398 DANIliL I. ARNON 



CO2 assimilation can occur only in the light), non-cyclic photophos- 

 phorylation alone does not provide sufficient ATP for a reductive assimila- 

 tion of CO.^to the level of carbohydrate. Additional ATP must be supplied 

 by cyclic photophosphorylation. 



^^k OHYOROXYACETONE 

 i^^» PHOSPHATE 



PHOSFHOGLYCERATE 



fi0^" 



MONOPHOSPHATES 

 (GLUCOSE, FRUCTOSE) 



DIPHOSPHATES 

 (FRUCTOSE, RIBUUOSE) 



.( 



PHCMOL/\«ATlR- 



Fk;. 34. Radioautograph of a chromatogram showing products of photo- 

 synthetic ^^COo assimilation by ilkiminatctl cliloroplasts supplied with o-ooi 

 /jmoles FMN (Trebst, Losada, and Anion I34I). 



16. The energy conversion concept in photosynthesis 



The concept of photosynthesis to which we w-ere led in the 6 years 

 since the process was first completely localized in isolated chloroplasts 

 [13-15] differs from the conventional view of photosynthesis that it is 

 mainly a process of CO2 assimilation. Photosynthesis appears to be first 

 and foremost a process for converting sunlight into chemical energy and 

 this conversion is more directly associated with phosphorus than with 

 carbon assimilation. In the light of present knowledge, photosynthesis may 

 be defined as the synthesis of cellular substances at the expense of chemical 

 energy formed by photochemical reactions. This definition inclucies, but 

 is not limited to, CO2 assimilation. 



In both bacterial and plant photosynthesis the photosynthetic events 

 proper are limited to the formation of adenosine triphosphate and reduced 



