Photosynthesis 

 ^<p vs. other forms 

 '^^ of biosynthesis 



Biosyntheic reactions in plants cannot be classified as 

 photosynthetic or nonphotosynthetic on the basis of direct 

 photochemical action because all reactions in the synthetic 

 pathways are probably "dark" reactions. However, we can 

 make a classification on the basis of the immediate source of 

 the required cofactors. The conversion of light energy results 

 in the formation of ATP and TPNH and perhaps other un- 

 known cofactors. When these cofactors are formed by the 

 light reaction and are used to bring about the synthesis of 

 carbon compounds, we may consider the reactions to be 

 photosynthetic. Also included in this category would be 

 preliminary and intermediate steps such as hydrations, con- 

 densations, and carboxylations. 



It may well be that all photosynthetic reactions, as just 

 defined, occur in the chloroplasts while the light is on. 

 If this is true, reactions outside the chloroplast would derive 

 their energy from substrate carbon compounds which diffuse 

 from the chloroplast to the extrachloroplastic spaces of the 

 cell. Such an interpretation is suggested by the report by 

 Tolbert (22), who found that chloroplasts isolated from Swiss 

 chard, when allowed to photosynthesize with HC^^Oa", ex- 



27 



