MECHANISM OF METABOLISM 217 



with quartz sand. They will then reduce nitrates to nitrites, sulphur 

 to hydrogen sulphide. The decolorization of litmus, methylene 

 blue, indigo, and other organic dyes is due in microbial cultures to 

 enzymes which are almost exclusively endo-enzymes. 



ENZYMIC THEORY OF KATABOLISM 



Regarding katabolism as the sum of all destructive processes of 

 the living cell substance, i.e., of the protoplasm, and considering the 

 cell substance to be decomposed and renewed constantly as long as 

 the cell is performing the normal functions of life, there must be a reno- 

 vating and a destructive process continuously going on in the proto- 

 plasmic molecules. If the food supply ceases, anabolism ceases with 

 it, but it has been demonstrated that katabolism may continue just 

 the same for some time. By this method, the products of katabolism 

 can be obtained separate from the products of food digestion which 

 would obscure the results of experiment on katabolism in normally fed 

 cells. 



It is difficult to determine to what extent katabolism is controlled 

 by endo-enzymes, the so-called autolytic enzymes, which have been men- 

 tioned in the above paragraph. Unquestionably, the katabolic processes 

 are similar to enzyme processes, since katabolism is checked by heat 

 or poison just like enzyme processes. 



ENZYMIC THEORY or ANABOLISM 



ANABOLISM AND INTRA-CELLULAR ENZYMES. All changes dis- 

 cussed in the previous chapters are processes in which organic or 

 inorganic compounds are broken up to smaller molecules. These 

 processes are exothermic, i.e., liberating heat or energy in other forms. 

 The opposite is true of the anabolic processes which build up complex 

 molecules from simple compounds. These synthetic processes are 

 endothermic, absorbing heat or other energy. Growth is the typical 

 manifestation of anabolism. It is the formation of new cells from dead 

 organic or inorganic matter, and it means the formation of all the com- 

 pounds necessary for cell life. Of all the substances found in the cell, 

 practically none are contained in the food, and it is wonderful that 

 in such a small unit as a microbial cell, there are contained the powers 

 of making protoplasm, enzymes, nuclear bodies, chromatin bodies, 

 the substance of the cell wall and probably many other unknown 



