Rajasthan Board RBSE Class 12 Biology Chapter 12 Nitrogen Metabolism and Nitrogen Cycle
RBSE Class 12 Biology Chapter 12 Multiple Choice Questions
Question 1.
In the root nodules of leguminous plants, the pigment, necessary for nitrogen fixation is?
(a) Haemoglobin
(b) Leghaemoglobin
(c) Chlorophyll
(d) Xanthophyll
Answer:
(b) Leghaemoglobin
Question 2.
The element, required for the activity of nitrogen-fixing microbes, is?
(a) Cobalt only
(b) Molybdenum only
(c) Iron only
(d) All the above
Answer:
(d) All the above
Question 3.
Plants absorb nitrogen from the soil in which form?
(a) Ammonia gas
(b) Nitrogen gas
(c) Nitrite
(d) Nitrate
Answer:
(d) Nitrate
Question 4.
The symbiotic nitrogen-fixing bacterium is –
(a) Nitrosomonas
(b) Nitrobacter
(c) Rhizobium
(d) All the above
Answer:
(c) Rhizobium
Question 5.
Which of the following is denitrifying bacterium?
(a) Rhizobium
(b) Bacillus
(c) Nitrobacter
(d) Nitrosomonas
Answer:
(b) Bacillus
RBSE Class 12 Biology Chapter 12 Very Short Answer Type Questions
Question 1.
In blue-green algae which specialised cell performs nitrogen fixation?
Answer:
Heterocyst
Question 2.
What is leghaemoglobin?
Answer:
Leghaemoglobin is a pink or reddish coloured protein pigment found in the functional nodules of leguminous plants. This pigment absorbs \({ O }_{ 2 }\) and protects the nitrogenase enzyme against inactivation due to the presence of \({ O }_{ 2 }\).
Question 3.
What role is played by Lectin glycoprotein?
Answer:
Glycoprotein named as lectin attracts a specific species of Rhizobium towards the root of specific species of leguminous plant.
Question 4.
Which two proteins are involved in symbiotic nitrogen fixation?
Answer:
- Leghaemoglobin
- Nodulin
Question 5.
What process is called nitrogen fixation?
Answer:
The process of conversion of free atmospheric \({ N }_{ 2 }\) into nitrogen compounds is called nitrogen fixation.
RBSE Class 12 Biology Chapter 12 Short Answer Type Questions
Question 1.
Why plants can’t use nitrogen directly in spite it’s a presence in the atmosphere up to 78% by volume?
Answer:
- A plant can not use atmospheric nitrogen directly because it is present in free gaseous form in the atmosphere whereas plants are capable of absorbing \({ N }_{ 2 }\) in the form of nitrogen compounds only from the soil.
- In soil, nitrogen is found in the form of organic and inorganic compounds and is absorbed by the roots.
Question 2.
What do you understand by symbiotic nitrogen fixation?
Answer:
When nitrogen fixation is performed by a mutualistic relation established between a microbe and plant, the process is called symbiotic \({ N }_{ 2 }\) fixation. Here the two alone independently can not carry out the process and \({ N }_{ 2 }\) fixation has an outcome of the interaction of the microbe and the plant.
Example: Root nodule of leguminous plants containing Rhizobium bacteria. Anabaena alga present in the leaves of Azolla (A pteridophyte).
Question 3.
Write notes on:
- Nitrification
- Denitrification
- NIf-gene
- Leghaemoglobin
Answer:
1. Nitrification:
The process of conversion of ammonia into nitrate compounds by oxidation is called nitrification. The process is expressed by the following reactions.
2. Denitrification:
- The process of degradation of nitrate compounds into nitrite and nitrogen gas by microbes is called denitrification.
- Denitrification is brought about by Thiobacillus denitrifying bacterium present in the soil.
3. NIf Genes:
The genes found in the genome of some bacteria and having a significant role in \({ N }_{ 2 }\) fixation are called Nif genes.
4. Leghaemoglobin:
The pink or reddish coloured pigment-protein found in the functional root nodules of leguminous plants is called leghaemoglobin. This absorbs \({ O }_{ 2 }\) and prevents inactivation of the nitrogenase enzyme.
Question 4.
Explain the mechanism of the formation of root nodules in leguminous plants.
Answer:
The mechanism of root nodule formation in leguminous plants can be explained by the following steps:
- The roots of legume plants secrete lectin protein which attracts specific species of Rhizobium towards it.
