26 



The Nature of Biological Diversity 



would be a rather recent evolutionary addition to an already highly 

 developed biosynthetic energy-manipulating system. 



C=C-OH 







II 

 + HO-P-OH 



I 

 0~ 



hv 



II 



I I 

 H-C-C=0 



OH 



H-C-C 



/ 



T>P0 3 H 2 



H 2 



> c=c 



ADP 



T)PO,H, 



> C=C-OH + ATP 



I 



IT 

 I I 

 HC-C=0 



I 



Pyrophosphate Linkage 



in Yon phot osnnthetic Processes 



The appearance of pyrophosphate linkage in a variety of organ- 

 isms is well known. In practically all organisms, there are mechanisms 

 for producing ATP which do not involve photosynthetic mechanisms 

 at all. One of them is a reversal of one reaction in which ATP is used 

 in the photosynthetic cycle (triose phosphate dehydrogenase). By 

 running the reaction backward (Step 2, Fig. 1) one can make ATP. 

 A more important source is a reaction which apparently involves 

 iron — the cytochromes, involving also the oxidation and reduction 

 of the pyridine nucleotide. The two reactions together are involved in 

 the creation of ATP in nonphotosynthetic organisms. This process of 

 the oxidation of pyridine nucleotide by the passage of electrons from 

 pyridine nucleotide back to oxygen through the iron cytochromes, 

 with the concomitant formation of ATP, is known as oxidative phos- 

 phorylation. It leads to the creation of more ATP than does the sub- 

 strate oxidation process. The return of a photoexcited electron of 

 chlorophyll through all or part of a similar chain could produce the 

 necessary ATP (see Fig. 3) . 



Thus the creation of both the reduced pyridine nucleotide and the 

 ATP are not unique to photosynthetic processes. These processes also 

 occur in nonphotosynthetic organisms. 6 We know something about 



