METABOLISM 53 



This can be done in a variety of ways. 

 The source of energy is usually an elec- 

 tric current, radium, or ultra-violet 

 light. Ordinary light can be used in the 

 presence of certain metals such as iron 

 (which also occurs in the plant). By 

 subjecting a mixture of ammonia, car- 

 bon dioxide and water, to ultra-violet 

 light nitrogenous compounds can be 

 produced. Some of these appear to be 

 amino-acids from which proteins can be 

 built up. 



It may be added that certain bacteria 

 are able by oxidizing compounds of iron 

 or sulphm* to gain suflScient energy to 

 utilize the carbon dioxide of the air with- 

 out the aid of sunlight or chlorophyll. 

 Hence it has been suggested they may 

 have been the precursors of chlorophyll- 

 bearing organisms in the process of evo- 

 lution. 



Since many of the most important 

 steps in constructive metabolism have 

 already been imitated in the laboratory 

 it seems probable that others will share 



