PHOTOSYNTHESIS 



reagent necessary to permit the reaction to proceed. This 

 reagent may be ATP, and the oxidant required may be either 

 0-2 or another oxidant produced photochemically. It is the 

 reaction of this oxidant (or Oo) with the activated (Ho) which 

 can produce ATP by the mechanism almost certainly similar to 

 that ordinarily known as "oxidative phosphorylation." Only 

 under special conditions such as these, in which a high level of 

 H (from Ho) and ATP are present simultaneously, may we expect 

 the photosynthetic carbon cycle to operate (53). 



The precise detailed mechanism of the carboxylation re- 

 action is not yet known with certainty. The first step may be 

 the enolization of the ribulose diphosphate to form a double 

 bond between carbon atoms 2 and 3. An addition of the carbon 

 dioxide may then take place with the shift of an electron from 

 tlie hydroxyl hydrogen on carbon 3 to one of the carboxyl 

 oxygens. This hydrogen would come off as hydrogen ion leaving 

 a carbonyl group at carbon 3. The resulting compound would 

 be a /3-keto acid. 



H2CO(P) HsCOCD 



f O ^^C C-OH 



CO2 + HC-OH »► C-OH *■ 



HC-OH HC-OH 



H2C-0(P) H2G-0(P) 



H.C-OCP) H2C-0(P) 



^^C--C-OH -O2C-C-OH 



\ ^C^OH ^ C=0+H+ 



' HC-OH HC-OH 



H2CO® H2C-0(P) 



Once formed, the j8-keto acid appears to undergo "acid 

 splitting" into two molecules of PGA. This is analogous to the 

 splitting of acetoacetate to two molecules of acetate. "Ketone 



39 



