NIOS Class 12 Biology Chapter 10 Nitrogen Metabolism

NIOS Class 12 Biology Chapter 10 Nitrogen Metabolism Solutions to each chapter is provided in the list so that you can easily browse throughout different chapters NIOS Class 12 Biology Chapter 10 Nitrogen Metabolism Notes and select need one. NIOS Class 12 Biology Chapter 10 Nitrogen Metabolism Question Answers Download PDF. NIOS Study Material of Class 12 Biology Notes Paper 314.

NIOS Class 12 Biology Chapter 10 Nitrogen Metabolism

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Also, you can read the NIOS book online in these sections Solutions by Expert Teachers as per National Institute of Open Schooling (NIOS) Book guidelines. These solutions are part of NIOS All Subject Solutions. Here we have given NIOS Class 12 Biology Chapter 10 Nitrogen Metabolism Solutions, NIOS Senior Secondary Course Biology Solutions for All Chapter, You can practice these here.

Nitrogen Metabolism

Chapter: 10




1. What is the percent by volume of nitrogen gas in the atmosphere? 

Ans: N₂ gas in atmosphere is 78.03%.

2. Name two biomolecules that contain nitrogen in plants.

Ans: (i) Enzymes.

(ii) Proteins.

3. Why nitrogen is a stable molecule?

Ans: Since it is triple (≡) bonded molecule. 

4. What is the percentage of nitrogen in protein?

Ans: 16%.

5. What is the boiling point of nitrogen?

Ans: 195.8°C.

6. Choose the correct option: Nitrogen fixation is the conversion of:

(a) atmospheric Nitrogen ⁱⁿᵗᵒ→ Nitrates.

(b) atmospheric Nitrogen ⁱⁿᵗᵒ→ Ammonia. 

(c) atmospheric Nitrogenⁱⁿᵗᵒ→ into Amino acids. 

Ans: (b) atmospheric Nitrogen ⁱⁿᵗᵒ→ Ammonia.

7. Nitrogen content of biosphere remains constant because of:

(a) Nitrogen cycle.

(b) Nitrogen fixation. 

(c) Industrial pollution.

(d) Absorption of nitrogen. 

Ans: (a) Nitrogen cycle.

8. Nitrates are converted into nitrogen by microbes called __________.

Ans: Denitrifying Bacteria convert No, into nitrogen.


1. Define nitrogen fixation.

Ans: It is defined as “the conversion of molecular nitrogen into NH3.”

2. Which industrial process is utilised for converting nitrogen to ammonia?

Ans: Haber’s Process.

3. Distinguish between biological and abiological nitrogen fixation.

Ans: The biological N₂ fixation takes place in a living cell but a biological N₂ fixation occurs living cell.

4. Name the enzyme that helps in nitrogen fixation in lining cells.

Ans: Nitrogenase enzyme.

5. Which gas prevents nitrogen fixation?

Ans: Oxygen (O₂).


1. Match the following: 

Column AColumn B
(i) Azotobacter(a) anaerobic nitrogen fixer.
(ii) Clostridium(b) aerobic nitrogen fixer.
(iii) Lichens(c) nitrogen fixing cyano bacter-ium.
(iv) Anabaena(d) symbiotic nitrogen fixer.


Column AColumn B
(i) Azotobacter(b) aerobic nitrogen fixer
(ii) Clostridium(a) anaerobic nitrogen fixer.
(iii) Lichens(d) symbiotic nitrogen fixer.
(iv) Anabaena(c) nitrogen fixing cyano bacter-ium.

2. Which gymnospermous plant fixes nitrogen?

Ans: Cycas plant.

3. Is there any other gas evolved during nitrogen fixation? If yes, name the gas evolved. 

Ans: Yes; H₂ (hydrogen gas).

4. How many ATP molecules are required to reduce a single molecule of nitrogen?

Ans: 16 ATP are required.

5. What is the major source of electrons for reduction of nitrogen?

Ans: Reduced coenzymes, e.g. Ferrodoxin.

6. Match the following:

Column AColumn B
(i) Leghemoglobin(a) Cyanobacterium
(ii) Anabaena(b) Legumes
(iii) Reductive process(c) Nitrogen fixation


Column AColumn B
(i) Leghemoglobin(b) Legumes
(ii) Anabaena(a) Cyanobacterium
(iii) Reductive process(c) Nitrogen fixation

7. Name the proteins the help in establishing symbiosis and maintain nodule functioning.

Ans: Nodulins.