- The root hair becomes hook-like under the influence of hormone secreted by the plant root and the ‘Nod’ factor secreted by the Rhizobium.
- The cell wall of hook-shaped root hair breaks near the tip and the bacterial cells enter into root hair along with mucilaginous substance.
- The plasma membrane of root hair becomes folded and forms infection thread which penetrates the cortex region of root.
- The bacteria continue to divide and induce polyploidy in the nucleus of cells of outer cortex. These cells divide continuously and form a nodule.
- IAA secreted by the bacteria cause growth in the size of the nodule and soon vascular tissue differentiates in it which gets connected with the main vascular tissue of root.
- The functional nodules appear pink in colour due to the presence of leghaemoglobin.
RBSE Class 12 Biology Chapter 12 Essay Type Questions
Question 1.
What is meant by nitrogen fixation? Explain biological nitrogen fixation in plants.
Answer:
The process of conversion of free atmospheric nitrogen into nitrogen compounds which may be absorbed and utilized by plants is called nitrogen fixation. Biotic \({ N }_{ 2 }\) fixation can be, a symbiotic where free-living microbes such as Azotobacter, Clostridium (Bacteria) Nostoc and Anabaena.
(Blue-green algae) carry out \({ N }_{ 2 }\) fixation and, symbiotic where Rhizobium and Bradyrhizobium bacteria form nodule in the root of leguminous plants and carry out \({ N }_{ 2 }\) fixation.
- The process of conversion of atmospheric nitrogen into organic or inorganic nitrogenous compounds by the micro-organisms is called biotic nitrogen fixation.
- These microorganisms are called Di-azotrophs because they convert atmospheric dinitrogen (N ≡ N) in the form of nitrogen compounds.
Biotic \({ N }_{ 2 }\) fixation is of two types:
(1) Asymbiotic \({ N }_{ 2 }\) Fixation:
This type of \({ N }_{ 2 }\) fixation is performed by free-living microbes living in the soil. The following free-living microbes participate in the process of symbiotic nitrogen fixation.
- Aerobic bacteria: Azotobacter, Azomonas
- Anaerobic bacteria: Clostridium
- Photosynthetic bacteria: Chlorobium, Rhodopseudomonas
- Fungi: Yeast, Actinomycetes
- Blue-green algae: Nostoc, Anabaena, or Cyanobacteria Oscillatoria, Plectonema
Note: In members belonging to blue-green algae I (Cyanobacteria), a special cell called Heterocyst is found which performs nitrogen fixation. Molybdenum element is required for the activity of these microbes.
(2) Symbiotic \({ N }_{ 2 }\) Fixation:
- In this type of \({ N }_{ 2 }\) fixation, the microbes establish a mutualistic relationship with the plant.
- Rhizobium and Bradyrhizobium SPS enters the roots of Leguminous plants and develop root nodules where they fix atmospheric nitrogen in nitrogen compounds.
- Similarly Azorizobium SPS. form stem nodules in Sesbania SPS. and carry out nitrogen fixation.
- The microbe Frankia forms nitrogen-fixing nodules in Almus (A non-leguminous plant).
- In the leaves of Azolla (a pteridophyte) \({ N }_{ 2 }\) fixation is carried out by Anabaena SPS.
Mechanism of Bacterial infection and Nodulation:
Rhizobium is a Gram-negative, rod-shaped bacterium found in soil. The bacterium accumulates near the roots of a specific leguminous plant and induces nodule formation. The establishment of a symbiotic relationship between the root and Rhizobium is a complex process.
Formation of nodules is completed by the following steps:
-
- To begin with, the roots of the leguminous plant secrete a specific glycoprotein (Sugar binding protein)-lectin. This lectin attracts specific Rhizobium SPS. towards roots.
- Under the influence of plant hormone, auxin and cytokinins secreted by the root and a Nod factor secreted by the bacterium attached to root hair, curling of root hair are induced and the root hair becomes hook-shaped.
- The cell wall near the tip of the hook-shaped root hair becomes damaged (breaks) and through the broken wall the bacterial cells enter the root hair along with mucilaginous substance. After entry into the root hair, these bacteria are called bacteroids.
- The plasma membrane of the invaded root hair becomes folded and forms a thread-like structure called infection threads. The bacteroid keeps on multiplying in the infection thread and the infection thread penetrates the cortex cells.
- As the infection thread penetrates the cortex cells, the amount of DNA in the nucleus of the cells of outer cortex increases due to polyploidy induced in the cells of outer cortex.
