24 The Nature of Biological Diversity 



electron located in an as yet unknown place. It is an important recog- 

 nition that this phenomenon occurs and occurs very quickly after the 

 chlorophyll absorbs the light.* The excited chlorophyll in some way 

 is able to extract an electron from the ferrous iron compound, at 

 present associated with the chlorophyll in modern organisms in the 

 form of cytochrome, to produce the ferricytochrome and an electron 

 in some molecules as yet undesignated. 19 - 4 This appears to be an im- 

 portant connection between a molecule that is unique to photosyn- 

 thetic plants, namely, chlorophyll, and certain kinds of molecules 

 which are not unique to photosynthetic plants, namely, the iron 

 cytochromes (iron hemes). The iron hemes have universal distribu- 

 tion and this is an important fact to remember. 



In addition, we now know that electrons must ultimately find their 

 way to pyridine nucleotide, which picks up a proton from the water, 

 making reduced pyridine nucleotide. The oxidized iron, or something 

 close to it, will eventually take electrons from water, giving rise to the 

 ferrous iron and molecular oxygen. 



At the same time that all these things are happening, somewhere 

 along the line, either on the way from the intermediate oxidant to 

 oxygen (reaction 1, Fig. 3), or on the way from the intermediate 

 reductant to the pyridine nucleotide (reaction 2, Fig. 3), or, perhaps, 

 in a recombination reaction in which the electron falls back into the 

 hole (reaction 3, Fig. 3), we also create adenosine triphosphate. The 

 ATP is designated by ~P?, which represents "high-energy phos- 

 phate" linkages. The reactions in Fig. 3 indicate possibilities only, 

 and not knowledge of three different ways (places) in which pyro- 

 phosphate could be created: (1) The fall of the intermediate oxidant 

 toward oxygen; (2) the fall of the intermediate reductant (perhaps 

 a sulfhydryl group) to the pyridine nucleotide which would, per- 

 haps, give rise to pyrophosphate; or (3 ) perhaps the energy of recom- 

 bination of the hydrogen-hydroxyl (electron-hole), which could also 

 give rise to a number of pyrophosphate linkages. 



2. Photoinduced Dehydration 



A more profound departure from the basic redox primary photo 

 process is possible, particularly in the light of the recently indicated 2o 



* A recent modification 14 of the van Niel generalization inserts a ferrocyto- 

 chrome ahead of the water molecule as the primary electron donor to the excited 

 chlorophyll, hut it does not specify the primary fate of the excited electron which 

 must be removed from chlorophyll. The oxidized cytochrome is presumed capable 

 of oxidizing water to oxygen, with the concomitant formation of ATP. a suggestion 

 similar to that of Bassham 23 and corresponding to reaction 1 in Fig. 3. 