1. Which is the most reduced form of inorganic nitrogen?

Ans: NH.

2. Match the following:

Column AColumn B
(i) Nitrate reductase(a) Nitrogen fixation
(ii) Nitrite reductase(b) Nitrate reduction
(iii) Nitrogenase(c) Nitrite reduction


Column AColumn B
(i) Nitrate reductase(b) Nitrate reduction
(ii) Nitrite reductase(c) Nitrite reduction
(iii) Nitrogenase(a) Nitrogen fixation

3. In which part of the cell, reduction of nitrate to nitrite occurs?

Ans: Cytol.

4. Which is the most oxidised form of inorganic nitrogen?

Ans: Nitrate.

5. In which plant organelle reduction of nitrite to ammonia is catalysed by the enzyme?

Ans: Chloroplast.


1. Match the following:

Column AColumn B
(i) Amino acid(a) Keto acid
(ii) Glutamic acid(b) Amino group and carboxylic group
(iii) a-ketoglutaric acid(c) Amino acid 


Column AColumn B
(i) Amino acid(b) Amino group and carboxylic group
(ii) Glutamic acid(c) Amino acid 
(iii) a-ketoglutaric acid(a) Keto acid

2. Name two biochemical reactions for biosynthesis of amino acids in plants.

And: (i) Reductive Deamination. and 

(ii) Transamination.

3. Which group of enzymes catalyses transamination reaction?

Ans: Transaminases.

4. What is the source of amino group for amino acid synthesis in reductive amination reaction?

Ans. Ammonia (NH₂).

5. Which keto acid is the source for synthesis of glutamic acid? 

Ans: Alpha (α) ketoglutaric acid.


1. Define nitrogen fixation.

Ans: Nitrogen fixation: It is defined “as the conversion of molecular N₂ into NH₃”.

2. Which form of combined nitrogen may be formed during lightening storms?

Ans: Oxides of nitrogen are formed as NO and NO₂.

3. Name three biomolecules other than enzymes and proteins, which contain nitrogen. 

Ans: (i) Nucleic acids.

(ii) pigments.

(iii) vitamins.

4. Name one aerobic and one anaerobic bacterium, which fixes nitrogen.

Ans: (i) Aerobic bacterium: Azotobacter.

(ii) Anaerobic bacterium: Clostridium.

5. Which amino acid is synthesised due to reductive amination of a-ketoglutaric acid?

Ans: Glutamic acid.

6. Differentiate between biological and abiological nitrogen fixation.

Ans: Differences between Biological and Abiological Nitrogen Fixation:

Biological Nitrogen FixationAbiological Nitrogen Fixation
It occurs in a living cell.It occurs without a living cell.

7. What is required for biological nitrogen fixation?

Ans. It requires:

(i) the molecular oxygen.

(ii) a strong reducing power to reduce nitrogen such as FAD.

(iii) a source of energy (ATP). and 

(iv) enzyme nitrogenase as well as.

(v) a compound to trap NH₃ formed.

8. How does human haemoglobin differ from leghaemoglobin?

Ans. Human haemoglobin (Hb): It occurs in red blood cells or erythrocytes. Leghaemoglobin (Lb) contained in root nodules of legume plants. Hb is bright red in colour but Lb is reddish pink in colour. Lb protects the nitrogenase enzyme but Hb transport O₂ in body of animals.

9. What is the function of leghaemoglobin?

Ans: Function of leghaemo-globin: It lowers the partial pressure of oxygen. It helps in N₂ fixation. It acts as oxygen scavenger and protects nitrogenase enzyme.

10. What are the functional differences between nitrate reductase and nitrite reductase?

Ans: Nitrate reductase is inducible. It is capable to increase in light. Nitrate is converted into nitrite and this reaction is catalysed by nitrate reductase enzyme.

The nitrite formed is reduced to NH₃ and nitrite reductase enzyme catalyse this reaction.It is capable to accept electrons from NADPH orFADH₂, etc.

11. What is the difference between nitrogen fixation and nitrogen assimilation? Describe in brief the process of abiological nitrogen fixation.

Ans: Nitrogen fixation is “the reduction of nitrogen into NH₃”. It is a reductive process. Nitrogen assimilation is the synthesis of nitrogen compounds from nitrogen.