- These polyploid cells produce the nodule by repeated cell divisions. The size of the nodules increases by the impact of IAA (Indole acetic acid) secreted by the bacterial cells.
- After the formation of the nodule, vascular tissue develops in it and connects with the vascular tissue of the root. The bacteroid present in the active and functional nodule contains a pigment called leg haemoglobin in the peri-bacteroid membrane.
- Due to this pigment, these nodules appear pink coloured. The nonfunctional (dead) nodules are white or light yellow coloured.
Note: In symbiotic \({ N }_{ 2 }\) fixation two types of proteins have special significance. These are
1. Leg haemoglobin:
- The active nodules of leguminous plants (which fix nitrogen) have a pink or red coloured pigment called leg haemoglobin.
- This pigment absorbs oxygen during nitrogen fixation.
- The nitrogen-fixing enzyme nitrogenase is highly sensitive to the presence of oxygen and is active only under anaerobic conditions (in the absence of \({ O }_{ 2 }\)) and becomes inactive in the presence of oxygen.
- Thus by absorbing \({ O }_{ 2 }\), leg haemoglobin prevents inactivation of the nitrogenase enzyme.
- Hence in the absence of leg haemoglobin, fixation of nitrogen is not possible because oxygen inactivates nitrogenase enzyme.
2. Nodulin Protein:
- This protein can be of several forms depending upon the structure of nodule and nitrogen and carbohydrate metabolism.
Note: Symbiotic \({ N }_{ 2 }\) fixation is the outcome of mutual co-operation and expression of genes found in the genome of two symbiotics. - In fixation of atmospheric \({ N }_{ 2 }\) into nitrogen compounds the Nod gene of the host and Nod, Nif and Fix genes of the bacterium play a special role. “The Nod gene plays an important role in nodule formation whereas Nif and Fix genes play an important role in \({ N }_{ 2 }\) fixation”.
Mechanism of Symbiotic \({ N }_{ 2 }\) Fixation:
- Symbiotic nitrogen fixation is completed in the root nodules of leguminous plants.
- These nodules contain nitrogenase enzyme and leghaemoglobin protein pigment is necessary for \({ N }_{ 2 }\) fixation.
- Synthesis of the nitrogenase enzyme is controlled by the Nif gene of Rhizobium bacterium.
- Nitrogenase is a complex enzyme and besides protein, it contains molybdenum (Mo) and Iron (Fe) elements.
- Nitrogenase enzyme converts free atmospheric nitrogen into ammonia and uses ATP energy in the process. The process can be represented by the following reaction.
Assimilation of Ammonia:
- Ammonia and ammonium ions formed during biotic \({ N }_{ 2 }\). fixation is toxic for plants, hence Ammonia formed in this process is used in the formation of amino acids.
- This is completed by two methods:
- Reducing amination: In this process, ammonia first reacts with α-ketoglutaric acid and forms glutamic acid.
- Transaminase reaction: In this process, an amino group of one amino acid is transferred to one keto-acid. As a result, a new amino acid is formed.
- Reaction:
- Reducing amination: In this process, ammonia first reacts with α-ketoglutaric acid and forms glutamic acid.
- By transamination, about 17 types of amino acids can be synthesized.
Question 2.
Write an essay on different events of the nitrogen cycle.
Answer:
Nitrogen cycle involves the following four events:
- \({ N }_{ 2 }\) fixation
- Ammonification
- Nitrification
- Denitrification
1. Nitrogen Fixation:
- In one molecule of nitrogen, two atoms are bonded by powerful trivalent bond and so nitrogen molecules are inert.
- Breaking this bond and forming bonds with another molecule(s) requires a high amount of energy.
- The process by which free atmospheric nitrogen is converted into nitrogen compound is called nitrogen fixation.
- The fixation of atmospheric free nitrogen is carried out by different methods. This can be explained with the help of the following chart:
I. Abiotic Nitrogen Fixation:
This type of nitrogen fixation takes place by natural processes due to environmental factors. This can be of two types:
(1) Atmospheric \({ N }_{ 2 }\) Fixation:
During electric discharge, and thunder and due to ultraviolet radiations, atmospheric nitrogen reacts with water and forms nitric acid.
- Nitric acid further reacts with oxygen to form nitrogen dioxide. Nitrogen dioxide reacts with water and forms nitrous acid (\({ HNO }_{ 2 }\)) and nitric acid (\({ HNO }_{ 3 }\)).
