ACTION OF ENZYMES AND CELLS 169 



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 depen- 

 dent 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. 186). 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 disappeared 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 been replaced by a mixture of dextrin and maltose, the resistances, 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 farther, and converts 

 the dextrin and also the maltose into dextrose; it possesses the 

 power of breaking down the resistance 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 induce, 



