Nitrogen Metabolism

Nitrogen is found in the atmosphere in the form of dinitrogen or nitrogen gas. Because it is triple bonded, molecular nitrogen or diatomic nitrogen (N₂), is extremely stable. As a result of its stability, molecular nitrogen is not extremely reactive in the atmosphere under normal circumstances. The Nitrogen Cycle is the most important part of nitrogen metabolism. The atmosphere, soil, and biomass are the three main sources of nitrogen. Nitrogen cycles between these sources in the following way:

Atmospheric source:

‘Nitrogen fixation’ is the process of converting atmospheric nitrogen (N₂) to soluble salts like nitrites and nitrates. Biological, industrial, and electrical processes fix nitrogen in the atmosphere. Biological nitrogen fixation is the conversion of nitrogen to ammonia by living organisms. Industrial nitrogen fixation uses nitrogen from sources such as industrial combustion, automotive exhaust, forest fires, and power plants. When natural factors such as lightning and ultraviolet radiation supply energy to convert nitrogen to nitrogen oxides, this is referred to as “electrical nitrogen fixation.”

Soil source:

The methods described above fix nitrogen from the atmosphere into the soil. Plants and animals, in turn, take up this nitrogen.

Biomass source:

The organic nitrogen in plants and animals decomposes to ammonia when they die. Ammonification is the process of returning nitrogen to the soil. Some of the ammonia evaporates and returns to the atmosphere, but the majority of it is turned to nitrate by soil bacteria, as follows: First, the bacteria Nitrosomonas and/or Nitro coccus oxidize ammonia to nitrite. 2NH₃ + 3O₂ → 2NO₂^– + 2H⁺ + 2H₂O (ii) Nitrobacter then further oxidizes nitrite to nitrate. NO₂^– + H₂O → 2NO₃– + 2H⁺. These reactions are known as ‘Nitrification,’ and the bacteria that cause them are known as ‘Chemoautotrophs.’ Plants absorb the produced nitrate and carry it to their leaves, where it is converted to ammonia. The amine group of amino acids is formed by this ammonia. Pseudomonas and Thiobacillus ‘Denitrify’ nitrates in the soil, converting them to nitrogen. As a result, nitrogen continues to cycle through the environment. Biological Nitrogen Fixation: Biological nitrogen fixation is the conversion of nitrogen to ammonia by living organisms. Nitrogenase, a nitrogen reduction enzyme, is only found in prokaryotes. N₂ fixers are microorganisms that fixes oxides of nitrogen. Nitrogen-fixing microorganisms can be Free-living or symbiotic. Azotobacter and Beijerinckia are examples of free-living nitrogen-fixing aerobic microorganisms, whereas Rhodospirillum is anaerobic and free-living. A variety of cyanobacteria, such as Anabaena and Nostoc, are also nitrogen-fixing free-living bacteria.

Nitrogen metabolism in plants

Nitrogen is required by all plants since it plays a major function in their overall metabolism. But nitrogen fixation occurs only in a few microorganisms and plants. As a result, plants that do not fix nitrogen depend on alternative nitrogen sources, such as nitrate and ammonia, to keep metabolic activity going. Most plants take nitrate, which is then converted to ammonia by two separate enzymes. The enzyme nitrate reductase catalyzes the initial step in the conversion of nitrate to nitrite. Other key components of this enzyme are FAD, cytochrome, NADPH or NADH, and molybdenum. The entire nitrate reduction process occurs in the cytosol and is an energy-dependent reaction in both plants and animals. Many plants have been examined for nitrate reductase, and it has been discovered that the enzyme is continuously generated and destroyed. Nitrate reductase is an inducible enzyme. This indicates that as the concentration of nitrate in the cytosol rises, more nitrate reductase is generated. Excess NH₄ +, on the other hand, has a negative impact on the production of nitrate reductase. When nitrate is available, light has also been reported to enhance nitrate reductase in plants. The nitrite formed in the first stage is reduced to ammonia in the second step, which is catalyzed by the enzyme nitrite reductase. Nitrite is carried from the cytosol to the chloroplast or plastids, where it is converted to ammonia. The enzyme nitrite reductase can accept electrons from NADH, NADPH, or FADH₂ as a source of energy. Furthermore, reduced ferredoxin has been shown to donate electrons to nitrite reductase, which is responsible for converting nitrite to ammonia. Because ammonia buildup is harmful, plants must quickly use the ammonia that has been generated. Excess ammonia is leached out by some plants, particularly algae, which is then converted to nitrite and nitrate by microorganisms in the soil or water.