212 



MELVIN CALVIN 



We can therefore follow van Niel and reaffirm the suggestion that the photo- 

 chemical apparatus in the green plants is primarily concerned with the splitting 

 of the water molecule and the generation of reducing agent and intermediate 

 oxidant. The latter (intermediate oxidant) will eventually find its way into 

 molecular oxygen. The former (reducing agent) will be used to reduce the com- 

 pounds formed by combination of carbon dioxide with suitable intermediates 

 in the cell. From our knowledge of the nature of the carbon cycle it is already 

 clear that ATP is also required for this latter process, and this ATP may either 

 be generated by oxidative phosphorylation or by a recombination of the 'inter- 

 mediate oxidant' with some 'intermediate reductant' that has been formed by 

 direct photolysis. Thus, we can suppose that the reaction sequence which is 

 now used by green plants for the reduction of carbon dioxide was developed 

 independently of the photosynthetic apparatus, and presumably later coupled 

 to the energy-capturing and transforming mechanism which the photosynthetic 

 apparatus represents. 



CH3 



j N /N— I 



CHz GHz (CH=0 b) 



CH H pHj CH H CHj 



l\ / 

 HO ^Fe° CH > HC 



'V-^c-A^^- cH3H>r ^5- 



ÇH2 H CH2 '^^^CHz M-t C=0 



ÇH2 ÇH2 ÇHz COzCHî 



COzH 



COzCzoHjg 



HAEM (os inHoemoglobin ond 

 Cylochrom) 



CHLOROPHYLL a 



Fe -PROTOPORPHYRIN NO 9 



Fig. 3. Structural relationships between haem and chlorophyll a. 



We must now examine the nature of the photochemical apparatus to see if we 

 can discover something of its origin. The primary energy-trapping molecule is, 

 of course, a porphyrin, chlorophyll. In recent years, the work of Granick has 

 clearly indicated, if not conclusively demonstrated, that present-day synthesis 

 of chlorophyll is a branch in the synthetic sequence leading also to the haems. 

 This branch seems to occur just before the insertion of the iron into the por- 

 phyrin ring (Fig. 3). 



We can thus see that the most likely course for the evolution of the photo- 

 synthetic apparatus involved the prior existence of the porphyrin type of cata- 

 lytic structure. This catalytic structure was evolved not in response to the 

 existence of sunlight, but rather in réponse to the other evolutionary driving 

 force, which we have already seen operative in the development of the por- 

 phyrin nucleus, namely, the oxidative catalysis at which the iron compounds of 

 the porphyrins are so efficient. This leads to the suggestion that the development 

 of the porphyrin either was not directly dependent upon the existence of oxygen 



