KESP1KATION 305 



Turning from the question of respiration to study other 

 changes which subserve a similar purpose with regard to 

 the local supply of energy, we may first examine such pro- 

 cesses as are oxidative. In them all we cannot fail to mark 

 the activity of the protoplasm in carrying them out. The 

 living substance does not, however, act as a general oxidising 

 agent, but different protoplasts possess specific powers. 

 Certain micro-organisms can cause the oxidation of ammonia 

 and the consequent formation of a nitrite ; others can 

 convert the nitrite into a nitrate, but neither can do the 

 work of the other. Others have not such limited powers ; 

 a certain bacterium can cause the oxidation of alcohol 

 to acetic acid, and after the exhaustion of what alcohol 

 may be present, can further oxidise the acetic acid to carbon 

 dioxide and water. The exact way in which the protoplasm 

 acts as a carrier of the oxygen without apparently undergoing 

 decomposition is very obscure. It may perhaps combine 

 with the oxygen and pass it on to these oxidisable substances, 

 acting as a carrier only. 



It has recently been found that besides exerting a direct 

 oxidative power, protoplasm can secrete an enzyme, or 

 perhaps a variety of enzymes, each with a special peculiarity, 

 through whose instrumentality the oxidation is effected. 

 These enzymes have been termed oxidases, and they are 

 probably widespread in the vegetable kingdom. A dis- 

 cussion of their peculiarities would be beyond the scope of 

 this volume, but we may call attention to their general 

 features. 



The first one discovered is known as laccase ; it has a 

 very wide distribution, occurring in the roots, stems, and 

 leaves of various plants, and in a very large number of 

 fungi. It appears to oxidise various constituents of plants, 

 but particularly the colouring matters. Another, known 

 as tyrosinase, occurs in other fungi, and oxidises chiefly 

 tyrosin. Others oxidise various colouring matters, together 

 with tannin. Most of them require the presence of a 

 peroxide in the cells to enable them to act. 



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