LECTURE XII. 



METABOLISM (continued}. 



WE have now to enquire more closely into the mode of 

 origin and the chemical nature of the products which, as we 

 have seen, are formed in connexion with the metabolism of 

 the plant. 



8. The Products of Metabolism. 



We gather from what has been said in previous lectures 

 that the products of metabolism may be classified into two 

 groups ; those, namely, which can be used in the constructive 

 processes, and those which can not : the former we term 

 plastic products, the latter waste-products. We find, also, that 

 these may be further subdivided into non-nitrogenous and 

 nitrogenous. 



^\\^ plastic products may be formed in either of two ways : 

 either constructively, when they are built up from simpler 

 substances (as when carbohydrate is formed from carbon 

 dioxide and water, see p. 145), or destructively, when they 

 are formed as the result of the dissociation of the molecules 

 of a more complex substance (as when starch is formed from 

 protoplasm). 



Beginning now with the non-nitrogenous plastic sub- 

 stances, we find them to be principally carbohydrates and 

 fats. To these we may perhaps add the more complex 



