Metabolism of Nitrogen: Nitrogen Cycle, Nitrogen Fixation

Metabolism of Nitrogen is a process in which nitrogen circulates between the atmosphere and living organisms. Nitrogen is another essential element in the body of a living organism. The proportion of nitrogen in the atmosphere is 78% which is the highest among all other elements. Nitrogen is one of the most important constituents of proteins, enzymes, DNA, RNA, ATP molecules, chlorophyll, hormones and vitamins and is absorbed by plants in certain usable forms. 

In this lesson, you’ll study many aspects of nitrogen metabolism in plants, such as nitrogen fixation and assimilation. Living organisms consist of many elements like oxygen, hydrogen, carbon, etc.

What is Metabolism of Nitrogen?

Metabolism of Nitrogen is a cycle of chemical reactions. Nitrogen is one of the essential elements of the living cell. Nitrogen exists as two nitrogen atoms joined by a powerful triple bond (N≡N or N₂). Plants need it to produce protein, nucleic acid, chlorophyll, and many other vitamins. Its availability in the soil is limited, being inert. Plants absorb N₂ mainly as ammonia (NH₃) or in a significantly less amount as nitrate (NO₃) or nitrite (NO₂). Nitrogen fixation refers to the conversion of nitrogen into ammonia in a stepwise manner. The nitrogen in the atmosphere is converted into a usable form in soil or water, which plants and animals assimilate. The decomposition of the dead living organisms releases the nitrogen back to soil and water and returns to the atmosphere through denitrification.

Nitrogen Cycle

Fig: Nitrogen Cycle

The Nitrogen Cycle has the following four steps:

Step I: Nitrogen Fixation

Fig: Nitrogen Fixation

Nitrogen Fixation refers to the conversion of nitrogen (N₂) into ammonia (NH₃) or nitrate (NO₃), or Nitrite (NO₂). This process is again two types: physical nitrogen fixation and biological nitrogen fixation.

(a) Non-biological or physical nitrogen fixation- In this conversion of N₂ to inorganic nitrogenous compounds takes place. It does not involve any microorganisms. It takes place in the following ways:

I. Atmospheric nitrogen fixation: Atmospheric nitrogen fixation is a natural process. In this process, nitrogen can be fixed by electrical discharge in the atmosphere. Atmospheric nitrogen fixation occurs during lightning, thundering volcanic eruptions or UV radiations when nitrogen in the atmosphere can combine with oxygen to form nitrogen oxides like N₂O, NO₂, NO, etc.

II. Industrial nitrogen fixation: It takes place during the combustion, automobile exhausts, cement industries, etc. In industrial fixation, nitrogen gets reduced to ammonia. For example: In Haber’s process, synthetic ammonia is formed by a mixture of nitrogen and hydrogen at a very high temperature of 500⁰C and 1000 atmospheric pressure.

III. Together, these two can fix not more than 10-15% of total nitrogen.

(b) Biological nitrogen fixation- In this process of conversion of N₂ to inorganic nitrogenous compounds like ammonia (NH₃) or nitrate (NO₃), or Nitrite (NO₂) takes place. It takes place through microorganisms. It takes place in the following ways:

Some-living microbes which fix nitrogen are given in the table below:

Some symbiotic nitrogen fixing organisms are given in the below table:

Mechanism of Biological fixation of Nitrogen:
The basic needs for nitrogen fixation are:

I. Nitrogenase enzyme.
II. A mechanism that protects nitrogenase from oxygen.
III. Ferredoxin protein, which is an electron acceptor.
IV. Constant supply of ATP.
V. The Coenzymes and co-factors like TPP, Co-A, inorganic phosphate, and Mg⁺² are required.
VI. Cobalt and Molybdenum as co-factors for the enzyme.
VII. A carbon compound to trap released ammonia.
The nitrogenase enzyme is vital for this process. Nitrogenase is a Fe-Mo protein. This enzyme needs Mo as a co-factor. This enzyme works in anaerobic conditions and is affected by oxygen.

1. It can add H to the dinitrogen in the presence of some strong reducing agents, like FMNH₂, NADH₂, etc.
2. It produces, stepwise, diamide (N₂H₂), hydrazine (N₂H₄) and finally ammonia (NH₃).
3. 8 moles of ATP are needed to produce one mole of ammonia.
   N₂ + 8e⁻ + 8H⁺ + 16 ATP → 2NH₃ + 16 ADP + 16P_i + H₂
4. This energy is provided by the host cell (in the form of ATP).
5. Leghaemoglobin is a potent oxygen scavenger. This protein protects the nitrogenase from oxidation damage and allows fixing nitrogen in anaerobic conditions.

Rhizobium is a rod-shaped, soil-borne bacteria that live symbiotically with the roots of leguminous plants (Fabaceae family). Garden pea, groundnut, sweet pea, soyabean, clover bean, broad bean, alfalfa, lentil, etc. Rhizobium leguminosarum, R. phaseoleae are important species that form root nodules. They produce root nodules, an ovoid- to spherical outgrowth in the roots. Formation of nodules involves the following steps:
a. Rhizobium bacteria multiply and colonise the surrounding roots. Roots secrete specific chemicals to attract these bacteria.
b. They get attached to the epidermis of the root and root hairs.
c. They collect over the root hairs and release the ‘Nod’ factor. This causes curling of the root hairs.
d. An infection thread containing the bacteria is produced. This thread grows into and along with the multiplication of the bacteria.
e. Bacteria reach inside the roots through the infection thread and start nodule formation.
f. Inside the cortical layer, the bacteria stop dividing and become bacteroids. These bacteria release signals to initiate the division of the cells of the cortical and pericycle layer. Each enlarged, non-motile bacterium is called a bacteroid.
g. Nodule formation occurs.
h. The nodule contains nitrogenase enzyme and red-pink pigment called leghaemoglobin.