Abiological Nitrogen Fixation: Reduction of nitrogen into NH₃ takes place without a living cell. It is of two kinds 

(i) natural. and 

(ii) industrial.

In industries, it is done by Haber’s process. The synthetic NH₃ is produced by passing a mixture of H₂ and N₂ through iron oxides (catalyst) at very high temperature and the pressure. See reaction below:

12. Describe in brief various steps involved in biological nitrogen fixation.

Ans: Biological nitrogen fixation: It occurs in a living cell. It is reduction of N₂ into NH₂. “The process of converting free-atmospheric nitrogen by living organisms (nitrogen fixers) into those forms which plants can absorb, is called biological nitrogen fixation”.

The nitrogen-fixers may be free-living, e.g. Azotobacter, Clostridium and cyanobacteria like Nostoc, Anabaena and Aulosira or they may be symbionts like Rhizobium bacterium living in the root nodules of leguminous plants.

The nodules are the sites of nitrogen fixation which contain leghaemoglobin and the enzyme nitrogenase.

The leghaemoglobin (Lb) is a pink colour pigment similar to haemoglobin of vertebrates and functions as an oxygen scavenger and protects the nitrogenase from oxygen. During this process, a dinitrogen molecule is reduced by the addition of hydrogen atoms into two molecules of ammonia catalysed by the enzyme nitrogenase.

This requires:

(i) a strong reducing agent.

(ii) energy in the form of ATP to transfer the hydrogen atom. and 

(iii) the enzyme systems.

13. Enumerate various free living and symbiotic nitrogen fixing system with suitable examples.

Ans: (i) Some Free Living nitrogen Fixing System:

S. NoOrganisms/ Examples Status/Examples
1AnabaenaPhotosynthetic, cyano-bacteria.
2ClostridiumNon-photosynthetic, anaerobic bacteria.
3AzotobacterNon-photosynthetic, anaerobic bacteria.
4KlebsiellaNon-photosynthetic, anaerobic bacteria.

(ii) Some Symbiotic Nitrogen Fixing System with Example:

S. No.SystemSymbionts
1AngiospermsDigitaria (Brazilian grass), Azospirillum and corn.
2LichensCyanobacteria and fungi.
3bryophyteCyanobacteria and Antho-ceros (a bryophyte).

14. What are the major differences between free living and leguminous nitrogen-fixing organisms?

Ans: The free living bacteria: They are clostridium, Azotobacter, Klebsiella, Anabaena and Rhodospirillum. They fix nitrogen.

Legume nitrogen fixing organisms are Rhizobia which live in the root nodules of legum plants like Cicer, pea, soyabean, etc. They live symbiotically. So their association is mutual and called mutualism/symbiosis that results in symbiotic N₂ fixation. Symbiotic N₂ fixing organisms include cyanobacteria and fungus; non-legume and actinomycete member, etc.

15. Describe in brief nitrate and nitrite reduction in plants.

Ans: (a) Nitrate Reduction in Plants: It is absorbed by plants from soil. It is reduced to NH3. This reaction is catalysed by enzyme nitrate reductase. It occurs in the cytol.

(b) Nitrite Reduction in Plants: Nitrite is reduced to NH₃ and reaction is catalysed by nitrite reductase enzyme. Nitrite is found in cytol. It is transported to plastids say chloroplast. Here it is reduced to NH₃.

16. Describe in brief the reductive amination reactions for synthesis of amino acids in plants.

Ans: In Reductive Amination Reaction: NH3 unites with a keto acid, e.g. a-ketoglutaric acid (produced in Kreb’s cycle). This keto acid undergoes enzymatic reductive amination and amino acid (glutamic acid) is formed in plants.

17. Describe the transamination reaction for synthesis of amino acids in plants. How does this differ from reductive amination?

Ans: Transamination Reaction: It includes the transfer of amino group to the keto acid.

Oxalo acetic acid + Glutamic acid The aspartic acid transfers its NH₂ group (amino group) to a-ketoglutaric acid for synthesis of glutamic acid as well as for release of a keto acid also.

Difference from Reductive Amination: Several amino acids are produced by transamination reaction. From ammonium, the amino group is incorporated into the keto acids.

By reductive amination, NH3 enters metabolic stream of the plants to initiate glutaric acid synthesis.

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