Nitrogen in Living Things

one of the basic biogenic elements that constitute the most important substances of living cells, proteins, and nucleic acids. However, the quantity of nitrogen in living things is small: 1–3 percent of the dry mass.

Molecular nitrogen, which is found in the atmosphere, can be assimilated only by certain microorganisms and the blue-green algae. Significant reserves of nitrogen are concentrated in the soil in the form of various minerals, such as ammonium salts and nitrates, and organic compounds, including the nitrogen from proteins, nucleic acids, and the products of their decomposition—that is, the still incompletely decomposed remains of plants and animals.

Plants assimilate nitrogen from the soil in the form of inorganic, as well as certain organic, compounds. Under natural conditions, the soil microorganisms, or am-monifiers, which mineralize the organic nitrogen of the soil to ammonium salts are of great significance for plant nutrition. The nitrate nitrogen of the soil is formed as a result of the vital activity of the nitrifying bacteria, discovered in 1890 by S. N. Vinogradskii, that acidify the ammonia and ammonium salts into nitrates. A portion of the nitrate nitrogen assimilable by the microorganisms and plants is lost, changing into molecular nitrogen under the effect of the denitrifying bacteria. The plants and microorganisms assimilate well both ammonium and nitrate nitrogens, reducing the latter to ammonia and ammonium salts. The microorganisms and the plants actively convert the inorganic ammonium nitrogen into organic nitrogen compounds, amides (asparagine and glutamine), and amino acids.

As was shown by D. N. Prianishnikov and V. S. But-kevich, nitrogen in plants is stored and transported in the form of asparagine and glutamine. With the formation of these amides, the ammonia is rendered harmless. High concentrations of ammonia are toxic not only for animals but for plants. The amides become part of many proteins in microorganisms, plants, and animals. The synthesis of glutamine and asparagine by fermentation amidation of glutaminic and asparaginic acids is carried out in microorganisms and plants, and, within certain limits, in animals.

The synthesis of amino acids occurs by the reduction amination of a number of aldehyde acids and ketoacids which arise as a result of the oxidation of carbohydrates (V. L. Kretovich) or by fermentation reamination (A. E. Braunshtein and M. G. Kritsman, 1937). The end products of the assimilation of ammonia by microorganisms and plants are proteins which become part of the protoplasm and nucleus of cells and which also are deposited in the form of reserve proteins. Animals and man are capable of only limited amino acid synthesis. They cannot synthesize eight irreplaceable amino acids—valine, isoleucine, leucine, phenylalanine, tryptophan, methionine, threonine, and lysine—and for this reason, their basic nitrogen source is the proteins consumed with food—that is, ultimately, the proteins of plants and microorganisms.

Proteins in all organisms are subject to fermentation decomposition, the end products of which are amino acids. In the following stage, as a result of deamination, the organic nitrogen of the amino acids is again converted into an inorganic ammoniacal nitrogen. In microorganisms and particularly in plants, the ammonium nitrogen can be used for a new systhesis of amides and amino acids. In animals, the ammonia formed with the lysis of proteins and nucleic acids is rendered harmless by the synthesis of uric acid in reptiles and birds and urea in mammals, including man; these are then eliminated from the organism. From the standpoint of nitrogen exchange, plants, on the one hand, and animals (and man) on the other, differ in the fact that in animals the formed ammonia is only slightly utilized, while a larger portion is eliminated from the organism. In plants, nitrogen exchange is “locked in”—that is, the nitrogen entering the plant returns to the soil only along with the plant itself.