We have seen that carbon dioxide is the source from which green plants derive the carbon which they synthesize into carbohydrates, fats, and proteins. Animals directly or indirectly feed on plants so that the ultimate source of the carbon of animals is likewise the carbon dioxide of the atmosphere. Although both plants and animals by their respiratory process are continually returning to the outer world some of this carbon as carbon dioxide, it is evident that relatively enormous amounts of carbon are nevertheless being taken out of circulation and locked up in the bodies of the plants and animals. For example, it has been estimated that about one half the weight of a dried tree trunk is contributed by carbon.
The same general segregation is going on in regard to nitrogen. The green plants take it in the form of nitrates, for instance, and store it away in the proteins; and again animals get their nitrogen from plant proteins, so that the ultimate source of the animal nitrogen is the same. In a somewhat similar manner we might trace the fate of the other chemical elements necessary for protoplasm, but that of carbon and nitrogen is particularly striking and instructive, and is sufficient to illustrate the fact that although both green plants and animals are continually taking elements from and returning them to their environment, nevertheless more is taken away than is returned.
The agents which restore to the inorganic world the elements removed by green plants and animals are the colorless plants, chief among which are the Bacteria. As we know, when an animal or plant dies, decay sets in almost immediately; that is, the complex chemical compounds are slowly but surely reduced to simpler and simpler forms until ' dust ' remains. Although undoubtedly many of these compounds would automatically, so to speak, tend to simplify, nevertheless this is not only hastened, but chiefly carried out by organisms of decay such as the Bacteria. Through enzymes, or ferments, which they form, FERMENTATION occurs. The carbohydrates and fats are resolved into carbon dioxide and water, and the proteins are reduced to carbon dioxide, water, and ammonia (NH 3 ) or free nitrogen, while the nitrogenous waste (urea, etc.) of animals is similarly broken down. Practically all of these long series of chemical reactions are carried on by different kinds of Bacteria. Most green plants, how ever, take their nitrogen chiefly in the form of nitrates and Accordingly we find that another type of Bacteria (NITRITE BACTERIA) acts upon the ammonia and transforms it into nitrous acid (HNO 2 ). After certain chemical reactions in the soil, forming, e.g., potassium nitrite or ammonium nitrite, still another type (NITRATE BACTERIA) oxidizes the nitrites into nitrates (e.g., KNO 3 or NH 4 N0 3 ), so that again this nitrogen is in a form which is available for green plants.
But, still confining our attention to the nitrogen, it is obvious that there is a leak from this cycle, since some of the nitrogen in the form of ammonia or free nitrogen escapes to the atmosphere. The greatest loss, however, is brought about by a group of DENITRIFYING BACTERIA whose activities are largely spent in changing nitrates into gaseous nitrogen which escapes into the air, and so putting it beyond the reach of green plants and animals. Fortunately there is also a special group of NITROGEN-FIXING BACTERIA which rescue Animal, Proteins Proteins of Green Plants Ammonia.
The Nitrogen Cycle. A schematic representation of the circulation of nitrogen in nature.
The nitrogen from the atmosphere and return it to the cycle of elements in living nature. These organisms inhabit the soil or little nodules which they produce on the rootlets of leguminous plants, such as beans, clover, and alfalfa; and this accounts for the fact, long known but not understood, that these plants when plowed under are particularly efficient in enriching the soil. In brief, there is a cycle of the elements in nature through green plants and animals and back again to the inorganic world through the Bacteria and other colorless plants. Such is the reciprocal nature of the nutritive processes of living organisms.
It is hardly necessary to state that the chemical changes produced by the Bacteria are either the direct results of, or are incidental to, the process of nutrition in these organisms.
Therefore the material taken as food by certain groups is relatively complex, for example, by those which bring about the early putrefactive changes in proteins; while that em ployed by others is very simple since they find adequate chemical combinations less complex than those needed by green plants. Indeed, it is now known that one group of Bacteria is able to utilize carbon dioxide and water just as do green plants. But instead of obtaining energy for the synthesis from sunlight, these Bacteria derive it from chemical energy liberated by the oxidation of substances in their environment. This process is referred to as CHEMOSYNTHESIS, in contrast with photosynthesis, and although it is apparently restricted to a relatively small group of organ isms, may well represent the most primitive method of nutrition from which all the others have been derived in the evolution of life.