RBSE Solutions For Class 12 Biology Chapter 12: Nitrogen Metabolism and Nitrogen Cycle | Textbook Important Questions & Answers

RBSE Class 12 Biology Chapter 12- Nitrogen Metabolism and Nitrogen Cycle, provides complete information related to nitrogen fixation, its types, mechanism of nitrogen fixation, nitrification, denitrification and ammonification. Other terminologies include essential pigments, elements, bacteria and proteins, required in the nitrogen cycle process.

RBSE Solutions for Class 12 help students to perform their best in the exams. By practising these important questions, students can analyze their preparation, get a thorough knowledge about all the important terminologies and at the same time, it helps them assess their understanding of the material.

The Rajasthan Board Class 12 Biology Chapter 12 important questions are the best study material for both class assignments and other board examinations. By practising these important questions, students can gain deep knowledge about the topics explained in this chapter and also help them to be well prepared for their upcoming examinations.

RBSE Class 12 Biology Chapter 12 Important Questions

RBSE Biology Chapter 12: MCQ Type Questions

Q.1. Which of the following is identified as a denitrifying bacterium?

(a) Rhizobium.

(b) Bacillus.

(c) Nitrobacter.

(d) Nitrosomonas.

Sol:(b) Bacillus.

Q.2. Heterocyst is a specialized cell performing nitrogen fixation in _________.

(a) Blue-Green algae.

(b) Root nodules.

(c) Aerobic bacteria.

(d) Anaerobic bacteria.

Sol: (a) Blue-Green algae.

Q.3. The symbiotic nitrogen-fixing bacterium is _________.

(a) Rhizobium.

(b) Nitrobacter.

(c) Nitrosomonas.

(d) All of the above.

Sol: (a) Rhizobium.

Q.4. What form of nitrogen is absorbed by plants?

(a) Ammonia gas.

(b) Nitrogen gas.

(c) Nitrate.

(d) Nitrite.

Sol: (c) Nitrate.

Q.5. The element required for the activity of nitrogen-fixing in microbes is _________.

(a) Iron.

(b) Cobalt.

(c) Molybdenum.

(d) All of the above.

Sol:(d) All of the above.

Q.6. Which of the following pigments is necessary for nitrogen fixation in root nodules of leguminous plants?

(a) Chlorophyll.

(b) Xanthophyll.

(c) Haemoglobin.

(d) Leghaemoglobin.

Sol:(d) Leghaemoglobin.

Q.7. The overall amount of nitrogen found in the atmosphere is _________.

(a) 78 percent

(b) 21 percent

(c) 0.04 percent

(d) 0.93 per cent.

Sol: (a) 78 percent

Q.8. Which of the following statements are true?

(a) Rhizobium, a gram-negative bacillus bacterium is found in the air.

(b) Rhizobium, a gram-negative bacillus bacterium is found in soil.

(c) Rhizobium, a gram-negative bacillus bacterium is found in water.

(d) Rhizobium, a gram-negative bacillus bacterium is found within the plant cells.

Sol: (b) Rhizobium, a gram-negative bacillus bacterium is found in soil.

Q.9. How many stages are there in the nitrogen cycle?

(a) Three.

(b) Four.

(c) Six.

(d) Five.

Sol:(b) Four.

Q.10. Oxidation of ammonia into nitrate is called _________.

(a) N2 fixation.

(b) Nitrification.

(c) Denitrification.

(d) Ammonification

Sol: (a) N2 fixation.

RBSE Biology Chapter 12: Short Answer Type Questions.

Q.1. What is leghaemoglobin?

Sol: Leghaemoglobin is defined as reddish or pink coloured pigment present within the functional nodules of leguminous plants. These pigments function by absorbing oxygen and protect the nitrogenase enzymes against inactivation due to the presence of oxygen.

Q.2. What are lectins?

Sol: Lectins are a group of cell-agglutinating proteins or glycoprotein, which are present in various living organisms. Glycoprotein named as lectin attracts a specific species of Rhizobium towards the root of a specific species of leguminous plant.

Q.3. What is symbiotic nitrogen fixation? Name the proteins involved in the symbiotic nitrogen fixation.

Sol: Symbiotic nitrogen fixation is a type of nitrogen fixation, in which a symbiotic relationship is established between the microorganism and plant. There are different species of Rhizobium involved in the symbiotic nitrogen fixation. Leghaemoglobin and nodulin are the two types of proteins involved in the symbiotic nitrogen fixation.

