Other Modes of Nutrition; Conservation of Food Elements - 1 83 



organisms and many radical departures from 

 the main lines of evolution. In short, many 

 methods of gaining matter and energy for 

 growth and activity are found among simpler 

 living bodies, but only the holophytic and 

 holozoic modes of nutrition proved adequate 

 for the sustenance of larger and more com- 

 plex organisms. 



CYCLIC USAGE OF THE FOOD 

 ELEMENTS 



Each organism, regardless of its mode of 

 nutrition, grows at the expense of specific 

 substances taken from the environment. Con- 

 sequently, the life of a species cannot con- 

 tinue if any substance essential to its growth 

 becomes exhausted. In the total economy of 

 living things, however, the various food sub- 

 stances keep returning to the environment, 

 ultimately in their original form and quan- 

 tity. The earth is like a sealed but balanced 

 aquarium. It receives no significant quantity 

 of matter from other parts of the universe. 

 But despite this material isolation, the earth 

 can supply the material needs of all its in- 

 habitants indefinitely — so long as light from 

 the sun maintains its strength and continues 

 to energize, directly or indirectly, the meta- 

 bolic processes of existing organisms. 



The simplest way to realize the inexhausti- 

 bility of the material requirements of life is 

 to trace the cycle of chemical changes by 

 which each element returns to the environ- 

 ment after participating in the metabolism 

 of various organisms. Each element partici- 

 pates in the formation of many protoplasmic 

 compounds, but finally each returns to the 

 environment in its original form. Thus the 

 carbon cycle, the hydrogen cycle, and the 

 nitrogen cycle describe the specific series of 

 chemical changes by which each of these ele- 

 ments is utilized by different organisms and 

 finally restored to the environment in the 

 original form. Each element exhibits a dif- 

 ferent cycle, but only the carbon and nitro- 

 gen cycles will be described. 



The Carbon Cycle. The environmental 



source of carbon for all organic syntheses is 

 the carbon dioxide present in the atmosphere 

 and natural waters of the earth (Fig. 10-9). 

 The green plants (by photosynthesis) and to 

 a much smaller extent the autotrophic bac- 

 teria (by chemosynthesis) take this carbon 

 and build it into the molecular structure of 

 carbohydrates; and these carbohydrates fur- 

 nish all the energy, and a large part of the 

 matter needed by plants for the synthesis of 

 proteins, lipids, and other organic com- 

 pounds in the protoplasm. The potential en- 

 ergy stored by green plants in various organic 

 compounds is derived initially from the sun, 

 and this energy becomes the driving force in 

 the metabolism of virtually all other organ- 

 isms. Organic compounds, created by plants 

 and appropriated by animals, saprophytes, 

 and parasites, furnish energy and matter for 

 the synthesis of all essential organic compo- 

 nents in heterotrophic organisms. Eventually, 

 however, all organic compounds suffer de- 

 composition: sooner or later all organic car- 

 bon returns to the environment as carbon 

 dioxide, via the respiration of the organisms 

 wherein the energy is utilized (Fig. 10-9). 



The Nitrogen Cycle. The main environ- 

 mental source of nitrogen used by green 

 plants for the synthesis of the amino acids 

 and other essential nitrogen compounds is 

 nitrate ( — N0 3 ) nitrogen, in the soil and 

 natural waters of the earth (Fig. 10-10). The 

 organic nitrogen compounds formed by 

 plants are used later by animals and other 

 heterotrophic organisms for the synthesis of 

 their proteins. In living animals, proteins 

 and other complex nitrogen compounds may 

 be catabolized — in which case the nitrogen 

 is excreted as urea and other simple nitro- 

 genous wastes; or such protein may pass to 

 the environment as a remnant of some ani- 

 mal. In any event, organic nitrogen in the 

 environment is utilized by the putrefactive 

 bacteria (Fig. 10-10), which excrete the nitro- 

 gen mainly in the form of ammonia (NH 3 ) 

 but partially in the form of free nitrogen 

 (N L ,). Ammonia and ammonium compounds 

 in the soil are utilized as a source of energy 



