Assimilation in Biology

(anabolism), a process present in all living organisms, one facet of the metabolic process. Assimilation is the process of forming from simpler materials those complex substances making up an organism (in the last analysis, from the elements in the external environment). It is one of the most characteristic properties of living things. The process of assimilation supports growth, development, the renewal of the organism, and the storing up of reserves used as a source of energy. From the point of view of thermodynamics, living organisms are open systems—that is, they can exist only with the uninterrupted input of energy from outside.

The primary source of energy for all living nature is solar radiation. All of the various organisms inhabiting the earth can be divided into two basic groups distinguished by their use of different sources of energy: autotrophic organisms and heterotrophic organisms. Only the first group—that is, primarily green plants—is capable of exploiting the radiant energy of the sun directly in the process of photosynthesis by forming organic compounds (carbohydrates, amino acids, protein, and others) from inorganic substances. The remaining living organisms, with the exception of certain microorganisms capable of obtaining energy by means of chemical reactions, assimilate already formed organic matter, using it as a source of energy or as structural material for the building of their bodies. Thus, during the assimilation of the proteins in food by heterotrophs (which include animals), the breakdown of proteins to amino acids takes place at first—that is, the proteins lose their biological individuality—and then the re-synthesis of proteins characteristic of a particular organism takes place. In living organisms the process of renewal of component parts owing to the destruction (dissimilation) and the creation of organic material—that is, assimilation—takes place continually. Thus, for example, the complete renewal of the proteins in the body of an adult human takes place in approximately two and a half years. The intensity of assimilation and its correlation with the reverse process— dissimilation or catabolism—varies significantly both in different organisms and in the course of one individual’s life. The most intensive assimilation takes place during periods of growth: in animals, at a young age; in plants, during the vegetative period.