16 CHEMICAL TRANSFORMATIONS 



in its absence, or could only occur incomparably more slowly. In 

 this process, the changes in the chlorophyll itself are so insignifi- 

 cant, in comparison with the amount of changes in light energy 

 and chemical energy, that they may be left out of consideration, 

 and we may regard the chlorophyll as a mechanism for carrying 

 on the energy exchanges, just like a chemical catalyst or an enzyme, 

 although the types of energy between which it works are different, 

 as well as the limitations of its action. In the chemical modifi- 

 cations subsequent to the action of the chlorophyll, the various 

 cells may be similarly regarded as catalysing agents differing 

 somewhat in their mode of action upon the chemical materials 

 supplied to them, and in the products formed, but agreeing in 

 that they cause large amounts of interchange between chemical 

 energy and other forms, without being themselves altered per- 

 manently, or proportionately to the reactions which they induce. 

 The same is true of the enzymes formed by the action of the cells, 

 which induce various reactions either within the cells, or after 

 separation from them in the secretions. 



Hence we may regard the processes occurring in plants and 

 animals as energy reactions induced by the cells or their enzymes 

 acting as energy-transformers. 



Excepting in the green cell in the presence of sunlight, the net 

 result of the energy interchanges induced by the cells and their 

 secretions is that the chemical energy produced by the absorption 

 and disappearance of the light energy is used up and converted 

 into other forms, such as heat, mechanical energy, osmotic energy, 

 electrical energy, &c. The process is accompanied by absorp- 

 tion of oxygen, and the matter passes back again into inorganic 

 forms identical with or closely resembling those with which the pro- 

 cess began, and containing little or no more energy than at the 

 start. 



In the intermediate reactions of metabolism the process is 

 not, however, purely one of oxidation; the cell, on account of ."its 

 peculiar properties as an energy- transformer, and probably by 

 the production intermediately of its own peculiar type of energy, 

 is capable of inducing synthetic processes in which chemical 

 energy is taken up. The supply of energy for such a synthetic 

 process is obtained from energy given out by other chemical pro- 

 cesses running concurrently in which energy is set free. 



It is in this respect that the more complex transformer which 



