Heterotrophic Organisms

or heterotrophs, organisms that utilize organic compounds for nourishment (in contrast to autotrophic organisms, which are able to synthesize necessary organic substances from inorganic carbon, nitrogen, and sulfur compounds).

The heterotrophic organisms include all animals and man, as well as some plants (fungi and many parasitic and saprophytic angiosperms) and microorganisms. The division of plants and microorganisms into heterotrophic and autotrophic is rather arbitrary, however, despite the central difference in their type of metabolism. Even typical autotrophs, such as photosynthesizing green plants, can assimilate a certain amount of organic material from the soil through the roots, but their growth and development are better served by inorganic sources of nitrogen. Some green plants, which are capable of photosynthesis, are also insectivorous (for example, the sundew and the bladderwort); basically they use organic nitrogen, while their carbon nourishment is a product of photosynthesis. Some autotrophs require the presence of vitamin-like substances, which are necessary for autotrophic synthesis. In 1921 the Russian scientist A. F. Lebedev showed that even pronouncedly heterotrophic organisms (for example, certain bacteria and fungi) can assimilate CO₂ carbon. This was confirmed isotopically in 1933 by the American scientists H. Wood and C. Werkman. Heterotrophic synthesis provides for minor accumulation of organic substances (up to 10 percent of the organism’s total carbon).

The possibility of CO₂ assimilation by a cell without green (or other) pigment is of major significance in understanding the evolution of chemosynthesis and photosynthesis. The ability of animal tissues also to use CO₂ has been shown. In this connection a tendency has arisen to differentiate organisms as autotrophs or heterotrophs not by the type of carbon nutrition but rather according to the character of the source of vital energy. In accordance with this tendency, those organisms for which the oxidation of complex organic compounds (hydrocarbons, fats, and proteins) serves as the carbon source are considered heterotrophic; those organisms which effect photochemical reactions are considered photo-autotrophs; and those organisms for which the source of energy is the oxidation of inorganic substances are considered chemoautotrophs. Animals and man are strictly heterotrophic organisms, using organic compounds to replace expended energy, to build and renew body tissues, and to regulate vital functions. These kinds of heterotrophic organisms differ in their need for one or another organic compound (depending upon the degree of its participation in the organism’s metabolism) and in the possibility of synthesis of these compounds by the organisms themselves. The so-called essential amino acids, the vitamins, and closely allied compounds are among those necessary substances which can not be synthesized by heterotrophic organisms. By decomposing complex organic substances and converting them to inorganic substances, heterotrophic organisms play an important role in the natural recycling of substances.