J, A. BASSHAM AND M. CALVIN 



then used in part to produce ATP and in part to produce a 

 reducing agent of the level of TPNH through the expenditure 

 of ATP in a coupled reaction. Since the formation of reduced 

 vitamin K and oxygen from water and vitamin K is said to 

 require 39 kcal., the energy of one quantum would have to be 

 used with nearly 100% efficiency. Such an efficient mechanism 

 will be especially attractive if the very low quantum require- 

 ments reported by Warburg prove correct. The presence of 

 vitamin K in chloroplasts and its concurrent formation with 

 chlorophyll (23) are also favorable to this suggestion. Finally, 

 its oxidation-reduction potential is close to that measured with 

 illuminated chloroplast preparations (50). The mechanism 

 proposed by Wessels for the formation of ATP from reduced 

 vitamin K is reasonable but energetically very inefficient in that 

 only one ATP is formed for each molecule of reducing agent 

 used. No specific mechanism was proposed for the formation 

 of TPNH from TPN+ by the oxidation of reduced vitamin K 

 and the conversion of ATP to ADP and inorganic phosphate, but 

 this reaction would be a bit uphill energetically, since the dif- 

 ference in redox-free energies between TPNH and reduced 

 vitamin K is apparently about 12.5 kcal., slightly more than the 

 10 or 11 kcal. now thought to be available from ATP hydrolysis. 

 Besides this, the proposal of Wessels requires the steps in the 

 liberation of O2 from whatever intermediates may be formed in 

 the reaction of oxidized chlorophyll with water to proceed with 

 virtually no change in free energy. In other words, this proposed 

 mechanism includes one very inefficient step and a number of 

 steps which are nearly 100% efficient in energy transfer. Al- 

 though this is entirely possible, we find it slightly more satisfying, 

 from a thermodyamic viewpoint, to suppose that most of the 

 steps involved in the energy transfer from one system to another 

 proceed efficiently but with a small loss of energy in each, thus 

 providing a smooth driving force throughout the entire process 

 which will not require the enzymes at any stage to cope with 

 infinitesimal concentrations of substrates. 



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