The Origin of Life 1 1 



variation involving small changes in the porphyrins led eventually 

 to the construction of chlorophyll and the invention of photo- 

 synthesis. In addition, Granick believes that all the colored com- 

 pounds in the sequence that leads to chlorophyll might have had the 

 same function as chlorophyll. In the early stages, metallic ions, 

 present as constituents of minerals, might have served to catalvze 

 the same reactions that they now catalvze as metalloenzvmes. 



Photosynthesis is the result of a complex scries of reactions. Some 

 of these can take place in the dark, whereas others can occur only 

 with illumination. Most of the many reactions usually included under 

 the rubric photosynthesis, in the broad sense, are actually dark reac- 

 tions, involving the addition of CO2 to — C — C — C — chains. These 

 dark reactions can be carried out by most cells. It seems likely that 

 many of these reactions evolved independently, perhaps earlier than 



Random synthesis from C, compounds by radiation 



■ Succinic acid 



CO2H 

 I 



CHj 

 I 



CH2 

 1 



CO;H 



CHj— CO,H 



/ 

 NH2 



"^^ Glycine 



CO2H 



CH2 

 C = 



CO2H 



CO2H CH2 



■CO, 



CH-CO.H 



/ 



NH2 



a-NH2-j3-Keto- 

 adipic acid 



I 



H2C-0=C 

 I I 



/C,, /CH2 

 NH,— CH, II "N 

 ^0 H2 



8-Amino- 

 levulinic acid 



Fe 

 Porphyrins 



n-steps 



Possible points for catalytic function of Fe 



CH2 



CH H CH3 

 C 



HC {Fe-. CH 



2 H 



CHj 

 I 



CO2H 



CHj 

 CHj 

 COjH 



Protoporphyrin No. 9 



Fig. 1.2 I Possible steps in the synthesis of porphyrin compounds from 

 molecules produced randomly under the influence of radiation. Iron 

 may act as a catalyst at the points marked by arrows but is a much 

 better catalyst when combined with porphyrin. Thus the production ot 

 protoporphyrin 9 is facilitated by the presence of protoporphyrin 9. 

 {From Calvin, 1959, Evolution 13.) 



