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. 
Tf 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. 
