Biological Nitrogen Fixation Questions in English

(D) $Clostridium$ is a free-living,anaerobic,nitrogen-fixing bacterium.
It is a saprophytic organism that possesses the ability to fix atmospheric $N_2$ into $NH_3$ in the soil.
$Rhizobium$ is a symbiotic nitrogen fixer,while $Azotobacter$ is a free-living aerobic nitrogen fixer.

(C) The correct answer is $(c)$. Bacteria are highly useful in increasing soil fertility because many species of bacteria,such as $Rhizobium$,$Azotobacter$,and $Azospirillum$,are capable of biological nitrogen fixation. They convert atmospheric nitrogen $(N_2)$ into ammonia $(NH_3)$,which can be absorbed by plants,thereby enriching the soil with essential nutrients.

(B) Bacteria that convert atmospheric nitrogen $(N_2)$ into nitrogenous compounds like ammonia $(NH_3)$ or amino acids are known as nitrogen-fixing bacteria.
This process is called nitrogen fixation.
Examples include $Rhizobium$,$Azotobacter$,and $Anabaena$.

(C) Leghemoglobin is an oxygen-binding protein found in the nitrogen-fixing root nodules of leguminous plants. It is a red-colored pigment that acts as an oxygen scavenger,maintaining a low oxygen concentration in the nodules to protect the oxygen-sensitive enzyme nitrogenase,which is essential for nitrogen fixation.

(C) $Rhizobium$ is a symbiotic bacterium that lives in the root nodules of leguminous plants.
It possesses the enzyme nitrogenase,which catalyzes the conversion of atmospheric nitrogen $(N_2)$ into ammonia $(NH_3)$.
This process of converting atmospheric nitrogen into a usable form for plants is known as biological nitrogen fixation.

(C) Nitrogen fixation is the process by which atmospheric nitrogen $(N_2)$ is converted into ammonia $(NH_3)$ or other nitrogenous compounds.
Bacteria that perform this process are known as nitrogen-fixing bacteria.
Examples include $Rhizobium$,$Azotobacter$,and $Clostridium$.
Therefore,the correct option is $C$.

(C) The correct answer is $C$.
Biological nitrogen fixation is primarily carried out by organisms that possess specialized cells called heterocysts,which provide an anaerobic environment for the enzyme nitrogenase to function.
$Anabaena$,$Nostoc$,and $Rhizobium$ are well-known nitrogen-fixing organisms.
$Oscillatoria$ lacks heterocysts,which are essential for protecting the nitrogenase enzyme from oxygen inhibition; therefore,it cannot perform nitrogen fixation.

(C) $Frankia$ is a filamentous soil bacterium that forms symbiotic nitrogen-fixing root nodules in actinorhizal plants.
$1$. $Frankia$ is capable of inducing root nodules on many plant species (actinorhizal plants).
$2$. It is known that $Frankia$ can fix nitrogen in the free-living state,although the rate is often lower than in symbiosis.
$3$. The statement in option $C$ is incorrect because $Frankia$ $DOES$ form specialized structures called vesicles. These vesicles provide an anaerobic environment for the enzyme nitrogenase,protecting it from oxygen using a chemical barrier composed of triterpene hopanoids.
$4$. Like $Rhizobium$,$Frankia$ infects host plants via root hair deformation and triggers cortical cell division to form nodules.
Therefore,the incorrect statement is $C$.

(C) $Azotobacter$ is a free-living,aerobic,nitrogen-fixing bacterium found in the soil.
$Rhizobium$ is a symbiotic nitrogen-fixing bacterium that forms root nodules in leguminous plants.
$Pseudomonas$ and $Nitrosomonas$ are involved in the nitrogen cycle (denitrification and nitrification,respectively) but are not primarily known as free-living nitrogen fixers in the same context as $Azotobacter$.

