PHOTOSYNTHESIS 



quiring one quantum of light energy. This corresponds to the 

 well-known four-quantum theory in the primary step, since the 

 transfer of four electrons, requiring four quanta, are required to 

 form one molecule of O2. 



Reactions between a single molecule of chlorophyll, water, 

 and hydrogen or electron acceptor are difficult to formulate, 

 since one is faced in that case with the necessity either of absorb- 

 ing two consecutive quanta in one chlorophyll molecule in order 

 to have enough energy to form oxygen and reducing agents of the 

 strength of TPNH (about 51 kcal.) or else of forming oxygen and 

 a reducing agent of insufficient strength which could be used in 

 subsequent reactions involving dismutations of energy to form 

 better reducing agents. Another alternative is that a single 

 molecule of chlorophyll, activated by one quantum, reacts with 

 some activated, hydrated compound in which the O — H bond 

 could be more readily broken, and thus has left over enough 

 energy to form a good reducing agent. It may well be that 

 water is in fact incorporated into some compound to weaken 

 the O— H bond before the O — H bond is broken regardless of 

 the mechanism of electron transfer, but it is doubtful if this 

 activation is sufficient to permit the formation of oxygen and 

 reducing power of the strength of TPNH at the same time by 

 one quantum. 



Recent proposals involving reactions in which one molecule 

 of chlorophyll provides both electrons in a given H2O photolysis 

 include the one by Levitt (39), who suggests that a chlorophyll 

 molecule, excited by one quatum of light, gives up an electron 

 to thioctic acid forming oxidized chlorophyll, after which the 

 molecule of oxidized chlorophyll absorbs a second quantum 

 and transfers a second electron. The resulting dipositive chloro- 

 phyll then reacts with water to produce hydrogen ion and oxygen. 

 In the meantime, the thioctic acid has been reduced by the two 

 electrons to the dithiol. 



On the other hand, Wessels (64) has proposed a one quan- 

 tum per two electrons reaction in which a much weaker reducing 

 agent, reduced vitamin K, is produced. This reducing agent is 



53 



