30 The Nature of Biological Diversity 









FIG. 6. Frozen dried spinach chloroplast sonicate; 880 A diameter PSL (poly- 

 styrene latex) markers. 



Chlorophyll structure 



The actual detailed structure of the one molecule unequivocally 

 associated with the capture of light and its transformation, i.e., chloro- 

 phyll, is shown in Fig. 7. This shows the structure of some of the 

 different kinds of chlorophyll that are known: The first is proto- 

 chlorophyll, which appears in etiolated plants grown in the dark. 

 When such plants are placed in the light, the protochlorophyll is 

 converted to chlorophyll. The principal difference between proto- 

 chlorophyll and chlorophyll is the addition of two extra hydrogen 

 atoms at the double bond in ring D. Bacteriochlorophyll is the mole- 

 cule which is responsible for the capture and conversion of light in 

 the purple and green bacteria; it differs from green plant chlorophyll 

 in having a second, dihydropyrrole ring in it. 



We must devise some way of making those ordered chloroplast 

 structures which were seen in Figs. 4, 5, and 6, and we must envisage 

 some way of evolving this particular molecule, chlorophyll, belonging 

 to the general class of tetrapyrrollic substances known as porphyrins. 

 These two things — ordered array within the cells and the development 

 of chlorophyll itself — are two essential features of our evolutionary 

 scheme for the process of photosynthesis. 



The structural feature, the appearance of order and structure, is 

 something common to the evolution of all living organisms, and be- 

 longs to the general discussion of how ordered structures may be 

 evolved from nonliving materials. This is really part of the problem 

 of the origin of life. 