Q.4. What is Nitrogen Fixation?

Sol: Nitrogen Fixation is a biological process in which the nitrogen gas is converted into a usable and observable form for plants and other microbes. In this process, atmospheric free nitrogen gas is converted into ammonia and other related nitrogenous compounds.

Q.5. What is nitrification?

Sol: Nitrification is defined as the process of converting ammonia into nitrate compounds by the process of oxidation. This process is completed in the presence of Nitrosomonas bacteria – a genus of Gram-negative rod-shaped, group of chemotrophic bacteria.

The process of nitrification is expressed by the following chemical reactions:

2NH3 + 3O2 → 2HNO2 + 2H2O + Energy

Q.6. Explain why plants cannot use nitrogen directly from the atmosphere.

Sol: A plant cannot use atmospheric nitrogen directly because it is present in free gaseous form in the atmosphere, whereas the plants are capable of absorbing nitrogen gas in the form of nitrogen compounds only from the soil. In soil, nitrogen compounds are found in the form of both organic and inorganic compounds and are absorbed by the plant’s roots system.

Q.7.What is NIf-gene?

Sol: NIf-gene is a group of genes found in the genome of some bacteria having a significant role in the nitrogen fixation process. These genes encode enzymes involved in the fixation of atmospheric nitrogen. Nitrogenase complex is an enzyme that fixes atmospheric nitrogen and is encoded by the NIf-genes.

Q.8.What is the Nitrogen cycle?

Sol: Nitrogen cycle is a biogeochemical process by which atmospheric nitrogen is converted into chemical forms to make it available for living organisms and is returned into the atmosphere. This process is carried out through several biological and physical processes.

Q.9.Define denitrification?

Sol: The process of degradation of nitrate compounds into nitrate and nitrogen gas by microbes is called denitrification. This process is brought about by Thiobacillus denitrifying bacteria present in the soil. The process is carried out during the absence of oxygen by bacterial species such as Clostridium and Pseudomonas.

Q.10.What is the importance of nitrogen?

Sol: Nitrogen is an important component of all life. It is a building block of proteins, nucleic acids and other cellular constituents which are essential for all forms of life.

Q.11.What is Nitrogen Metabolism?

Sol: Nitrogen metabolism is a set of chemical processes, which is carried on to convert the substance into a usable energy form. This chemical process is mainly based on the recycling of ammonia (NH3) into the neutral or charged form ammonium ion (NH4+). The main part of nitrogen metabolism is the nitrogen cycle.

Q.12. What are the types of Nitrogen Fixation?

Sol. There are three different types of nitrogen fixation.

  1. Atmospheric fixation: It is a natural phenomenon, where the energy of lightning breaks nitrogen into nitrogen oxides and is used by plants.
  2. Industrial nitrogen fixation: It is a man-made alternative that aids in nitrogen fixation by the use of ammonia. Ammonia is produced by the direct combination of nitrogen and hydrogen, and later it is converted into various fertilisers such as urea.
  3. Biological nitrogen fixation: As plants cannot use nitrogen directly from the atmosphere, bacteria like Rhizobium and blue-green algae transform the unusable form of nitrogen into other compounds that are more readily and directly usable by leguminous plants. These nitrogen compounds get fixed in the soil by these microbes.

Q.13. What is assimilation?

Sol: Assimilation is the process by which plants take in the nitrogen compounds from the soil with the help of their roots. The available nitrogen is in the form of ammonia ions, nitrate ions and nitrite ions. These nitrogen compounds are used in the formation of the plant proteins.

Q.14.What is ammonification?

Sol: Ammonification is the biological process by which the organically bound nitrogen of plants biomass is recycled after their death. During the process of ammonification, the nitrogen present in the organic matter is released back into the soil. The decomposers, namely bacteria or fungi present in the soil, convert the organic matter back into ammonium. This process of decomposition produces ammonia which is further used for other biological processes.

Q.15. What is nodulin protein?

Sol: The organ-specific plant proteins related to the structure of the nodules and metabolism of nitrogen and carbohydrate is called the nodulin protein. These nodulin proteins are induced during symbiotic nitrogen fixation.

Q.16. Define proteolysis and deamination.

