30 GENERAL PHYSIOLOGY 



f erent constituents with oxygen. Obviously, this constant Uberation of 

 energy in its various forms, must be compensated for, i.e., Uving sub- 

 stance must either generate it or obtain it from some outside source. 

 The law of the conservation of energy, however, teaches us that energy 

 is not created but is merely transformed from one kind into another 

 and hence, living matter must derive it from somewhere, namely, from 

 the medium in which it lives. Various substances are here at hand 

 which contain stored or potential energy. When assimilated by liv- 

 ing matter, either through its respiratory or digestive channel, these 

 chemical bodies are converted into kinetic energy. 



The metabolism of a cell consists in a continuous decomposition 

 and new formation of its protoplasmic material. The former process 

 is designated as dissimilation or catabolism, and the latter as assimila- 

 tion or anabolism. It is true, however, that the metabolism is uniform 

 only in principle, because practically every type of cell has its own 

 peculiar work to perform and hence, a number of special varieties of 

 metabolism are obtained. Expressed in another way, it may be said 

 that the fundamental interchange of material between the cell and its 

 surroundings is modified in many cases to suit particular purposes. 

 Thus, a certain group of cells may be destined to give rise to a digestive 

 secretion, while another furnishes chiefly contractile reactions, and so 

 on. This specificity, however, is not so clearly marked in free-living 

 unicellular organisms as it is in the more complex animals and plants, 

 because the function of the former is not so diversified. 



The catabolic processes occurring in a cell necessitate a constant 

 acquisition of new material to replenish that which has been lost. 

 It is true, however, that the manner in which this assimilation is ac- 

 complished, differs somewhat in different animals and plants. An 

 especially tedious process is in existence in the green plants, because 

 their protoplasm is built up from the simplest possible compounds, 

 such as carbon dioxid, water and various inorganic salts. The animal 

 cell, on the other hand, is constituted differently so that it can also 

 make use of the more complex foods held in the form of organic com- 

 binations. It must be evident, however, that the former can no longer 

 be regarded as synthetic and the latter as decomposition organisms, 

 because the metabolism of both types of cells is dependent upon proc- 

 esses of dissociation and synthesis. It is true, however, that the life 

 of the animals depends upon that of the plants, because only the latter 

 are capable of producing carbohydrates, fats and proteids from inor- 

 ganic material. These are the essentials of animal life. Animals, 

 therefore, are the parasites of the plants. There is, however, one ex- 

 ception to this rule, because those plants which contain no chlorophyl, 

 such as the fungi, must make use of organic substances in order to 

 obtain their requirement in carbon. The fungi, however, are capable 

 of forming nitrogen from the inorganic constitutents of the soil, while 

 animals must derive their supply of nitrogen exclusively from proteids 

 and derivative compounds. As far as their metabolism is concerned, 



