PHOTOSYNTHESIS 

 INTERMEDIATE TRANSFER SYSTEMS 



We have already anticipated, in the discussion of both the 

 carbon reduction cycle and the light reaction, some of the re- 

 actions involved in the transfer of reducing power and energy 

 from the light reaction to carbon reduction and in the evolution 

 of oxygen from whatever products are formed in the breaking of 

 the O — H bond. Let us assume, for the time being, that the 

 theories above, which require that tlie absorption of a quantum 

 for each electron taken from water (whether by quantum con- 

 version by thioctic acid or by transfer of an electron by the 

 chlorophyll aggregate from water to reducing agent) are correct. 

 Then there is ample energy in four quanta of 6800 A light (168 

 kcal./mole) to bring about the photolysis of two water molecules 

 and the formation of two molecules of reducing agent of strength 

 equal to TPNH: 



2[H20 -f TPN+ = 1/2 Oo + TPNH + H+] AF - +103 kcal 



The entire excess of energy, some 65 kcal. minus whatever 

 was lost in the primary absorption and conversion processes, 

 will be available for the evolution of oxygen from the inter- 

 mediates formed from the oxidation of water. It is possible that 

 some of this energy might be used in the formation of some ATP 

 from ADP and inorganic phosphate. Whether or not this occurs 

 is very important in the evaluation of possible quantum require- 

 ments. We have already arrived at a quantum requirement of 

 two molecules of TPNH and three molecules of ATP for each 

 molecule of carbon dioxide reduced. If one molecule of the 

 primary reducing agent must be oxidized in order to form two or 

 three molecules of the required ATP by some reaction similar 

 to that which couples the energy of the oxidation of DPNH to 

 the formation of ATP (38), tlien the total requirement for 

 equivalents of reducing agent will be seven or six, requiring 

 seven or six quanta (4). If all the ATP molecules could be 

 supplied from the energy left over from the evolution of oxygen 

 as suggested above, then the quantum requirement will obviously 

 be only four. 



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