(B) Nitrogen fixation is the process of converting atmospheric nitrogen into ammonia or other nitrogenous compounds.
$Anabaena$ is a genus of filamentous cyanobacteria that exists as plankton.
It is well-known for its ability to fix atmospheric nitrogen through specialized cells called heterocysts.

(C) Belt's corpuscles are specialized,protein-rich structures found at the tips of leaflets in certain species of the subfamily $Mimosoideae$,such as $Acacia$ $sphaerocephala$,which serve to attract ants for protection.
Leghaemoglobin is an oxygen-binding protein (pigment) found in the root nodules of leguminous plants,specifically those in the subfamily $Papilionatae$ (also known as $Faboideae$),which helps in maintaining low oxygen levels for nitrogen fixation by $Rhizobium$ bacteria.

(A) The enzyme $Nitrogenase$ is responsible for the biological fixation of atmospheric nitrogen into ammonia. This enzyme is found exclusively in certain prokaryotes,such as $Rhizobium$ and $Azotobacter$. It catalyzes the reduction of $N_2$ to $NH_3$ using $ATP$ and electrons.

(A) Molybdenum is an essential component of the enzyme nitrogenase,which is responsible for biological nitrogen fixation.
It is also a key component of nitrate reductase,an enzyme involved in nitrogen metabolism.
Therefore,$Molybdenum$ $(Mo)$ plays a crucial role in the process of biological nitrogen fixation.

(B) Molybdenum is an essential micronutrient for plants.
It is a structural component of the enzyme nitrogenase,which is responsible for the biological nitrogen fixation process.
Nitrogenase catalyzes the conversion of atmospheric nitrogen $(N_2)$ into ammonia $(NH_3)$.
Therefore,molybdenum plays a critical role in nitrogen metabolism and nitrogen fixation in plants.

(B) The correct answer is $B$ (Castor).
Nitrogen fixation is the process of converting atmospheric nitrogen into ammonia,which is primarily carried out by symbiotic bacteria like $Rhizobium$ in the root nodules of leguminous plants.
Bean,Gram,and Pea are all members of the family $Fabaceae$ (legumes),which form symbiotic relationships with nitrogen-fixing bacteria.
Castor ($Ricinus$ $communis$) belongs to the family $Euphorbiaceae$ and does not possess the ability to form symbiotic associations with nitrogen-fixing bacteria; therefore,it cannot fix atmospheric nitrogen directly.

(B) Nitrogen fixation is the biological or chemical process of converting atmospheric nitrogen $(N_2)$ into nitrogenous compounds.
In biological nitrogen fixation,atmospheric nitrogen is reduced to ammonia $(NH_3)$ by the enzyme nitrogenase,which is found in certain prokaryotes.
Therefore,the correct representation of nitrogen fixation is the conversion of $N_2$ into $NH_3$.

(A) $Nif$ genes are present in $Rhizobium$.
These genes are responsible for the synthesis of the enzyme nitrogenase,which is essential for fixing atmospheric nitrogen.
$Rhizobium$ $leguminosarum$ bacteria are found in a symbiotic association within the root nodules of leguminous plants.

(D) Leghaemoglobin is a red-colored pigment found in the root nodules of leguminous plants.
It acts as an oxygen scavenger,creating an anaerobic environment within the nodules.
This anaerobic condition is essential because the enzyme nitrogenase,which is responsible for nitrogen fixation,is highly sensitive to oxygen and is inactivated by it.
Therefore,Leghaemoglobin protects nitrogenase,allowing the process of biological nitrogen fixation to occur efficiently.

(A) $N_2$ fixation is the process of converting atmospheric dinitrogen $(N_2)$ into ammonia $(NH_3)$.
Biological nitrogen fixation is carried out by prokaryotes using the enzyme nitrogenase,which reduces $N_2$ to $NH_3$.
While $N_2$ can also be converted to nitrates $(NO_3^-)$ via industrial or atmospheric processes (like lightning),the term '$N_2$ fixation' in biological contexts specifically refers to the reduction of $N_2$ to $NH_3$ as the primary stable product available for assimilation.
Therefore,option $(a)$ is the most accurate representation of biological $N_2$ fixation.

