48 VELOCITY OF REACTION, AND 



produces its remarkable effect upon the duration and course of 

 the reaction. 



If the catalyst is not capable of acting upon external forms 

 of energy, such as light energy in the case of the green plant cell, 

 but can only utilise chemical energy and convert this into heat 

 energy and osmotic energy, then the catalyst can only work 

 towards the equilibrium point and not away from it. It can start 

 a reaction held stationary by molecular attractions, or can modify 

 a reaction already running by hastening or slowing it, but it must 

 act in all cases towards the equilibrium point from either end. 

 Also dependent upon the power of the enzyme, the reaction may 

 be slowed and stopped at other points, usually corresponding to 

 some definite stage in the reaction, which are called false equilibrium 

 points (see p. 66). This follows, because as the reaction comes 

 nearer the equilibrium point, the chemical potential aiding the 

 catalyst in its work becomes less, and hence always the velocity 

 becomes less, but also the resistance may become so great as to 

 be insuperable for a particular enzyme, and the reaction may come 

 to a dead stop. Take, for example, the relative action as catalysts 

 upon starch solutions of dilute acid, and of diastase of malt. The 

 diastase is powerless as a catalyst when all the starch has dis- 

 appeared and been replaced by a mixture of dextrin and maltose, 

 the resistances, for this catalyst, have been increased beyond 

 the power it possesses and the reaction ceases. But the acid 

 proceeds further, and converts the dextrin and also the maltose 

 into dextrose ; it possesses the power of breaking down the resist- 

 ance which was insuperable for the diastase. 



In both these cases, however, the movement is towards the 

 point of chemical equilibrium, and it is by acting as a transformer 

 of chemical energy into other forms that the catalyst does its work. 

 The only difference is that at a certain stage the potential factor 

 of the molecular energy of the substance being broken up, becomes 

 too powerful for the diastase, and the reaction is stopped so far 

 as that catalyst is concerned ; but the more powerful hydrogen 

 ion of the acid is still able to overcome the molecular cohesion, 

 and to continue the reaction a stage farther. 



But there are catalysts or transformers which can convert 

 other forms of energy into chemical energy, and these form a 

 distinct class from the others ; for although they are similar to 

 the former in not being themselves altered by the reaction they 