- Subsequently, this acid reaches soil with rainwater and reacts with alkaline radicals to form nitrite and nitrates.
- The nitrite and nitrates are soluble in water and are absorbed by roots.
- This type of \({ N }_{ 2 }\) fixation may account for about 10% of the total nitrogen fixation. Reactions:
(2) Industrial \({ N }_{ 2 }\) Fixation:
At very high temperature (200°C) and pressure, in the presence of a catalyst, nitrogen combines with hydrogen to form ammonia (\({ NH }_{ 3 }\)). This method is used at an industrial level to produce \({ NH }_{ 3 }\) which is used in the manufacture of chemical fertilizer.
- This method of ammonia (\({ N/h }_{ 3 }\)) formation is called Haiber method.
- This method of \({ N }_{ 2 }\) fixation is also called as chemical nitrogen fixation.
II. Biotic Nitrogen fixation:
- The process of conversion of atmospheric nitrogen into organic or inorganic nitrogenous compounds by the micro-organisms is called biotic nitrogen fixation.
- These microorganisms are called Di-azotrophs because they convert atmospheric dinitrogen (N ≡ N) in the form of nitrogen compounds.
Biotic \({ N }_{ 2 }\) fixation is of two types:
(1) Asymbiotic \({ N }_{ 2 }\) Fixation:
This type of \({ N }_{ 2 }\) fixation is performed by free-living microbes living in the soil. The following free-living microbes participate in the process of symbiotic nitrogen fixation.
- Aerobic bacteria: Azotobacter, Azomonas
- Anaerobic bacteria: Clostridium
- Photosynthetic bacteria: Chlorobium, Rhodopseudomonas
- Fungi: Yeast, Actinomycetes
- Blue-green algae: Nostoc, Anabaena, or Cyanobacteria Oscillatoria, Plectonema
Note: In members belonging to blue-green algae I (Cyanobacteria), a special cell called Heterocyst is found which performs nitrogen fixation. Molybdenum element is required for the activity of these microbes.
(2) Symbiotic \({ N }_{ 2 }\) Fixation:
- In this type of \({ N }_{ 2 }\) fixation, the microbes establish a mutualistic relationship with the plant.
- Rhizobium and Bradyrhizobium SPS enters the roots of Leguminous plants and develop root nodules where they fix atmospheric nitrogen in nitrogen compounds.
- Similarly Azorizobium SPS. form stem nodules in Sesbania SPS. and carry out nitrogen fixation.
- The microbe Frankia forms nitrogen-fixing nodules in Almus (A non-leguminous plant).
- In the leaves of Azolla (a pteridophyte) \({ N }_{ 2 }\) fixation is carried out by Anabaena SPS.
Mechanism of Bacterial infection and Nodulation:
Rhizobium is a Gram-negative, rod-shaped bacterium found in soil. The bacterium accumulates near the roots of the specific leguminous plant and induces nodule formation. The establishment of the symbiotic relationship between the root and Rhizobium is a complex process.
Formation of nodules is completed by the following steps:
- To begin with, the roots of the leguminous plant secrete a specific glycoprotein (Sugar binding protein)-lectin. This lectin attracts specific Rhizobium SPS. towards roots.
- Under the influence of plant hormone, auxin and cytokinins secreted by the root and a Nod factor secreted by the bacterium attached to root hair, curling of root hair are induced
and the root hair becomes hook-shaped. - The cell wall near the tip of the hook-shaped root hair becomes damaged (breaks) and through the broken wall the bacterial cells enter the root hair along with mucilaginous substance. After entry into the root hair, these bacteria are called bacteroids.
- The plasma membrane of the invaded root hair becomes folded and forms a thread-like structure called infection threads. The bacteroid keeps on multiplying in the infection thread and the infection thread penetrates the cortex cells.
- As the infection thread penetrates the cortex cells, the amount of DNA in the nucleus of the cells of outer cortex increases due to polyploidy induced in the cells of outer cortex.
- These polyploid cells produce the nodule by repeated cell divisions. The size of the nodules increases by the impact of IAA (Indole acetic acid) secreted by the bacterial cells.
- After the formation of the nodule, vascular tissue develops in it and connects with the vascular tissue of the root. The bacteroid present in the active and functional nodule contains a pigment called leg haemoglobin in the peri-bacteroid membrane. Due to this pigment, these nodules appear pink coloured. The nonfunctional (dead) nodules are white or light yellow coloured.
