CHAPTER XII 



ANIMAL FOOD AND NUTRITION {Continued) 



METABOLISM 



We have emphasized the fact that the distinctive character 

 of animals as compared with plants is that they are predomi- 

 nantly energy-liberating organisms. The energy is stored up 

 in complex compounds and the tearing down of these com- 

 pounds by oxidation sets free the energy contained in them. 

 We have also given as a definition that any substance which is 

 used by the animal to build up body substance or to yield 

 energy is a food. Body substance in this connection means 

 not only cell substance and muscular tissue, essentially protein 

 in composition, and body fat, but also any substance more 

 complex than the absorbed food material. Such substances 

 may be reserve or stored, or they may be transition material 

 on the way to still more complex forms. 



The food substances which we have been considering and 

 which we have followed through their absorption into the 

 animal circulation are, therefore, to undergo further change, 

 and this change probably always occurs within the living cell. 

 They are to be built up into body materials, and they or the 

 body materials formed from them are to be torn down by 

 oxidation that the energy resulting therefrom may be set free. 

 This energy in its various forms is the manifestation of the life 

 of the cell and the animal. 



Metabolism, Anabolism, Katabolism. — The two processes, 

 viz. the one building up, the other tearing down, are most inti- 

 mately connected. The building-up process, by which the 

 body substance is formed from absorbed food material is known 



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