170 VELOCITY OF REACTION, AND 



they differ in that they require external energy in order to do their 

 work; instead of causing energy to be given out from the chemical 

 system, they cause the system to take up energy, and instead of 

 assisting the system towards the equilibrium point towards which 

 it tends in their absence, they cause a movement away from the 

 equilibrium point. 



To this class of catalysts the living cells of plants and animals 

 belong, and although the process is most clearly seen where chloro- 

 phyll is present, and is masked in other cells by preponderating 

 action in the opposite direction, there is probably no cell in which 

 anabolic processes do not occur, as shown by the building up, 

 accompanied by storage of chemical energy, of complex organic 

 substances, such as the organised proteid or protoplasm of the 

 cell, and the granular deposits in the cell of amyloses, fats, and 

 other reserve foodstuffs, from the soluble constituents of the 

 plasma or of the circulating fluids by which the cell is nourished. 

 In most types of cell the energy required for the anabolic processes 

 is derived from chemical energy obtained by an oxidation process 

 affecting a portion of the nutrient matter, the energy obtained from 

 this reaction being used to run the anabolic reaction. 



The linkage together in this way of a variety of complex chemical 

 reactions is what distinguishes the cell as an energy transformer from 

 the simpler soluble enzyme, which is so often a product of its activity, 

 Such linkage of reaction is never seen in the case of enzymes, which 

 are exceedingly fixed and selective in their action. The enzyme 

 acts usually upon one type of molecular arrangement only, often 

 failing in attacking even the stereo-isomer, but the cell carries on a 

 wide commerce of reaction with many types of matter, and modifies 

 the reactions in varied ways ; and also differently at different periods 

 according to its condition, and the manner in which it is affected by 

 concurrent reactions taking place in other cells in the body, or by 

 the influence of the nervous system upon it. 



Actions similar to those of the cell in storing up chemical energy 

 are also seen in physical transformers; an example of such is the 

 synthesis of compounds by the electric current and the electrolysis 

 of conducting solutions. Here the electrodes, two conductors at 

 different potentials, act as energy transformers for converting elec- 

 trical into chemical energy. 



The analogy between chemical energy transformations and 

 those of other forms of energy is so clear and the action is so obviously 