Sol: Proteolysis is the process of ammonification, which functions by converting the dissolved proteins into smaller polypeptides or amino acids.

Deamination is the process of ammonification, in which the amino acids are disintegrated by bacteria and releases the produced ammonia into the atmosphere.

Q.17. What is Nitrogen Fixation by Lightning

Sol: Nitrogen Fixation by Lightning is another process which helps in nitrogen fixation in lightning. It is a natural phenomenon where the energy of lightning breaks and converts the non-absorbable form of nitrogen into a usable form. Even though the contribution of lightning in the nitrogen fixation is small, they save plants from the deficiency of essential elements.

Q.18. What is the importance of Nitrogen?

Sol: Nitrogen gas is vital but a limiting element. All organisms including humans, plants, and certain protozoans, demand nitrogen for their daily cellular activities. Many cellular components are also made up of nitrogen. Despite all of this, the concentration of a usable form of nitrogen is very less. Nitrogen gas is vital but a limiting element. The earth’s atmosphere consists of 78 per cent of nitrogen gas.

Q.19.What is non-symbiotic nitrogen fixation?

Sol: Non-symbiotic nitrogen fixation is a type of nitrogen fixation done by individual microorganisms present in the soil. These microorganisms include:

  1. Aerobic bacteria- Azotobacter and Azomonas.
  2. Anaerobic bacteria – Clostridium
  3. Fungi – Yeast cells.
  4. Cyanobacteria- Nostoc and Anabaena, etc.
  5. Photosynthetic bacteria – Chlorobium and Rhodopseudomonas.

Q.20. Draw the flowchart of nitrogen fixation.

Sol: Flowchart of Nitrogen Fixation

RBSE Biology Class 12: Long Answer Type Questions

Q.1. What is Nitrogen Fixation? Explain in detail about Abiotic nitrogen fixation.

Sol. Nitrogen is a vital element for all plants. This element plays a vital role in a variety of plant processes. It is the major component of the chlorophyll, a photosynthetic pigment, found in all green plants and other photosynthetic organisms. Since plants cannot use free atmospheric nitrogen, it depends on some microbes, which are capable of fixing atmospheric nitrogen.

Nitrogen fixation is the biological process in which the free atmospheric nitrogen is converted into a usable and observable form for plants. Plants use ammonia and other related nitrogenous compounds.

The nitrogen fixation is of different types. The below flowchart explains in detail about the different types of nitrogen fixation by the symbiotic microorganisms.

Stages of Nitrogen Fixation

The two main types of nitrogen fixation are Abiotic nitrogen fixation and Biotic nitrogen fixation.

Abiotic nitrogen fixation

It is also called Abiological nitrogen fixation. This type of nitrogen fixation is mainly done with the effect of environmental factors. There are two types of A-biological nitrogen fixation:

Atmospheric nitrogen fixation

The effect of lightning and UV rays present in the atmosphere combines with the free nitrogen molecules and produces nitric oxide. Nitric oxide (NO) recombines with oxygen gas (O2) and produces nitrogen dioxide (NO2). This nitrogen dioxide produces nitrous acid (HNO2) and nitric acid (HNO3) by reacting with water molecules. Thus, the formed nitric acid goes into the soil with the rainwater, where it again reacts with alkaline substances and produces nitrate which is directly absorbed by the plants.

The reaction of atmospheric nitrogen fixation is as follows:

N2 + O2 2NO (Nitric oxide).

2NO + O2 2NO2 (Nitrogen dioxide).

2NO2+ H2O HNO2 + HNO3(Nitric acid).

Industrial nitrogen fixation

In the presence of excessive heat, pressure and catalyst, the atmospheric hydrogen and nitrogen combine together and produce ammonia, which is used to manufacture fertilizers in the industries which are later applied to the soil.

The reaction of industrial nitrogen fixation is:

N2 + 3H2 2NH3 (Ammonia).

Q.2. Explain the mechanism of the formation of root nodules in leguminous plants.

