PROTOPLASM 7 



be absolutely definite, but in all probability the yeast forms 

 something which acts upon glucose, since, by applying 

 pressure to it, a fluid can be extracted which contains a 

 substance having the same action as the living yeast. Such 

 a substance is called an Enzyme or Zymin. These Zymins 

 act only in the presence of water, and at suitable tempera- 

 tures, and their action is to accelerate changes which occur 

 more slowly without their presence. Very minute quantities 

 can bring about changes in large quantities of the substance 

 on which they act, and in acting they undergo no marked 

 change. 



Many complex substances besides sugar very readily break 

 down and liberate energy. For instance, formate of lime 

 breaks into carbonate of lime, carbonic acid, and hydrogen, 

 under different conditions : first, under the influence of certain 

 bacteria, minute living organisms resembling yeast in many 

 particulars; second, under the influence of some substance 

 contained in the bacteria after they are killed; and lastly, 

 in the presence of finely divided iridium, rhodium, and 

 ruthenium i.e. the same change is brought about by a 

 living organism, by a substance contained in it, and by a 

 metal. 



Hence mere vibrations of molecules, occurring in different 

 ways and in different substances, may be sufficient to bring 

 about these changes, and however the change is brought 

 about the result is to set free energy. Such a process has 

 been termed Catalysis. 



Living yeast differs from these dead substances simply in 

 the fact that it uses the energy liberated from the glucose. 

 In virtue of this, the yeast has the power of repair and of 

 growth. 



But protoplasm is also constantly breaking down, and if 



yeast be kept at a suitable temperature in water without any 

 supply of material for construction, it gives off carbon dioxide 

 and decreases in bulk on account of these disintegrative 

 changes. These are as essential a part of the life of living 

 matter as the building-up changes, and it is only when they 

 are in progress that the latter are possible. 



Protoplasm (living matter) is living only in virtue of its 



