PHOTOSYNTHETIC PIGMENTS 39 



solution. In the red and blue-green algae the fluorescence of 

 the phycobilins is also reduced, the band being shifted 

 towards the red. 



In the living cell the fluorescence of chlorophyll has been 

 shown to be independent of the wavelength of the exciting 

 light. Thus in Chlorella the yield of fluorescence changes 

 very little when the wavelength of the exciting light is 

 changed from a region where it is almost completely ab- 

 sorbed by chlorophyll to a region where carotenoids absorb 

 almost 50% of the incident energy. Thus energy must be 

 transferred from excited carotenoids to chlorophyll. Simi- 

 larly it has been shown that in diatoms energy can be trans- 

 ferred from fucoxanthol to chlorophyll, and in the red algae 

 from phycoerythrin to chlorophyll. In the latter case it 

 would appear that more than 90% of the energy is trans- 

 ferred and other experiments in which the efficiency of photo - 

 synthesis in light of different wavelengths was measured 

 (Chapter 4) confirm this result. 



BIOSYNTHESIS OF TETRAPYRROLIC PIGMENTS 



In classical work on the biosynthesis of haemoglobin by 

 Shemin, Rittenburg, and others, it was shown how the 

 pyrrol rings of the porphyrin originated from the amino- 

 acid glycine and a four-carbon compound. An immediate 

 precursor of porphyrins is the pyrrol derivative porpho- 

 bilinogen whose structure was elucidated by Cookson and 



'active' 

 succinate 

 COOH COOH COOH 



CH2 CH2 COOH CH, COOH 



1 II I " I 



rcH, 



1 



CO— 



zi 



CH<> CHo CH2 CHo 



r + ir -^1 I " 



CO CHo C-^^ C 



CH2NH2 



I 

 LCOOH 



CH2 COCH2NH, CH CCH2NH2 



NH2 NH 



glycine S-aminolaevulinic acid porphobilinogen 



FIG. 3.7 



