to perform essentially the same functions as TPNH and ATP 

 and would presumably be about as effective as carriers of 

 chemical potential. In all discussions of the role of TPNH 

 and ATP, the possibility of their replacement by as-yet- 

 unidentified cofactors should be kept in mind. 



For the purpose of discussion, let us consider the photo- 

 synthesis of carbon compounds as an isolated set of reactions. 

 The principal substrates for this set of reactions are CO2, 

 hydrogen (as TPNH), phosphate (as ATP), and NH4 + . The 

 ammonium ion may be contained in the plant nutrient or 

 it may be derived from the reduction of nitrate. If nitrate 

 reduction is the source of NH4 + , the energy for the reduc- 

 tion must also come from the light, at least indirectly. Other 

 probable inorganic substrates for photosynthesis of organic 

 compounds include sulfate, magnesium ion, and a number 

 of trace elements. Many of these are required for growth in 

 plants but may or may not be incorporated in organic com- 

 pounds by photosynthesis. 



