Carbon 

 ^^ reduction cycle 

 "^i^r of photosynthesis 



We believe the principal pathways for the photosyn- 

 thesis of simple organic compounds from CO2 to be those 

 shown in Figure 2 (1,2). The points at which ATP and 

 TPNH act in these pathways are indicated. Kinetic studies 

 (3) show that these pathways account for nearly all the car- 

 bon dioxide reduced during photosynthesis, at least in the 

 unicellular algae Chlorella pyrenoidosa. From other inves- 

 tigations (4) it appears that the general metabolic sequence 

 is the same in most respects for all photosynthetic organisms. 

 (We shall discuss the recently proposed role of glycolic acid 

 in CO2 reduction in the section on Carboxylic Acids.) 



The central feature of carbon-compound metabolism 

 in photosynthesis is the carbon reduction cycle. Most of the 

 carbon dioxide used is incorporated via this cycle. Pathways 

 lead from intermediates in the cycle to various other impor- 

 tant metabolites. A few of these pathways are shown in Fig- 

 ure 2. 



The initial step for carbon dioxide incorporation in 

 the cycle is the carboxylation of ribulose-l,5-diphosphate at 

 the number 2 carbon atom of the sugar to give a highly 



