Principles of Biochemistry Volume 2 - A. Lehninger 1985

Bioenergetics and Metabolism
Biosynthesis of Amino Acids and Nucleotides
The Nitrogen Cycle

Nitrogen is required for The Biosynthesis of both Amino Acids and NUCLEOTIDES. In nature, however, soluble nitrogen compounds in a biologically available form are scarce. Therefore, most organisms conserve ammonia, amino acids, and nucleotides, especially since all these compounds serve as precursors for vital Biomolecules such as Nucleic Acids and Proteins. Indeed, as we already know, free amino acids, Purines, and Pyrimidines generated during metabolic turnover are frequently salvaged and reused.

The largest reservoir of nitrogen is in the air, which consists of nearly 80% molecular nitrogen (N2). However, only a relatively few species are capable of converting atmospheric nitrogen into forms accessible to living organisms; it is these organisms that drive the preservation and recycling of biologically available nitrogen in the massive global nitrogen cycle (Fig. 22-26). The First stage of this cycle is the Fixation of Atmospheric nitrogen by nitrogen-fixing organisms to yield ammonia. Ammonia can be utilized by almost all living forms. Yet, certain important species of soil Bacteria derive their required energy by oxidizing ammonia to nitrites and ultimately to nitrates. These bacteria are ubiquitous and exceptionally active, causing nearly all ammonia entering the soil to be eventually oxidized to nitrates—a process known as nitrification. Plants and many species of bacteria readily reduce nitrates back to ammonia using nitrate reductase. From this derived ammonia, plants synthesize amino acids, whereas herbivorous animals utilize these amino acids (both nonessential and essential) to build their own proteins. Upon the death of animals, their remains undergo microbial decomposition. The ammonia released during protein breakdown returns to the soil, where nitrifying bacteria convert it back into nitrites (NO-2) and nitrates NO-3.

Let us now examine The process of Nitrogen Fixation, which is essential for all life forms.



Last update: 06/08/2026

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