(A) Rhizobium is a symbiotic nitrogen-fixing bacterium that forms nodules in the roots of leguminous plants.
Nitrogen fixation is an energy-intensive process that requires a significant amount of $ATP$.
Phosphorus is a crucial component of $ATP$ and is essential for the development of root nodules,which in turn enhances the process of biological nitrogen fixation.
Therefore,the application of phosphorus-based fertilizers is required for better rhizobial nitrogen fixation.

(D) $Azospirillum$ is a nitrogen-fixing bacterium that lives in the rhizosphere of various crop plants.
It forms a loose association with the roots of plants like maize,wheat,and some Brazilian grasses.
This association is known as associative nitrogen fixation,where the bacterium fixes atmospheric nitrogen and provides it to the plant,thereby increasing crop yield significantly.

(B) The correct answer is $B$.
$Phosphorus$ is essential for the energy-intensive process of nitrogen fixation.
$Rhizobium$ requires $ATP$ to break the triple bond of atmospheric nitrogen $(N_2)$.
Since $ATP$ synthesis is dependent on $Phosphorus$,the application of phosphate fertilizers alongside $Rhizobium$ culture is recommended to enhance nitrogen fixation efficiency in leguminous plants.

(D) Biological nitrogen fixation is the process by which atmospheric nitrogen $(N_2)$ is converted into ammonia $(NH_3)$ by living organisms like bacteria (e.g.,Rhizobium).
Additionally,in the nitrogen cycle,nitrogen can also be converted into nitrates $(NO_3^-)$ through biological or atmospheric processes.
Since both conversions represent pathways of nitrogen fixation in different contexts,option $(d)$ is the most appropriate answer.

(B) $Azotobacter$ and $Bacillus \text{ } polymyxa$ are free-living or non-symbiotic nitrogen-fixing bacteria found in the soil.
These bacteria convert atmospheric nitrogen into ammonia, which is then utilized by plants.
By enriching the soil with nitrogen, they increase soil fertility and consequently improve the yield of crop plants.

(D) $Azotobacter$ is a free-living,aerobic,and non-photosynthetic nitrogen-fixing bacterium found in the soil.
$Rhizobium$ is a symbiotic nitrogen-fixing bacterium.
$Nostoc$ is a photosynthetic cyanobacterium that can fix nitrogen.
$Azospirillum$ is an associative symbiotic nitrogen-fixing bacterium.

(D) Green manure plants belong to the family $Leguminosae$ (also known as $Fabaceae$).
Young leguminous crops are used for green manuring because they possess root nodules containing nitrogen-fixing bacteria, which increase the nitrogen fertility of the soil.
Examples of such plants include $Crotalaria \text{ } juncea$ (Sunn hemp) and $Sesbania \text{ } aculeata$ (Dhaincha).

(C) $Rhizobium$ is a symbiotic bacterium that lives in the root nodules of leguminous plants.
It possesses the enzyme nitrogenase,which allows it to fix atmospheric (molecular) nitrogen $(N_2)$ into ammonia $(NH_3)$,a form that plants can utilize as a nutrient.
$Methanomonas$ is a methane-oxidizing bacterium,$Mucor$ is a fungus,and $Spirogyra$ is an alga; none of these are capable of biological nitrogen fixation.

(C) $Nostoc$ is a free-living,non-symbiotic nitrogen-fixing cyanobacterium (blue-green algae).
It fixes atmospheric nitrogen independently,often utilizing specialized cells called heterocysts.
The energy required for the nitrogen fixation process is derived from photosynthesis.

(D) The root nodule cells of leguminous plants contain a pink-colored pigment called leghaemoglobin.
Nitrogen fixation is catalyzed by the enzyme nitrogenase,which is highly sensitive to oxygen.
Leghaemoglobin acts as an oxygen scavenger,creating an anaerobic environment within the nodule to protect the nitrogenase enzyme from oxidative damage,thereby ensuring efficient nitrogen fixation.