Sol: The mechanism of the formation of root nodules in leguminous plants are explained in the following points:

  1. The roots of the legume plants secrete a protein called lectin, which functions by attracting a specific species of Rhizobium towards it.
  2. The root hair becomes hook-like under the influence of hormones secreted by the plant root and the “Nod” factor is secreted by Rhizobium.
  3. The cell wall of hook-shaped root hairs breaks near the tip and the bacterial cells enter into the root hair along with the mucilaginous substance.
  4. The plasma membrane of root hair becomes folded and forms a thread-like structure which penetrates into the cortex region of the plant’s root.
  5. The bacterium continues to divide polyploidy in the nucleus of cells of the cortex- an outer layer of a stem and root in a plant. These cells divide continuously and form a nodule.
  6. IAA secreted by the bacteria increases in the size of the nodule and soon differentiates into the vascular tissues, which gets connected with the main vascular tissue of roots.
  7. The functional nodules appear pink in colour due to the presence of leghaemoglobin pigments.

Q.3. What are the two main proteins used in symbiotic nitrogen fixation?

Sol: The two main proteins used in symbiotic nitrogen fixation are leghaemoglobin and nodulin protein. These two proteins have a special significance in symbiotic nitrogen fixation.

Leghaemoglobin: In the active and functional module, a red-pink colour pigment is found, which is called leghaemoglobin. This pigment absorbs oxygen from the atmosphere during the time of nitrogen fixation. This happens because the nitrogen fixation enzyme-nitrogenase is very sensitive to oxygen and it is only active in an anaerobic environment. It becomes inactive during the presence of oxygen. Hence, the leghaemoglobin absorbs oxygen to prevent nitrogenase from being inactive and therefore nitrogen fixation is not possible during the absence of oxygen.

Nodulin protein: There may be several types of nodulin proteins, which are related to the structure of the nodules and the metabolism of nitrogen and carbohydrates. Nitrogen fixation by symbiosis is the result of the mutual support and expression of the genes of two symbionts. Nitrogen fixation by symbiosis is the result of the mutual support and the expression of genes of the two symbionts.

In nitrogen fixation of nod genes of host and nod, nif and fix genes bacteria have a special contribution. The nod genes play a major role in the formation of nodules and nif genes are involved in the nitrogen fixation.

Q.4.Brief out the mechanism of bacterial infection and nodulation.

Sol: Rhizobium is a gram-negative, rod-shaped bacterium, which is found in the 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.

The formation of nodules is completed by the following steps:

Step- 1

The roots of the leguminous plant secrete glycoprotein – lectin, which functions by attracting the Rhizobium towards the roots.

Step- 2

With the help of plant hormones- auxin and cytokinins secreted by the roots, the Nod factor and root hairs change its shape to the hook-like structure.

Step- 3

The cell wall present near to the hook-shaped root hairs are damaged and through this damaged cell wall, bacteria enter the root hair along with mucilaginous substances. These bacteria, which have entered the root hair are called bacteroids.

Step- 4

Once after invading into the plant cells, these bacteria folds the plasma membrane and forms a thread-like structure, which is called infection threads. These bacteria keep on multiplying the infection thread, which penetrates into the cortex cells.

Step- 5

As the infection penetrates into the cortex cells, the total amount of the DNA present in the nucleus of the outer cortex cells increases due to the induced polyploidy within the cells of the cortex.

Step- 6

These polyploid cells produce nodules by the repeated cell division. Later, the size of the nodules increases by the impact of IAA – Induced Acetic Acid, a naturally occurring plant hormone, which is secreted by the bacterial cells.

Step- 7

Once after the formation of the nucleus, the vascular tissue develops and connects with the vascular tissue of the roots. The nodules appear pink in colour, which is due to the leghaemoglobin present in the peri bacteroid membrane. The non-functional nodules appear white or pale yellow in colour.

Mechanism of Bacterial infection and nodulation

Q.5.Explain the process of Nitrification, Denitrification and Ammonification.

Sol:

Nitrification.

It is the process of converting ammonia into nitrate compounds by the process of oxidation. This process is completed in the presence of Nitrosomonas bacteria – a genus of Gram-negative rod-shaped, group of chemotrophic bacteria.

This process is brought by different types of chemotrophic bacteria and is completed in two steps:

The first step is the conversion of Ammonia (NH3) into Nitrate. In this process, the Ammonia is first oxidized to nitrate by Nitrosomonas bacteria.

2NH3 + 3O2 → 2HNO2 + 2H2O + Energy

The second step is the conversion of nitrite into nitrate.

2HNO2 → 2HNO3 + Energy

Denitrification.

