20 : 6/ Photosynthesis 



373 



light photons absorbed by pigments other than chlorophyll are effective 

 in photosynthesis. 



(a). Carotenoid structure 



CH 3 CH 3 



H 2 



H 2 



Carotenoid Chain 

 -CH 3 ' 



H, 



(b). R and R' for ft-carotene 



Figure 8. Carotenoid structure. Different carotenoids have different R and R' 

 groups and exist as various cis and trans isomers. The all-trans isomer is illus- 

 trated. For /^-carotene, both R and R' have form shown in (b). 



Chlorophylls, when extracted, fluoresce very strongly, and each type 

 exhibits a characteristic fluorescence spectrum. Whole green cells also 

 fluoresce, always exhibiting the chlorophyll a fluorescence spectrum. 

 This is true no matter what wavelength is used to illuminate the cells. 

 Thus, all light energy absorbed which can be used in photosynthesis is in 

 some fashion coupled to the chlorophyll a molecules. How this occurs is 

 uncertain; the details of the light reaction are discussed in the next 

 section. 



(Chlorophyll a fluoresces much less in the bound state in the chloro- 

 plast than it does in extracts. In this sense, it is similar to flavin groups 

 which also lose most of their fluorescence upon binding to protein. 

 Pyridine nucleotides, however, do not fluoresce appreciably when free 

 but do so when bound to protein. No general interpretation for these 

 changes in fluorescence exists.) 



6. The Light Reaction 



Of the three parts of photosynthesis — photodissociation of water, con- 

 version of carbon dioxide to hexose, and photosynthetic phosphorylation 

 — the first is unique. Just how it occurs is not fully understood. Clearly, 