(D) $Rhizobium$ is a well-known symbiotic nitrogen-fixing bacterium that lives in the root nodules of leguminous plants. It converts atmospheric nitrogen into ammonia,which the plant can use for growth. $Nitrobacter$,$Nitrosomonas$,and $Nitrococcus$ are involved in the process of nitrification (converting ammonia to nitrites and nitrates) rather than nitrogen fixation.

(C) Free-living nitrogen-fixing bacteria,such as $Azotobacter$ and $Beijerinckia$,fix atmospheric nitrogen into the soil,which increases soil fertility.
These bacteria contribute to the nitrogen requirements of crops,thereby reducing the dependency on chemical nitrogen fertilizers.
It is estimated that the use of these biofertilizers can save approximately $20-30 \ kg/ha$ of nitrogen fertilizer application.

(B) $Aulosira$ $fertilissima$ is considered the most active nitrogen-fixing blue-green alga (cyanobacteria) found in rice fields.
It belongs to the class $Cyanophyceae$.
It significantly contributes to the nitrogen economy of paddy soils by fixing atmospheric nitrogen.

(A) $nif$ stands for nitrogen fixation genes. These genes are responsible for the synthesis of the nitrogenase enzyme complex,which facilitates the biological nitrogen fixation process. $nif$ genes are found in diazotrophic organisms,such as the symbiotic bacterium $Rhizobium$,which lives in the root nodules of leguminous plants.

(A) If the same crop is grown year after year,the fertility of the soil is lost due to the repeated utilization of the same minerals,which disturbs the soil structure and facilitates soil erosion.
Leguminous plants are particularly useful in crop rotation because they possess root nodules containing nitrogen-fixing bacteria.
These bacteria convert atmospheric nitrogen into nitrates,thereby enriching the soil with nitrates and restoring its fertility.

(C) $Azotobacter$ and $Clostridium$ are well-known examples of free-living nitrogen-fixing bacteria.
These bacteria live independently in the soil and fix atmospheric nitrogen into ammonia,which can be utilized by plants.
Unlike $Rhizobium$,which forms a symbiotic relationship within the root nodules of leguminous plants,$Azotobacter$ and $Clostridium$ do not require a host plant to perform nitrogen fixation.

(B) Leghemoglobin is an oxygen scavenger found in the root nodules of leguminous plants.
Nitrogenase,the enzyme responsible for nitrogen fixation,is highly sensitive to oxygen and is inactivated by it.
Leghemoglobin binds to oxygen,creating an anaerobic environment within the nodule,which protects the nitrogenase enzyme from oxidative damage and allows it to function efficiently in the process of biological nitrogen fixation.

(B) $Azotobacter$ is a well-known genus of free-living soil bacteria.
It is capable of fixing atmospheric nitrogen into ammonia,which can be utilized by plants.
Unlike $Rhizobium$,which is symbiotic,$Azotobacter$ lives independently in the soil.
It is not a cyanobacterium,as it does not perform oxygenic photosynthesis like $Anabaena$ or $Nostoc$.
Therefore,the correct classification is a free-living nitrogen-fixing bacterium.

(C) The enzyme nitrogenase,which is essential for biological nitrogen fixation,is highly sensitive to molecular oxygen $(O_2)$.
Leghemoglobin is a red-colored pigment found in the root nodules of leguminous plants.
It acts as an oxygen scavenger,binding to $O_2$ and maintaining a low oxygen concentration in the nodule environment.
This protection is crucial because it prevents the inactivation of the nitrogenase enzyme by $O_2$,thereby allowing the process of nitrogen fixation to occur efficiently.

(A) Molybdenum is an essential micronutrient for plants.
It is a structural component of the enzyme nitrogenase,which is responsible for biological nitrogen fixation in plants,particularly in legumes.
It also acts as a cofactor for nitrate reductase,which is involved in nitrogen metabolism.
Therefore,its primary role is associated with nitrogen fixation and metabolism.

