RESPIRATION 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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