creted mainly glycolic acid into the medium. Phosphate 

 esters, of importance to the carbon reduction cycle, were 

 retained in the chloroplasts. Isolated chloroplasts have a 

 carbon metabolism that is much more limited than photo- 

 synthesis in intact cells. This is probably due to loss of 

 enzymic activity by chloroplasts during the isolation process. 

 In all probability the carbon compounds excreted by intact 

 chloroplasts in vivo include substances other than glycolate. 

 There is more than a semantic reason for making a dis- 

 tinction between photosynthetic and nonphotosynthetic 

 pathways. The environment of the photosynthetic metab- 

 olism is unique. There is an abundance of the reduced and 

 energetic form of the coenzymes. Hence synthetic pathways 

 do not require energy derived from degradative reactions 

 such as decarboxylations and oxidations. For example, a 

 well-known biosynthetic pathway leading to glutamic acid 

 from acetate includes oxidative and decarboxylation steps. 

 Such a pathway is to be expected in a nonphotosynthetic 

 system, where degradation of part of the substrate is the 

 only means of obtaining the energy and reducing power for 

 synthetic reactions. In a photosynthetic system one might 

 expect instead a pathway involving only condensations, re 

 ductions, and carboxylations. We cannot say that this differ- 

 ence in type of reaction will always be borne out by the 

 actual mechanisms when they are known. This proposed dif- 

 ference in reaction type may be a useful working hypothesis 

 to those who attempt to map photosynthetic pathways from 

 experimental data. 



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