22 The Nature of Biological Diversity 



minal pyrophosphate link (Fig. 21. These are the two molecules that 

 are required in order to move the cycle around, and clearly these 

 must he manufactured as a result of the photochemical transforma- 

 tion. 



How much do we know about how the photochemical transforma- 

 tion manufactures those two substances? Here, we are not so thor- 

 oughly informed, but a good deal, nevertheless, is known and some 

 of it is of considerable importance in guiding our thinking as to what 

 the evolutionary relationships between the photosynthetic equipment 

 and other equipment of living organisms might be. 



I. Photoinduced Redox System 



The principal photochemical reaction we now know is, first, the 

 absorption of light by chlorophyll to produce some kind of an excited 

 chlorophyll, either a molecule or molecular aggregate. (I don't mean 

 this to be a separate chlorophyll molecule in solution, but simply the 

 chlorophyll as it exists in the photosynthetic equipment of the 

 organisms.) This electronically excited molecule must then undergo 

 some kind of transformation; for example, it may react with another 

 molecule or molecules to produce a separation of an oxidant from a 

 reductant. 1112 I am using this language first because of a bit of con- 

 fusion that has arisen in the meaning of these terms. The oxidant will 

 eventually become molecular oxygen; the reductant will eventually 

 become a reduced compound, pyridine nucleotide. The pyridine 

 nucleotide, together with the ATP, for which we have not yet de- 

 scribed a formation mechanism, will then go on to drive the carbon 

 cycle. 



Oxidant and reductant are the chemists' terms for what happens 

 after the excited chlorophyll loses its energy to some molecule, or 

 collection of molecules, if any redox system is directly involved. The 

 biologist has been accustomed to writing these two things in different 

 terms. Following van Niel, 13 " 16 he has generally associated the term M 

 (Fig. 3) with water and has called the oxidant ( [0] in Fig. 3) hy- 

 droxyl, or [OH], but he has been very careful to put a bracket around 

 it. (Those of you who know what the meaning of a bracket is will un- 

 derstand the significance of this; when you see a biochemist putting a 

 bracket around something of this sort it means that he doesn't really 

 know what he is talking about. It is a general representation, not a 

 chemical formula.) The reductant ( [R] in Fig. 3), according to the 

 biologists, has been called [H| (hydrogen) , and this had led many to 

 suppose that the primary process of quantum conversion involves the 

 splitting of the water molecule itself. What is meant by the van Niel 



