20 The Nature of Biological Diversity 



We recognize also that all of the 11 enzymes that are involved in 

 these transformations in the carbon reduction cycle are to he found 

 nearly everywhere widely distributed in the biological world — not 

 limited solely to organisms which are converting solar energy, but 

 also in organisms that have nothing whatever to do with the photo- 

 synthetic process. It therefore seems quite clear that at least this 

 sequence, that is, the carbon reduction sequence, undoubtedly evolved 

 in a separate chain of evolutionary events having little or nothing to 

 do in the early stages with the electromagnetic energy conversion 

 process itself. 4 The electromagnetic energy conversion process appears 

 to produce in a primary act, or very close to it, two materials, a reduc- 

 ing agent and a pyrophosphate linkage, which can then run the carbon 

 reduction cycle. 



We can already see the two quite independent evolutionary streams 

 which were joined only very recently in evolutionary history to pro- 

 duce the modern green plant. 5,6,7 The carbon reduction system was 

 one independent stream. These streams will, of course, break up into 

 finer parts as we go along, but this is our beginning. 



B. tyuantMim conversion in photosynthesis 



Let us now return to the photochemical process itself. Having sepa- 

 rated out the carbon reduction system as a distinct evolutionary 

 stream, I am going to leave it since there is nothing unique about it 

 for photosynthetic organisms except the combination of the product 

 of the light reaction with a certain collection of enzymes, all of which 

 can be found, either separately or in various combinations, in non- 

 photosynthetic organisms. 89 Therefore, the carbon reduction cycle 

 had a separate evolutionary history until the recent times. 



Let us now see what more we can say about the quantum conversion 

 process in photosynthesis. We can say a good deal about it, although 

 not nearly as much as we can about the carbon reduction process. We 

 do not have anywhere near the detailed knowledge of the quantum 

 conversion process that we do of the carbon reduction process. Figure 

 2 represents the structural formulas of the two molecules which we 

 believe to be essential for running the photosynthetic carbon reduc- 

 tion cycle. (There are undoubtedly others of which we are still un- 

 aware required for oxygen evolution as well. ) To run the carbon 

 cycle we need the reducing agent, which is a pyridine nucleotide in 

 its reduced form. An adenine and pyridine moiety are tied together 

 by two ribose sugars and a pyrophosphate link to give the molecule 

 known as diphosphopyridine nucleotide. Actually, in photosynthesis 



