150 PROBLEMS OF PHOTOSYNTHESIS 



or in the over-all reaction 



,S /SH 



L<^ I + H2O -^ L< + V2 O2 (c) 



S \SH 



(AG = +51300 cal) 



In the case of vitamin Ki 



HoO ^2H+ + 2e + 1/2 O. (a) 



(AG = +37500 cal) 



vitamin Ki + 2H+ + 2e -»► reduced vitamin Ki (d) 



(AG = +1500 cal) 



or in the over-all reaction 



vitamin Ki + HoO -^ reduced vitamin Ki + V2 O2 (e) 



(AG = +39000 cal) 



For reaction b (lipoic acid) the redox potential is, according to equation 44, 



13800 „ „„^ , 



E = — , . , ■ „ = -0.300 volt 

 46148 



and for reaction d (vitamin Kj) we find 



1500 „„.^ , 



^^ -46148= -0-032 volt 



It follows from these figures, which are not standard values, that, according 

 to the findings of Wessels and Havinga, vitamin Ki should have the preference 

 over lipoic acid. The latter has the same low potential as TPN+ and could 

 therefore not function as the primary hydrogen acceptor. 



Accepting the requirement of one quantum per two electrons in reactions 

 c and e, we find for the efficiencies in red 



,. . .^ 43000 o,^ 

 hpoicacid: -^y^ = ^^/o 



. . ., 43000 ...^ 

 vitamm Ki: ^qqoO = ^^^/o 



Of course, an efficiency of more than 1 00% is impossible as has been pointed 

 out by Bassham and Calvin (9). However, Wessels thinks that the reduc- 

 tion of TPN+ by reduced vitamin Ki proceeds with the aid of energy-rich 

 phosphate bonds produced in a partially back oxidation of reduced vitamin 

 Ki. The denominator will therefore be higher, and give more reasonable 

 figures. Figure 57 shows the various conversions in the vitamin Ki cycle. 



Generally speaking, there are some arguments in favor of the importance of 

 this vitamin, as can be seen from its role in oxidative phosphorylation and its 

 possible function in the respiratory chain. It is a striking fact that vitamin 

 Ki occurs in chloroplasts only and is not present in the cytoplasm. Anti- 

 vitamin-K substances inhibit Hill reactions. The light-sensitive vitamin Ki is 

 protected by CI ions. As Warburg and Luttgens (56) and Arnon and What- 



