Evolution of Photosynthetic Mechanisms 23 



theory, at least in chemical terms, is the creation of a reductant of 

 some general character, whose nature we do not know, and of an oxi- 

 dant, also of some general character whose nature we do not know as 

 yet. These two things are both very closely associated with and must 

 ultimately come from water, as given by the stoichiometry of the 

 primary reaction of photosynthesis in the first place. 



In more recent years, still another terminology has entered into 

 this discussion and it comes from quite a different source. The physi- 

 cist has called the reductant the "electron" and what is left after you 



Ch + hv 



^ Ch* 



Ch*+ M 



Or 



(CH 2 0), 



FIG. 3. Simplified photosynthesis scheme. The quantum is first absorbed by the 

 chlorophyll molecule: then something happens (p for primary) to the excited 

 chlorophyll to produce two chemical species, [O] and [Rl, for example, which can 

 go on, one of them [OJ to become molecular oxygen in some way (1). and the 

 other one [Rl leading to the reduction of CO- to carbohydrate (2). Along these 

 two routes, various other energy-containing species may be created (ATP or ~P). 

 ATP would be an energy-storage product. This may be created on either, or both, 

 sides. There may be back reaction (3) between the oxidants and reductants, which 

 also could create products of higher energy. The obvious one here is, of course, 

 the pyrophosphate linkage in ATP. 



take an electron away from a molecule is called a "hole/' 1 ' These 

 are the physicists' terms for the same phenomenon. You must not 

 become confused about the terminology because all of these — oxidant- 

 reductant, hydroxyl-hydrogen, electron-hole — are different names for 

 essentially the same thing. What we are trying to do now is to dis- 

 cover exactly the best way to describe these things in ultimate and 

 intimate detail. 



I introduce the terminology of the physicist because in the lasl few 

 years we have learned a number of the reactions of excited chloro- 

 phyll and one of them is an electron transfer reaction which is ob- 

 servable spectroscopically. An electron is transferred from iron in tin- 

 divalent state to give iron in the trivalent state, 18,19,2 21,22 with the 



