METABOLISM 145 



formations. On the one hand, molecules of resorbed digestion 

 products are being assimilated by the body cells and built up 

 into the structure of their protoplasm, while, on the other 

 hand, molecules of protoplasm or of cell enclosures are being 

 broken down and oxidized, yielding finally the relatively simple 

 excretory products. 



The term metabolism is commonly used to designate the to- 

 tality of the chemical changes which the constituents of the 

 resorbed feed undergo in the course of their conversion into 

 the corresponding excretory products. Similarly, one- may 

 speak in a more restricted sense of the metabolism of single 

 ingredients of the feed, as of the proteins, carbohydrates or 

 fats, protein metabolism, for example, signifying the chemical 

 changes undergone by the digestion products of the proteins 

 of the feed during their assimilation and subsequent transfor- 

 mation into excretory products. The adjective metabolic is 

 also used to describe these chemical changes. 



203. Anabolism and katabolism. The term metabolism, 

 as just defined, includes processes of two distinct kinds, viz., 

 those by which molecules of sugars, organic acids, amino acids, 

 etc., are built up into more complex compounds in the body 

 and those by which these complex compounds are broken down 

 again into simpler substances and finally into the excretory 

 products. 



The building up metabolism has received the name anabolism, 

 while the breaking down or oxidative phase is called katabolism. 

 Any change in the direction of greater molecular complexity 

 is spoken of as an anabolic change, while one in the direction of 

 greater molecular simplicity is a katabolic change. 



It must not be inferred from what has been said that anabolism 

 always precedes katabolism. Neither is the breaking down of cell 

 constituents by any means a process of uninterrupted katabolism. 

 On the contrary, many instances are known in which it is interrupted 

 at various stages by anabolic changes of one sort or another. While 

 the general direction of the change is towards simplification, there 

 are eddies in the current. Moreover, it is by no means probable that 

 all the resorbed substances are actually built up into protoplasm 

 before being katabolized. It is true that, to trie best of our knowl- 

 edge, the metabolic processes take place within the cells but it ap- 

 pears unlikely that the relatively large amounts of material some- 

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