Fig: Formation of Root Nodules

Step II: Ammonification

Fig: Ammonification

1. The decomposition of organic nitrogen (of protein, amino acids) into ammonia is ammonification. This process is a micro-dependent process.
2. Bacillus vulgaris, B. ramosus, various genera of Actinomycetes (ray fungi) help in this process.

Step III: Nitrification

Fig: Nitrification

1. Most of the ammonia formed during ammonification is converted into nitrate and nitrite. Some ammonia may escape to the atmosphere as a gas. This conversion is called nitrification.
2. In the first step, ammonia is oxidised to nitrite.
NH₃ + 3O₂ = 2NO₂⁻ + 2H⁺ + 2H₂O + Energy
Nitrosomonas, Nitrococcus, accomplish this process.
3. In the next step, nitrite is further oxidised to nitrate.
2NO₂⁻ + O₂ = 2NO₃⁻ + Energy
Nitrobacter, Nitrocystis, help in this process.
4. All these bacteria are chemoautotrophic forms. They utilise the energy liberated in these reactions for their metabolism.
5. The plants can use nitrates as it is.
a. However, it is reduced to ammonia in two steps:
b. In the first step, nitrite is formed by the enzyme nitrate reductase. This enzyme is a flavoprotein complex and needs Mo as a co-factor.
c. In the next step, nitrite is reduced to ammonia by the nitrite reductase enzyme. Ferredoxins donate electrons for this reaction. This process occurs in the leaves, where ferredoxins are available. Nitrite formed in other body parts is translocated to leaves to get further reduced. Nitrite reductase does not need Mo, but it needs Cu and Fe as co-factors.

Step IV: Denitrification

Fig: Denitrification

Some bacteria convert to nitrate, nitrite, or ammonia into dinitrogen and release it into the atmosphere. This process is called denitrification.
2NO₃⁻ → 2NO₂⁻ → 2NO → N₂O → N₂ ↑
Thiobacillus denitrificans, Pseudomonas denitrificans are examples of such bacteria.

Fate of Ammonia

1) Ammonia is protonated to form ammonium ions (NH₄⁺ ).

2) There are two basic ways ammonium ions are assimilated to plants:

a. Reductive Amination

In this process, ammonia reacts with α-Ketoglutaric acid to form glutamic acid.

b. Transamination

I. Enzyme is Aminotransferase.

II. Glutamic acid is the primary amino acid from which other amino acids are synthesised.

III. The amino group is transferred to some keto-acid.

IV. Thus, other amino acids are formed.

c) Amides are generally organic compounds derived from amino acids, in which the hydroxyl part of -COOH is replaced by -NH₂.

I. Thus, amides have two amino groups.

II. Amides perform the function of storage of nitrogen and transport of nitrogen.

III. Glutamine and asparagine are two amino acids with amide groups.

IV. Amides can be transported through the xylem vessels to any other part of the plant.

V. Soyabean (Glycine max) stores ureides (degraded urea) after fixing nitrogen. They maintain a C: N ratio.

Summary

Nitrogen is an essential component of all life forms. It is a principal constituent of proteins, nucleic acids, enzymes, chlorophyll, vitamins, ATP, etc. Nitrogen exists as two nitrogen atoms joined by a powerful triple bond (N≡N or N₂). Plants absorb N₂ mainly as ammonia (NH₃) or in a significantly less amount as nitrate (NO₃) or nitrite (NO₂). Nitrogen fixation refers to the conversion of nitrogen into ammonia mainly with the help of the microbe Rhizobium. The enzyme nitrogenase is the most crucial enzyme found in root nodules for N₂ fixation. The decomposition of the dead living organisms releases the nitrogen back to soil and water and returns to the atmosphere through denitrification.

FAQs

Q.1. What is denitrification?
Ans: Some bacteria convert nitrate, nitrite, or ammonia into dinitrogen and release it into the atmosphere. This process is called denitrification.

Q.2. What is nitrogen fixation?
Ans: Nitrogen Fixation refers to the conversion of nitrogen (N₂) into ammonia (NH₃) or nitrate (NO₃), or Nitrite (NO₂).

Q.3. Give one example of aerobic and anaerobic nitrogen-fixing bacteria.
Ans: One aerobic nitrogen-fixing bacteria is Azotobacter, and one anaerobic nitrogen-fixing bacteria is Clostridium.

Q.4. Name the most crucial enzyme found in root nodules for N₂ fixation.
Ans: Nitrogenase enzyme is the most crucial enzyme found in root nodules for N₂ fixation.

Q.5. What is the importance of nitrogen?
Ans: Nitrogen is an essential component of all life forms. It is a principal constituent of proteins, nucleic acids, enzymes, chlorophyll, vitamins, ATP, etc.

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