Biosynthesis: 



Carboxylic Acid Reducing 



acid anhydride Acyl derivative agent 



o o 



/ /■ TPNH 



(4) R— C + ADP— O— PO3H I > R— C > 



\ \ enzyme 



O- OPO3H- 



> ADP 



Aldehyde Acid 



O 



R— C + HOPO3H- 



\ 



o- 



Among the many other reactions of ATP in biosynthesis, one, 

 which is of considerable importance in photosynthesis, is the 

 formation of sugar phosphates from sugars: 



(5) H+ + ROH + ADP— O— POgH" -> 



R— OPO3H- + ADP + H2O 



The only known reactions of the carbon reduction cycle in 

 photosynthesis which would require the use of TPNH and 

 ATP are of the type shown in Eqs. (4) and (5). These re- 

 actions are the means by which chemical potential, derived 

 from the absorbed light, is used to bring about the reduction 

 and transformation of carbon from CO2 to organic com- 

 pounds. 



These two cofactors, ATP and TPNH, are at present 

 the only ones that are known to be generated by the light 

 reactions of photosynthesis and at the same time seem to be 

 required for steps in the carbon reduction cycle. The possi- 

 bility remains, however, that there are other energetic or 

 reduced cofactors acting as carriers of hydrogen and energy 

 from the light reactions to the carbon reduction cycle. Such 

 unknown cofactors might substitute for or replace TPNH 

 or ATP. They could, in fact, be more effective than the 

 known cofactors, particularly in vivo, where they might well 

 be built into the highly organized structure of the chloro- 

 plast. If such unknown cofactors do exist, they would have 



