294 A TEXTBOOK OF GENERAL BOTANY 
general category of catalysts. Catalysts are substances which 
change (usually accelerate) the rate of a reaction. Owing to the 
fact that a reaction which is accelerated by an enzyme might 
take place so slowly without the enzyme as to be imperceptible, 
it is customary, in order to avoid the use of cumbersome lan- 
guage, to speak of an enzyme as acting on a substance rather 
than as accelerating the rate 
of a reaction. 
Enzymes, like other catalysts, 
are characterized by not being 
a part of the initial substance 
in the reaction or of the final 
product, by not being altered 
in the reaction, by influencing 
a change in a relatively great 
volume of the reacting sub- 
stance, and by accelerating the 
effect in direct proportion to 
the amount of catalyst present. 
As an example of inorganic 

Fig. 298. Cells from seed of Croton 
tiglium with large, rounded oil glob- 
ules and irregularly shaped protein 
catalysts we may mention spongy 
platinum, which accelerates the 
oxidation of sulphur dioxide in 
the manufacture of sulphuric 
acid. Another example is finely 
divided nickel, which is used in the commercial hydrogenation 
of oils. By this means many inedible liquid oils are turned 
into solid edible fats. 
Plants contain many different kinds of enzymes, and each 
enzyme acts on only one substance or on a group of similar 
substances. The best-known reactions that are accelerated by 
enzymes are those in which complex substances are split into 
simpler ones. It is believed, however, that a given enzyme can 
accelerate a reaction in either direction; that is, if an enzyme 
splits up a complex substance, it can also influence the reverse 
process of the building up of the same complex substance. 
granules. (Xx 450) 
