268 C. p. WIIITTINGHAM 



smaller particles which in light showed " photophosphorylation" and 

 in dark oxidative phosphorylation. He suggests that since the same 

 particles now exhibit both types of phosphorylation it is probable that 

 "photophosphorylation" is simply the addition of photolysis and 

 oxidative phosphorylation. The lack of dark oxidative phosphoryl- 

 ation by "whole" chloroplasts might be attributed to inaccessibility 

 of the reaction sj^stem (compare the absence of cytochrome oxidase 

 activity); the nonrequirement for oxygen of photophosphorylation 

 suggests that the oxidized radical resulting from photolysis ("OH") 

 can terminate the oxidative chain resulting in phosphorylation in ab- 

 sence of oxygen. The "OH" radical may react directly with a cyto- 

 chrome. 



THE CHLOROPLAST REACTION IN VIVO 



Two types of mechanism have been suggested for the chloroplast 

 reaction. The first is that electrons are transferred directly to a 

 substance with an oxidation-reduction potential at least as negative 

 as coenzyme. Cahdn and colleagues have suggested that thioctic acid 

 might be the primary electron acceptor (free energy per electron 

 transfer 35 kcal.). The second type of mechanism is that a number of 

 electrons are transferred as the result of absorption of a single quan- 

 tum to a reagent of a potential relatively near that of oxygen. Thus 

 four electrons could be transferred to cytochrome / (free energy per 

 electron transfer 10 kcal.) with the energy from a single quantum. Sub- 

 sequent dark reactions are required to produce a reducing agent by 

 oxidative-reductive coupling until a potential near that of coenzyme 

 is attained. One such chain was proposed by Davenport, Hill, and 

 Whatley (la) and Hill (9) to include cytochrome/, cytochrome be, and 

 coenzyme. Wessels (12) has made the alternative suggestion that 

 one quantum may transfer two electrons to vitamin K (free energy 

 per electron transfer 19.5 kcal.) in the first stage. 



It has been argued from the fact that the difference spectra for 

 photosynthetic organisms indicate oxidation of at least some cyto- 

 chromes in light that the first type of mechanism is more probable. 



Discussion 



Lucile Smith : I was wondering if j'ou have been able to observe any oxidation- 

 reduction effects of cytochrome / on illumination. 