Note: In symbiotic \({ N }_{ 2 }\) fixation two types of proteins have special significance. These are:
1. Leg haemoglobin:
- The active nodules of leguminous plants (which fix nitrogen) have a pink or red coloured pigment called leg haemoglobin.
- This pigment absorbs oxygen during nitrogen fixation.
- The nitrogen-fixing enzyme nitrogenase is highly sensitive to the presence of oxygen and is active only under anaerobic conditions (in the absence of \({ O }_{ 2 }\)) and becomes inactive in the presence of oxygen.
- Thus by absorbing \({ O }_{ 2 }\), leg haemoglobin prevents inactivation of the nitrogenase enzyme.
- Hence in the absence of leg haemoglobin, fixation of nitrogen is not possible because oxygen inactivates nitrogenase enzyme.
2. Nodulin Protein:
- This protein can be of several forms depending upon the structure of nodule and nitrogen and carbohydrate metabolism.
Note: Symbiotic \({ N }_{ 2 }\) fixation is the outcome of mutual co-operation and expression of genes found in the genome of two symbiotics. - In fixation of atmospheric \({ N }_{ 2 }\) into nitrogen compounds the Nod gene of the host and Nod, Nif and Fix genes of the bacterium play a special role. “The Nod gene plays an important role in nodule formation whereas Nif and Fix genes play an important role in \({ N }_{ 2 }\) fixation”.
Mechanism of Symbiotic \({ N }_{ 2 }\) Fixation:
- Symbiotic nitrogen fixation is completed in the root nodules of leguminous plants.
- These nodules contain nitrogenase enzyme and leghaemoglobin protein pigment is necessary for \({ N }_{ 2 }\) fixation.
- Synthesis of the nitrogenase enzyme is controlled by the Nif gene of Rhizobium bacterium.
- Nitrogenase is a complex enzyme and besides protein, it contains molybdenum (Mo) and Iron (Fe) elements.
- Nitrogenase enzyme converts free atmospheric nitrogen into ammonia and uses ATP energy in the process. The process can be represented by the following reaction.
Assimilation of Ammonia:
- Ammonia and ammonium ions formed during biotic \({ N }_{ 2 }\). fixation is toxic for plants, hence Ammonia formed in this process is used in the formation of amino acids.
This is completed by two methods:- Reducing amination: In this process, ammonia first reacts with α-ketoglutaric acid and forms glutamic acid.
- Transaminase reaction: In this process, the amino group of one amino acid is transferred to one keto-acid. As a result, a new amino acid is formed.
- Reaction:
- Reducing amination: In this process, ammonia first reacts with α-ketoglutaric acid and forms glutamic acid.
- By transamination, about 17 types of amino acids can be synthesized.
2. Ammonification:
- The organic compounds present in soil decompose to form ammonia or ammonium compounds. This process is called ammonification.
- Nitrogen absorbed by plants in the form of inorganic compounds is converted into various organic compounds during nitrogen assimilation.
- From plants, these compounds may pass to animals.
- After death and decay of plant and animal, these organic compounds pass to the soil where these are decomposed by microorganisms and nitrogen is released in the environment in gaseous form.
- This decomposition is brought about by putrefying bacteria present in soil by the following steps:
- Proteolysis: In this process, proteins are broken down to amino acids.
Example: Clostridium, Pseudomonas - Deamination: In this process, amino acids are broken down and ammonia formed during the process is released to the atmosphere.
Example: Different species of Bacillus.
- Proteolysis: In this process, proteins are broken down to amino acids.
3. Nitrification:
- The process of oxidation of ammonia into nitrate is called nitrification.
- This is brought about by different types of chemosynthetic bacteria and the process is completed in two steps.
- Conversion of \({ NH }_{ 3 }\) into Nitrite:
Ammonia (\({ NH }_{ 3 }\)) is first oxidized to nitrite by Nitrosomonas bacterium.
- Conversion of Nitrite into Nitrate:
Nitrite is further oxidized to nitrate by the bacterium Nitrobacter
- Conversion of \({ NH }_{ 3 }\) into Nitrite:
4. Denitrification
- The process of degradation of nitrate present in soil into free nitrogen is called denitrification. This results in loss of nitrate compounds of soil and the soil fertility declines.
- Denitrification is brought about by Thiobacillus denitrify, Bacillus denitrify and Pseudomonas denitrify.
Note: The \({ N }_{ 2 }\) cycle completed in the above four steps can be represented by the following the figure.
