EFFECT OF CATALYST POISONS 



1911 



Since vitamin K is known to occur in green plant cells, and to have a re- 

 dox potential (about 0.025 volt) suitable for a Hill oxidant, Wessels sug- 

 gested that this vitamin is the intermediate (often designated in this 

 monograph as X) , to which hydrogen is primarily transferred from chloro- 

 phyll by light (in photosynthesis as well as in Hill reaction). Wessels 

 further suggested that vitamin K (which, like chlorophyll, contains a phy- 

 tyl side chain) may be associated with chlorophyll in monolayers on protein- 

 lipide interfaces (cf. chapter 3 7 A) in the proportion — made plausible by 



1.00 



0.75 



m 



X 



- 0.50 



u. 



O 



o 



UJ 



Q 



0.25 



^ I0-' 10" 



PARTIAL PRESSURE OF Og (atm.) 



10* 



Fig. .37D.9. Inhibition of photosynthesis by excess oxygen at different 

 carbon dioxide concentrations (after Taniiya and Huzisige 1949). 



vitamin K assays (4 X 10 ~^ g. per g. dry matter in spinach, as compared 

 to about 10~^ g. chlorophyll)— of one molecule vitamin K per several hun- 

 dred molecules of chlorophyll (making it eligible to serve as Franck's 

 limiting "catalyst B"). Partial reoxidation of photochemically reduced 

 vitamin K by cytochrome (c or /) could produce high energy phosphates 

 needed as "boosters" to permit the reduction of pyridine nucleotide by 

 other molecules of reduced vitamin K. 



The stabilizing influence of Cl~ and Br~ ions on chloroplasts exposed to 

 light (mentioned in chapter 35, section B3(c)) can be related, in this pic- 

 ture, to a similar effect observed in photochemical destruction of vitamin K. 



This hypothesis is mentioned here because Wessel's paper was not avail- 



