FOOD USED IN METABOLISM 77 



tion, since it consists of a union of the material that is 

 being burned with oxygen. For a food substance to be 

 useful as a source of energy, it must be of a kind that will 

 oxidize within the protoplasm and so yield the energy 

 which the protoplasm must have if it is to live and func- 

 tion. Foodstuffs, both in plants and animals, must, then, 

 be materials that can be made over into living proto- 

 plasm, or into substances which the protoplasm manu- 

 factures for one purpose or another, or that can be burned 

 and so yield the energy which the protoplasm needs. 

 It is worth while to bear in mind that one important 

 difference between plants and animals is that in plants 

 the kind of metabolism that results in the formation of 

 materials predominates over that which consumes them, 

 while in animals the reverse is the case. Since most of 

 the material that is stored in plants has high energy 

 value, the result of this difference is that plants in general 

 gain in weight and potential energy supply throughout 

 their lifetime, while animals, as the adult stage is reached, 

 carry on a metabolism that is almost entirely concerned 

 with the consumption of energy; so that the metabolic 

 processes of animals tend to balance those of plants, 

 leaving'nature as a whole little if any the gainer in total 

 amount of stored-up energy. 



Kinds of Food Used in Metabolism. — It has just 

 been stated that any substance that can be oxidized 

 within protoplasm can serve as a basis for energy-yielding 

 metabolism. The three great classes of foodstuffs, carbo- 

 hydrates, fats, and proteins, can all do this, and so are 

 all useful as sources of energy. For the metabolism 

 which gives rise to materials only such substances are 

 useful as can be converted into the desired materials. 

 Thus for the manufacture of protoplasm, in growth or 

 in the repair of animal wastage, the protein-formers, 

 namely, the amino-acids, are absolutely essential, so that 

 unless they are available in suflficient amounts no proto- 

 plasm can be made, even though other kinds of food- 



