In the root nodules of leguminous plants,nitrogen fixation occurs in an anaerobic environment.
Leghemoglobin is an oxygen-scavenging pigment present in the root nodules that binds to oxygen,thereby maintaining a low oxygen concentration.
This protection ensures that the nitrogenase enzyme remains functional and is not inactivated by oxygen.

(A) The enzyme nitrogenase,which is responsible for biological nitrogen fixation,is highly sensitive to molecular oxygen $(O_2)$.
It requires anaerobic conditions to function effectively.
In root nodules of leguminous plants,the enzyme leghaemoglobin acts as an oxygen scavenger.
It creates an anaerobic environment by binding to oxygen,thereby protecting the nitrogenase enzyme from oxidative damage.
Therefore,both statement $A$ and statement $R$ are correct.

(D) $Azotobacter$ and $Beijerinckia$ are aerobic free-living nitrogen-fixing bacteria.
$Frankia$ is a symbiotic nitrogen-fixing bacterium that produces root nodules in non-leguminous plants like $Alnus$.
$Rhodospirillum$ is an anaerobic free-living nitrogen-fixing bacterium.

(B) The enzyme nitrogenase is essential for biological nitrogen fixation.
It is a complex metalloprotein that requires both $Mo$ (Molybdenum) and $Fe$ (Iron) as cofactors for its structure and catalytic activity.
Specifically,$Mo$ is a critical component of the $Fe-Mo$ protein (dinitrogenase) which is responsible for the reduction of atmospheric nitrogen $(N_2)$ to ammonia $(NH_3)$.
Therefore,Molybdenum $(Mo)$ is the key element required for the activation and proper functioning of the nitrogenase enzyme complex.

(C) The process of biological nitrogen fixation is catalyzed by the enzyme nitrogenase.
The overall equation for the reduction of nitrogen to ammonia is:
$N_2 + 8e^- + 8H^+ + 16ATP \rightarrow 2NH_3 + H_2 + 16ADP + 16Pi$.
According to this equation,$16$ $ATP$ molecules are required to produce $2$ molecules of $NH_3$.
Therefore,for the production of each $NH_3$ molecule,$16 / 2 = 8$ $ATP$ molecules are required.

(A) The given reaction represents the biological nitrogen fixation process catalyzed by the enzyme nitrogenase.
The balanced equation for the reduction of one molecule of nitrogen $(N_2)$ into two molecules of ammonia $(NH_3)$ is:
$N_2 + 8e^- + 8H^+ + 16 ATP \rightarrow 2NH_3 + H_2 + 16 ADP + 16Pi$
Here,$16$ molecules of $ATP$ are required for the reduction of each $N_2$ molecule,resulting in the production of $2$ molecules of $NH_3$ and $1$ molecule of $H_2$ gas.

(D) Let us analyze each pair:
$(I)$ Beijerinckia is a free-living,aerobic,nitrogen-fixing bacterium. This statement is correct.
$(II)$ Rhodospirillum is an anaerobic,nitrogen-fixing bacterium,not aerobic. This statement is incorrect.
$(III)$ Bacillus is a free-living,nitrogen-fixing bacterium. This statement is correct.
$(IV)$ Anabaena is a cyanobacterium,but Azotobacter is a free-living aerobic bacterium,not a cyanobacterium. This statement is incorrect.
Therefore,the incorrect pairs are $(II)$ and $(IV)$.

(D) $A-$ The statement is incorrect because $Frankia$ produces nitrogen-fixing nodules on the roots of non-leguminous plants like $Alnus$.
$R-$ The statement is correct. Both $Rhizobium$ and $Frankia$ are free-living bacteria in the soil,but they establish symbiotic relationships with specific plants to fix atmospheric nitrogen.
Therefore,$A$ is incorrect and $R$ is correct.

