METABOLISM 259 



special growing points or layers, is equally true of all 

 cells so long as they are living. In all cases, though 

 growth and division may not be evident, we have to do 

 with processes of repair of the inevitable wasting of the 

 living substance during the operations of its life. The 

 same kind of change is evident in all cells, though the 

 immediate results of such changes differ according to the 

 part any particular cell takes in promoting the well-being 

 of the whole organism. 



If we turn from these anabolic processes we find we 

 have proceeding, side by side with them, a decomposition 

 of the protoplasm, involving a separation from its complex 

 molecule of various substances which are of less com- 

 plexity than the living material itself. These often, 

 in the first instance, include such carbohydrates and nitro- 

 genous substances as it made use of in building itself up. 

 These can again be used in reconstruction of the proto- 

 plasm, or can be further broken down into simpler substances 

 still or can be retained unaltered. So long as the proto- 

 plasm is living, it is continually undergoing constant 

 reconstruction and decomposition. 



Besides initiating those chemical changes in which it 

 takes this prominent part, it is also the seat of a large 

 number of others into which its own molecule does not 

 immediately enter. Processes of both oxidation and reduc- 

 tion are continually going on in its substance, in which are 

 involved the various materials which are found there, either 

 in solution in the water which saturates it, or in amorphous 

 form ; substances which have been transported from other 

 cells, or have been formed in the processes of the self- 

 decomposition of the protoplasm. 



Two classes of enzyme have been discovered which 

 may, and probably do, assist in these changes. They are 

 the oxidases, to which allusion has been already made, 

 and reductases, which act in the opposite direction. The 

 former have been known for some time, the latter have 

 been observed only recently. 



