20 : 5/ Photosynthesis 



371 



all photosynthetic cells. The phycobilins are pigments found in some 

 algae and bacteria. In some fashion, all of these pigments act together 

 so that light photons absorbed by any of them are equally effective for 

 photosynthesis. (Some carotenoid pigments occur outside of chloro- 

 plasts and are completely ineffective for photosynthesis.) 



All chlorophylls contain a porphyrin structure, similar to that of the 

 heme groups discussed in Chapter 18. An atom of magnesium rather 

 than iron is found in chlorophyll. Attached to the hydrophilic por- 

 phyrin ring is a long hydrocarbon side chain which is soluble in lipids. 

 (Thus, chlorophyll should be a detergent!) All green plants have a type 

 of chlorophyll called chlorophyll a and most also have another, namely, 

 chlorophyll b. The two can be converted from one to the other in 

 extracts, but no such change has ever been demonstrated in whole 



H,C 



H,c 



-CH, 



H H H H H H H 



Q C C C C C G C CH 3 



V VVVVtVV 



H | H I H | H A 



CH 3 CH, / \ 



CH 3 H 



CH 3 



Figure 6. Structure of chlorophyll a. Chlorophyll £ differs in having a carboxyl 

 O 



group — C in place of the starred — CH 3 . 



\ 



OH 



