SOME ASPECTS OF BIOCHEMICAL DIVERSITY 



291 



the porphyropsin system. Porphyropsin, like rhodopsin, is a carotenoid- 

 protein complex and is purple in colour. Its spectrum resembles that of 

 rhodopsin, but with a maximum at 522m/^. On exposure to light a sub- 

 stance having properties similar to rhodopsin is liberated; it is called 

 retinene II. In chloroform it has an absorption maximum at 405m/x. In the 

 retina it is converted simultaneously to porphyropsin and to vitamin Ag. 



II. PORPHYRINS 



We have described the biosynthesis (p. 238) of porphyrins from 8- 

 aminole\ailinic acid via porphobilinogen. On to this basic process are 

 superimposed a considerable number of variations. By insertion of iron into 

 the protoporphyrin nucleus we obtain w^hat Granick has called the "iron 

 branch" of the biosynthetic chain (p. Ill), and a number of other variants 

 have been described in these pages. Protoporphyrin is also the starting 

 point for the biosynthesis of chlorophyll (the "magnesium branch" of 

 Granick) as well as for the biosynthesis of haem. 



A cell capable of photosynthesis (see p. 354) contains at least one 

 chlorophyll and at least one yellow pigment. In addition it often contains a 

 phycobilin. The chief pigment in photos}^nthesis, both in algae and in the 

 higher plants, is chlorophyll a. In the photosynthetic bacteria, on the other 

 hand, we find a different chlorophyll, bacteriochlorophyll (p. 122). Thus 

 whilst in green plants the chloroplasts contain chlorophyll a and chloro- 

 phyll b, the algae are much more variable and we find in them a number of 

 combinations .a -\- b, a -\- c, a -\- d, a + e. In addition we sometimes find 

 a phycobilin. The phycobilins are soluble in water and are proteins com- 

 bined with a chromophore belonging to the class of bile pigments. The 

 phycoerythrins are predominant in the red algae and the phycocyanins in 

 the blue-green algae. 



The chromophore of the phycoerythrins, phycoerythrobihn, is identical 

 wuth mesobilierythrin the formula of which is 



ME MP P M ME 



HO H 



CH, H 



CH 



N 



CH N OH 



(M = methyl group E = ethyl group P = propyl group) 

 The chromophore of the phycocyanins is mesobiliviolin 



MEMP PMME 



/\ /\C/\N/\ /\ /\C//\N\\ 

 HO N H H CH 



N 



H, 



H OH 



Various phycoerythrins are found in algae which differ in the structure of 

 the protein moiety. 2-Phycoerythrin is the most common and is found in the 