The process of degradation of nitrate compounds into nitrate and nitrogen gas by microbes is called denitrification. This process is brought about by Thiobacillus denitrifying bacteria present in the soil and is carried out during the absence of oxygen by bacterial species such as Clostridium and Pseudomonas.

Ammonification

The biological process during which the nitrogen present in the organic matter is released back into the soil. The decomposers, namely bacteria or fungi present in the soil, convert the organic matter back into ammonium. This process of decomposition produces ammonia which is further used for other biological processes.

Q.6. Brief out the mechanism of Symbiotic Nitrogen fixation.

Sol: Symbiotic nitrogen fixation is a type of nitrogen fixation, in which a symbiotic relationship is established between the microorganism and plant.

Symbiotic nitrogen fixation occurs in the roots nodules of leguminous plants because these nodules contain nitrogenase enzymes and leghaemoglobin pigments, which are necessary for the nitrogen fixation. Synthesis of symbiotic nitrogenase enzyme is controlled by nif genes of Rhizobium bacteria. The nitrogenase enzyme is made up of proteins and minerals-molybdenum and iron element, which functions by converting free atmospheric nitrogen molecules into ammonia and uses the produced energy molecules -ATP in this process. The complete process can be explained by the following chemical reaction.

N2 + 8H+ + 8e + 16 ATP → 2NH3 + H2 + 16ADP +16 Pi

Q.7. Brief out the different stages of Nitrogen Fixation.

Sol: The nitrogen cycle is completed in four stages:

Nitrogen fixation.

In this process of the nitrogen cycle, two atoms of nitrogen gas are inactive due to the presence of a triple bond. To break this triple bond and to form a new bond, this nitrogen molecule requires a large amount of energy. The process of nitrogen fixation in the biosphere takes place in different steps. The below flowcharts represent the different steps in nitrogen fixation.

Stages of Nitrogen Fixation

Ammonification.

Ammonification is the biological process of the formation of ammonium compounds from the disintegration of matter present in the soil. This process begins by absorbing inorganic nitrogen from plants and transferring it into organic matter by the process of assimilation. After the death and decay of plants, these organic matter is re-disintegrated by the microbes and released as inorganic nitrogen into the atmosphere in the form of gaseous. This action takes place by proteolysis and deamination process, which are completed by different types of active purifying bacteria present in the soil.

Nitrification.

Oxidation of ammonia into nitrates is called nitrification. This reaction is completed by various types of chemotrophic bacteria.

Here is the chemical equation of the nitrification process.

  • Conversion of ammonia into nitrate.

2NH3 + 3O2 → 2HNO2 + 2H2O + Energy

  • Conversion of nitrite into nitrate.

2HNO2 → 2HNO3 + Energy

Denitrification.

The conversion of nitrate into nitrogen in the soil by different types of bacteria is called denitrification. This is the final stage of the nitrogen fixation.

Q.9. Brief out the process of Biotic nitrogen fixation.

Sol: Biotic nitrogen fixation

It is a type of nitrogen fixation and is also called biological nitrogen fixation. The process of converting the atmospheric nitrogen into organic or inorganic compounds in the presence of microorganisms is called the biological nitrogen fixation. These microbes are called dia-diazotrophs because they fix the atmospheric dia nitrogen (N = N)

There are two types of biological nitrogen fixation:

Non-symbiotic nitrogen fixation

Non-symbiotic nitrogen fixation is a type of nitrogen fixation done by individual microorganisms present in the soil. These microorganisms include:

Aerobic bacteria- Azotobacter and Azomonas.

Anaerobic bacteria – Clostridium

Fungi – Yeast cells.

Cyanobacteria- Nostoc and Anabaena, etc.

Photosynthetic bacteria – Chlorobium and Rhodopseudomonas.

Symbiotic nitrogen fixation

In this type of nitrogen fixation, a symbiotic relationship is established between the microorganism and the plants. In the roots of leguminous plants, different species of Rhizobium and Brady rhizobium enter and form the root nodules and convert the nitrogen of the soil into nitrate compounds. There are different species of Rhizobium involved in the symbiotic nitrogen fixation. Leghaemoglobin and nodulin are the two types of proteins involved in the symbiotic nitrogen fixation

Q.10. Draw the detailed diagram of the nitrogen cycle representing the four different stages.

Sol:

Nitrogen fixation

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