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Overview of Nitrogen Metabolism:-The Nitrogen Cycle Maintains a Pool of Biologically Available Nitrogen

المؤلف:  David L. Nelson، Michael M. Cox

المصدر:  Lehninger Principles of Biochemistry

الجزء والصفحة:  834

2026-07-02

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Overview of Nitrogen Metabolism:-The Nitrogen Cycle Maintains a Pool of Biologically Available Nitrogen

The most important source of nitrogen is air, which is four-fifths molecular nitrogen (N₂). However, relatively few species can convert atmospheric nitrogen into forms useful to living organisms. In the biosphere, the metabolic processes of different species function interdependently to salvage and reuse biologically available nitrogen in a vast nitrogen cycle (Fig. 22–1). The first step in the cycle is fixation (reduction) of atmospheric nitrogen by nitrogen-fixing bacteria to yield ammonia (NH₃ or NH₄). Although ammonia can be used by most living organisms, soil bacteria that derive their energy by oxidizing ammonia to nitrite (NO^-₂) and ultimately nitrate (NO^-₃) are so abundant and active that nearly all ammonia reaching the soil is oxidized to nitrate. This process is known as nitrification. Plants and many bacteria can take up and readily reduce nitrate and nitrite through the action of nitrate and nitrite reductases. The ammonia so formed is incorporated into amino acids by plants. Animals then use plants as a source of amino acids, both nonessential and essential, to build their proteins. When organisms die, microbial degradation of their proteins returns ammonia to the soil, where nitrifying bacteria again convert it to nitrite and nitrate. A balance is maintained between fixed nitrogen and at mospheric nitrogen by bacteria that convert nitrate to N₂ under anaerobic conditions, a process called denitrification (Fig. 22–1). These soil bacteria use NO^-₃ rather than O2 as the ultimate electron acceptor in a series of reactions that (like oxidative phosphorylation) generates a transmembrane proton gradient, which is used to synthesize ATP. Now let’s examine the process of nitrogen fixation, the first step in the nitrogen cycle.

FIGURE 22–1 The nitrogen cycle. The total amount of nitrogen fixed annually in the biosphere exceeds 10¹¹ kg.