NIOS Class 12 Biology Nitrogen Metabolism Terminal Solutions

The Class 12 NIOS Biology Class 12 Chapter 10 provides students with solved terminal exercises for thorough exam preparation. This chapter explains important biological concepts in a simple and structured way. The solutions make it easier for learners to revise quickly and understand key topics with clarity.

By practicing these exercises, students build confidence and strengthen their foundation. This chapter is an essential tool for mastering NIOS Class 12 Biology and scoring well.

Class 12 NIOS Biology Class 12 Chapter 10

1. Define nitrogen fixation.

The conversion of molecular nitrogen into compounds of nitrogen, especially ammonia, is called nitrogen fixation. Nitrogen fixation is a reductive process, i.e., nitrogen fixation will stop if there is no reducing condition or if oxygen is present.

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

Nitric oxide and nitrogen dioxide. 

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

Amino acids, ammonia, and urea. 

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

• Azotobacter – Aerobic bacteria
• Clostridium – Anaerobic bacteria

5. Which amino acid is synthesized due to reductive amination of α-ketoglutaric acid?

Glutamic acid. 

6. Differentiate between biological and abiological nitrogen fixation.

• In abiological nitrogen fixation, the nitrogen is reduced to ammonia without involving any living cell.
• Biological nitrogen fixation is the reduction of molecular nitrogen to ammonia by a living cell in the presence of enzymes called nitrogenases.

7. What is required for biological nitrogen fixation?

Biological Nitrogen fixation requires

• (i) the molecular nitrogen
• (ii) a strong reducing power to reduce nitrogen, like reduced FAD (Flavin adenine dinucleotide) and reduced NAD (Nicotinamide Adenine Dinucleotide)
• (iii) a source of energy (ATP) to transfer hydrogen atoms from NADH2 or FADH2 to dinitrogen
• (iv) enzyme nitrogenase
• (v) compound for trapping the ammonia formed since it is toxic to cells.

8. How does human hemoglobin differ from leghemoglobin?

Leghemoglobin is produced as a result of the interaction between the bacterium and the legume roots. It is an oxygen scavenger. 

Hemoglobin in humans is found in the red blood cells. Here, it transports oxygen to all parts of the body. 

9. What is the function of leghemoglobin?

During N2-fixation, the function of Leghemoglobin is to act as an oxygen scavenger so that the enzymes, nitrogenase, can convert N2 to NH3 under anaerobic conditions.

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

• Nitrate reductase converts nitrate into nitrite. This process takes place in the cytosol.
• Nitrite reductase converts the nitrite into ammonia. This enzyme is present in the cytosol and is transported into the chloroplast to reduce nitrite into ammonia.

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

Nitrogen fixation is the process of reducing molecular nitrogen to ammonia and nitrates. Nitrogen assimilation is the process by which plants take in organic nitrogen (ammonium, nitrate) from the soil and incorporate it into various organic molecules. 

Abiological nitrogen fixation

In abiological nitrogen fixation, the nitrogen is reduced to ammonia without involving any living cell. Abiological fixation is of two types: industrial and natural.

• In industrial nitrogen fixation, a mixture of nitrogen and hydrogen is passed through a bed of catalyst at high temperature and pressure to generate ammonia.
• In natural nitrogen fixation, electrical discharges in the atmosphere convert atmospheric nitrogen into oxides by combining it with oxygen.

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

Biological nitrogen fixation is the reduction of molecular nitrogen to ammonia by a living cell in the presence of enzymes called nitrogenases. The process of nitrogen fixation is primarily confined to independent and free-living microbial cells like bacteria and cyanobacteria. The overall biochemical process involves stepwise reduction of nitrogen to ammonia.

• The enzyme nitrogenase binds with a molecule of nitrogen (N2) at its binding site.
• Hydrogen from reduced coenzymes reduces the nitrogen first into diamide (N2H2) and then into ammonia (NH3).
• Since ammonia is toxic to living cells, the organisms combine ammonia with organic acids to form amino acids.
• These steps require energy from ATP for the reduction, as molecular nitrogen is a stable molecule.

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

Free-living nitrogen-fixing systems

• Cyanobacteria, such as Anabaena, and bacteria called Rhodospirillum are photosynthetic, free-living nitrogen-fixing systems. The nitrogen-fixing process is confined to their cells as they possess the nitrogenase enzyme that can convert the molecular nitrogen into nitrates.
• Other bacteria, such as Clostridium, Klebsiella, and Azotobacter, are non-photosynthetic nitrogen-fixing bacteria. While Clostriidum is an anaerobic bacterium, Azotobacter is an aerobic bacterium.

Symbiotic nitrogen fixing systems

• Lichens- Cyanobacteria and Fungi.
• Bryophyte- Cyanobacteria and Anthoceros.
• Pteridophyte- Cyanobacteria and Azolla.
• Gymnosperm -Cyanobacteria and Cycas.
• Angiosperms- Legumes and Rhizobium.
• Angiosperms – Non-leguminous plants and actinomycete (Alnus, Myrica, Purshia).
• Angiosperm Brazilian grass (Digitaria), Corn, and Azospirillum.

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

In free-living nitrogen-fixing organisms, the nitrogen fixation is confined to their cells that contain the nitrogenase enzyme. 

• In leguminous nitrogen-fixing organisms, nitrogen fixation happens in the specialised bodies called root nodules.
• These nodules are formed by the symbiosis between the bacteria Rhizobium and the legume roots.
• Moreover, a group of proteins called nodulins helps establish the symbiosis between the bacteria and legume roots.
• In addition, leguminous nitrogen fixing requires the presence of leghaemoglobin to catch oxygen to allow the nitrogenase enzyme to convert molecular nitrogen into ammonia under anaerobic conditions.

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

Nitrate reduction

• Nitrate reduction happens in the cytosol.
• It is facilitated by the enzyme nitrate reductase.
• The enzyme synthesis is proportional to the nitrate concentration and light.
• The enzyme helps convert the nitrate into nitrite.

NO^–₃+ NADH + H⁺  Nitrate reductase  → NO^–₂ + NAD⁺ + H₂O 

Nitrite reduction

In the second step, the nitrite thus formed is reduced to ammonia in the chloroplast by the enzyme nitrite reductase. 

The nitrite is transported into the chloroplast, where the enzyme nitrite reductase accepts electrons from NADPH, NADH, and FADH2 to help reduce the nitrite into ammonia.

NO^–₂+ 3NADPH + 3H⁺  Nitrite reductase  → NH₂ + 3NADP⁺  

The produced ammonia is then converted into amino acids by the plant cells. 

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

In a reductive amination reaction, ammonia combines with a keto acid such as alpha-ketoglutaric acid, produced during the Krebs cycle. 

The keto acid then undergoes enzymatic reductive amination to produce the amino acid glutamic acid. 

α-ketoglutaric acid + NH₃ glutamate dehydrogenase  → Glutamic 

(keto acid) (amino acid)

Similarly, another amino acid called aspartic acid is produced by reductive amination of oxaloacetic acid.

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

The transamination reaction involves the transfer of an amino acid from the already synthesised amino acid to the keto acid. 

α-ketoglutaric acid + Aspartic acid Transaminase  → Glutamic    + Oxaloacetic acid

(keto acid) (amino acid) (amino acid) (keto acid)

Here, aspartic acid transfers its amino group into the α-ketoglutaric acid to form glutamic acid, releasing keto acid. This reaction is catalysed by the transaminase enzyme.

Terminal Questions Solutions