Biochemistry and Molecular Biology - Belyasova N.A. 2002

Metabolism. Processes Requiring Energy Input
Metabolism of Nitrogen-Containing Compounds
Nitrogen Cycle in Nature

In nature, inorganic forms of nitrogen exhibit considerable diversity, including molecular nitrogen—which constitutes the primary reservoir of nitrogen in the biosphere—oxidized forms (nitrates, nitrites, and various nitrogen oxides), and reduced forms (ammonia and ammonium salts). Within Organic compounds, nitrogen is predominantly found in the amino groups of Amino Acids. The Main Pathways of nitrogen transformation are illustrated in Fig. 16.1.

It is readily apparent that animals utilize solely organic nitrogen, which enters the body through food. These organic nitrogen reserves are replenished by plants and microorganisms capable of reducing oxidized forms of nitrogen and incorporating it into amino groups. However, oxidized nitrogen forms are easily leached from the soil into seas and oceans by precipitation; furthermore, they are reduced by denitrifiers into nitrogen oxides and N₂, which escape into the atmosphere. Consequently, the planet's reserves of fixed nitrogen could soon be depleted were it not for the continuous processes of MOLECULAR Nitrogen Fixation. This process is carried out partly by humans through The production of nitrogen fertilizers—a highly expensive and energy-intensive Procedure—and partly by nitrogen-fixing Bacteria, which include both free-living forms and those that fix nitrogen in Symbiosis with plants. A small quantity of oxidized nitrogen forms is produced in the atmosphere by lightning discharges and cosmic radiation. Thus, the enzymatic fixation of molecular nitrogen serves as the primary natural mechanism for renewing fixed nitrogen reserves, thereby sustaining life on Earth.

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Fig. 16.1. The Nitrogen Cycle in nature: 1 — industrial nitrogen fixation; 2 — activity of free-living and symbiotic nitrogen fixers; 3 — physical forces of nature