(D) Atmospheric nitrogen fixation is the process of converting atmospheric nitrogen $(N_2)$ into ammonia $(NH_3)$,which can be utilized by plants.
$Anabaena$ is a genus of filamentous cyanobacteria that exists as plankton.
These organisms possess specialized cells called heterocysts,which provide an anaerobic environment for the enzyme nitrogenase to function.
Nitrogenase is essential for the biological fixation of atmospheric nitrogen.
$Albugo$,$Cystopus$,and $Aprolegnia$ are fungi or fungus-like organisms that do not perform nitrogen fixation.

(D) The correct answer is $D$.
$Nostoc$ is a genus of cyanobacteria that is free-living,aerobic,and capable of photosynthesis.
It also possesses specialized cells called heterocysts which are the sites of nitrogen fixation.
$Rhizobium$ is a symbiotic nitrogen fixer.
$Azotobacter$ and $Azospirillum$ are free-living,aerobic nitrogen fixers but they are not photosynthetic.

(A) Nitrogen fixation is the process of converting atmospheric nitrogen $(N_2)$ into ammonia $(NH_3)$.
This reaction is catalyzed by the enzyme complex known as nitrogenase.
Nitrogenase is a molybdenum-iron protein and is found exclusively in certain prokaryotes (diazotrophs).
Therefore,the correct option is $A$.

(B) Paddy fields (rice fields) are waterlogged environments where specific nitrogen-fixing bacteria thrive.
$Azospirillum$ is a free-living,nitrogen-fixing bacterium that is commonly associated with the roots of grasses,including rice (paddy),where it fixes atmospheric nitrogen.
$Rhizobium$ is typically associated with leguminous plants.
$Oscillatoria$ is a cyanobacterium,but $Azospirillum$ is the most specific answer for nitrogen fixation in the rhizosphere of paddy crops.
$Frankia$ is associated with non-leguminous plants like $Alnus$.

(B) Nitrogen fixation is the process of converting atmospheric nitrogen $(N_2)$ into ammonia $(NH_3)$.
This process is catalyzed by the enzyme nitrogenase.
The enzyme nitrogenase is a complex metalloprotein that contains both iron $(Fe)$ and molybdenum $(Mo)$.
Molybdenum is an essential component of the nitrogenase enzyme complex,which is required for the reduction of atmospheric nitrogen.
Therefore,$Mo$ plays a vital role in nitrogen fixation.

(B) The enzyme nitrogenase,which is responsible for biological nitrogen fixation,is highly sensitive to molecular oxygen $(O_2)$.
In the root nodules of leguminous plants,the enzyme nitrogenase requires an anaerobic environment to function effectively.
Leghaemoglobin is a pink-colored pigment present in the root nodules that acts as an oxygen scavenger.
It binds to oxygen with high affinity,thereby maintaining a low concentration of free $O_2$ in the nodule,which protects the nitrogenase enzyme from oxidative damage and ensures its optimal activity.

(C) In leguminous plants,biological nitrogen fixation is carried out by the enzyme nitrogenase.
The overall equation for the reduction of nitrogen is: $N_2 + 8e^- + 8H^+ + 16ATP \rightarrow 2NH_3 + H_2 + 16ADP + 16Pi$.
As shown in the equation,the first stable product of nitrogen fixation is ammonia $(NH_3)$.
Ammonia is then further processed into amino acids like glutamate through the process of assimilation.

(B) The enzyme $Nitrogenase$ is highly sensitive to molecular oxygen $(O_2)$ and is inactivated by it.
In the root nodules of leguminous plants,a specialized oxygen scavenger called $Leghemoglobin$ is present.
$Leghemoglobin$ creates an anaerobic environment by binding to oxygen,thereby protecting the $Nitrogenase$ enzyme from oxidative damage and allowing it to function efficiently in the fixation of atmospheric nitrogen.

(B) The correct answer is $B$.
$Rhodospirillum$ is a free-living,anaerobic,nitrogen-fixing bacterium.
$Beijerinckia$ and $Azotobacter$ are free-living,aerobic,nitrogen-fixing bacteria.
$Rhizobium$ is a symbiotic,nitrogen-fixing bacterium found in the root nodules of leguminous plants.