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Nitrogen is an essential element for all living organisms, because o fit presence in proteins and nucleic acids. Most organisms cannot use the large quantity of nitrogen in the air. Before nitrogen can be used by living organisms it must be converted from N2 into some more reactive form. This conversion is called nitrogen fixation.
Nitrogen Fixation
Nitrogen fixation can take place naturally or synthetically.
Fixation by living organisms
Only prokaryotes are capable of fixing nitrogen. One of the best known nitrogen fixing bacteria is Rhizobium. This bacterium lives freely in the soil, and also in the roots of many species of plants. Rhizobium can only fix nitrogen to very a limited extent when living freely in the soil. Most nitrogen fixation by Rhizobium occurs when it is living in plant roots.
The bacteria fix nitrogen with the help of an enzyme called nitrogenase. This enzyme catalyzes the conversion if nitrogen gas, N2, to ammonium ion, NH4+. To do this, it needs:
Fixation in the atmosphere
When lightning passes through the atmosphere, the huge quantities of energy involved can cause nitrogen molecules to react with oxygen, forming nitrogen oxides. these dissolve in rain, and are carried to the ground.
Fixation by the Haber process
In the Haber process, nitrogen and hydrogen gases are reacted together to produce ammonia. This requires considerable energy inputs, so the resulting fertilizers are not cheap. The ammonia is often converted to ammonium nitrate, which is the most widely used inorganic fertilizer in the world.
Use of fixed nitrogen by plants
In legumes, the fixed nitrogen produced by Rhizobium in their root nodules is used to make amino acids. These are transported out of the nodules into xylem, distributed to all parts of the plants and used within cells to synthesize proteins. Other plants rely on supplies fixed nitrogen in the soil. Their root hairs take up nitrate ions by active transport.
Assimilation in nitrogen in animals
Animals can only use nitrogen when it is part of an organic molecule. Most of our nitrogen supply comes from proteins in the diet, with a small amount from nucleic acids.
Return of nitrate to the soil from living organisms
When an animal or plant dies, the proteins in its cells are gradually broken down to amino acids. The decomposers use some of the amino acids for their own growth, while some are broken down and the nitrogen is released as ammonia. The production of ammonia is called ammonification. Ammonia in the soil is rapidly converted to nitrite ions and then nitrate ions by a group of bacteria called nitrifying bacteria. They include Nitrosomoanas and Nitrobacter. These bacteria derive their energy from nitrification.
Denitrification
Denitrifying bacteria provide themselves with energy by reversing nitrogen fixation and converting nitrate to nitrogen gas, which is returned to the air.
Nitrogen Fixation
Nitrogen fixation can take place naturally or synthetically.
Fixation by living organisms
Only prokaryotes are capable of fixing nitrogen. One of the best known nitrogen fixing bacteria is Rhizobium. This bacterium lives freely in the soil, and also in the roots of many species of plants. Rhizobium can only fix nitrogen to very a limited extent when living freely in the soil. Most nitrogen fixation by Rhizobium occurs when it is living in plant roots.
The bacteria fix nitrogen with the help of an enzyme called nitrogenase. This enzyme catalyzes the conversion if nitrogen gas, N2, to ammonium ion, NH4+. To do this, it needs:
- a supply of hydrogen;
- a supply of ATP;
- anaerobic conditions, that is the absence of oxygen.
Fixation in the atmosphere
When lightning passes through the atmosphere, the huge quantities of energy involved can cause nitrogen molecules to react with oxygen, forming nitrogen oxides. these dissolve in rain, and are carried to the ground.
Fixation by the Haber process
In the Haber process, nitrogen and hydrogen gases are reacted together to produce ammonia. This requires considerable energy inputs, so the resulting fertilizers are not cheap. The ammonia is often converted to ammonium nitrate, which is the most widely used inorganic fertilizer in the world.
Use of fixed nitrogen by plants
In legumes, the fixed nitrogen produced by Rhizobium in their root nodules is used to make amino acids. These are transported out of the nodules into xylem, distributed to all parts of the plants and used within cells to synthesize proteins. Other plants rely on supplies fixed nitrogen in the soil. Their root hairs take up nitrate ions by active transport.
Assimilation in nitrogen in animals
Animals can only use nitrogen when it is part of an organic molecule. Most of our nitrogen supply comes from proteins in the diet, with a small amount from nucleic acids.
Return of nitrate to the soil from living organisms
When an animal or plant dies, the proteins in its cells are gradually broken down to amino acids. The decomposers use some of the amino acids for their own growth, while some are broken down and the nitrogen is released as ammonia. The production of ammonia is called ammonification. Ammonia in the soil is rapidly converted to nitrite ions and then nitrate ions by a group of bacteria called nitrifying bacteria. They include Nitrosomoanas and Nitrobacter. These bacteria derive their energy from nitrification.
Denitrification
Denitrifying bacteria provide themselves with energy by reversing nitrogen fixation and converting nitrate to nitrogen gas, which is returned to the air.