(D) Nitrogenase is a key enzyme responsible for biological nitrogen fixation,which is the process of converting atmospheric $N_2$ into ammonia $(NH_3)$.
Since leguminous plants rely on this enzyme within their root nodules to fix atmospheric nitrogen,inactivating nitrogenase would make both atmospheric nitrogen fixation and nitrogen fixation by leguminous plants impossible.
However,the conversion of ammonia to nitrate in the soil is a process known as nitrification,which is carried out by soil bacteria like $Nitrosomonas$ and $Nitrobacter$,not by the nitrogenase enzyme.
Therefore,the process that remains unaffected by the inactivation of nitrogenase is the conversion of ammonia to nitrate in the soil.

(B) Leghemoglobin is a red-colored pigment found in the root nodules of leguminous plants.
Its primary function is to act as an oxygen scavenger.
The enzyme $Nitrogenase$,which is responsible for biological nitrogen fixation,is highly sensitive to oxygen and is inactivated in its presence.
Leghemoglobin binds with oxygen to maintain a low oxygen concentration in the nodule,thereby protecting the $Nitrogenase$ enzyme and allowing the fixation process to occur efficiently.

(A) Molybdenum $(Mo)$ is a crucial component of the enzyme nitrogenase,which is essential for biological nitrogen fixation.
Nitrogenase catalyzes the conversion of atmospheric nitrogen $(N_2)$ into ammonia $(NH_3)$.
Copper $(Cu)$ is primarily involved in photosynthesis and respiration.
Zinc $(Zn)$ is involved in the synthesis of auxin (a plant growth hormone).
Manganese $(Mn)$ is involved in the splitting of water during photosynthesis and acts as an activator for various enzymes.

(C) Cobalt is an essential micronutrient for leguminous plants. It is a vital component of $Vitamin B_{12}$ (cyanocobalamin). In leguminous plants,it is required for the synthesis of leghemoglobin in root nodules,which is essential for the process of biological nitrogen fixation by the $Rhizobium$ bacteria.

(B) The enzyme nitrogenase,which is responsible for biological nitrogen fixation,is a molybdenum-iron protein.
It is highly sensitive to molecular oxygen $(O_2)$ because oxygen can irreversibly damage the enzyme complex.
To protect nitrogenase from oxygen,root nodules of leguminous plants contain a pigment called leghemoglobin,which acts as an oxygen scavenger.

(A) $Azotobacter$ is a free-living,aerobic bacterium that is capable of fixing atmospheric nitrogen.
$Rhizobium$ is a symbiotic nitrogen-fixing bacterium found in the root nodules of legumes.
$Clostridium$ is generally an anaerobic bacterium.
$Streptomyces$ are actinobacteria known for antibiotic production,not primary nitrogen fixation.
Therefore,the correct answer is $Azotobacter$.

(C) Nitrogen fixation is the process of converting atmospheric nitrogen $(N_2)$ into ammonia $(NH_3)$.
$Azotobacter$ and $Beijerinckia$ are free-living,aerobic nitrogen-fixing bacteria.
$Rhizobium$ is a symbiotic nitrogen-fixing bacterium found in the root nodules of legumes.
$Rhodospirillum$ is a free-living,anaerobic,photosynthetic nitrogen-fixing bacterium.
Therefore,the correct option is $C$.

(B) Nitrogen fixation is the process of converting atmospheric nitrogen $(N_2)$ into ammonia $(NH_3)$.
Free-living nitrogen-fixing bacteria,such as $Azotobacter$ and $Beijerinckia$,are aerobic microbes that live freely in the soil.
These bacteria fix atmospheric nitrogen independently without forming symbiotic associations with plants.
In contrast,root nodules are associated with symbiotic nitrogen-fixing bacteria like $Rhizobium$.
Therefore,the correct answer is soil.