THE LIVING CELL AND ITS FOOD 157 



energy is not, however, always the result. When the oxidation 

 occurs in the cell it is probably always due to the influence of 

 enzymes, and the energy set free may be in the form of heat 

 which is not accompanied with burning, as we ordinarily think 

 of it, but in the form of heat that maintains a certain temper- 

 ature such as the body heat of warm-blooded animals. The 

 energy may not, however, produce heat at all, but may be 

 manifested in the form of muscular work, as in the case of ani- 

 mals or of work not exactly muscular but which may be called 

 cellular, such as is required in the growth of large plants or trees. 



The fundamental chemical reaction of living cells, then, is the 

 oxidation of the complex organic compounds of the food to simple 

 compounds with the liberation of energy, which energy is the 

 basis of the living process manifesting itself as heat or work in 

 the various functions and properties of the living organism 

 whether the organism is plant or animal. 



Food as Building Material. — Not only must the living cell 

 be furnished with food which by oxidation yields energy, but it 

 must also utilize its food for building up its own substance. This 

 will necessarily be much greater in the cases of young and rapidly 

 growing organisms, but in all organisms cell substance is con- 

 tinually used up in the life process and new building material 

 must be supplied. We cannot state as a positive fact that all 

 food is first converted into cell substance or, as we say, body 

 substance, before it is oxidized, but it is at least probable that 

 this is so. We know that if an animal is deprived of food it 

 utilizes its own body substance, which is torn down and oxidized, 

 thus yielding the energy necessary for life. 



Function of Food. — Thus the function of food may be con- 

 sidered as twofold, i.e. to build body or cell substance and to 

 furnish energy. 



Plants and Animals Compared 



Each plant or animal of the higher forms, and it is only these 

 higher forms that we shall consider, is a very complex individual. 

 The single cells of which the organism is composed do not per- 



