28 PHOTOSYNTHESIS 



Etiolated seedlings showing a yellow colour due to caro- 

 tenoid pigments contain also the pigment protochlorophyll 

 which is converted to chlorophyll in the light. Protochloro- 

 phyll is more oxidized than chlorophyll, the two hydrogen 

 atoms in positions 7 and 8 being absent. 



Bacteriochlorophyll is the green pigment present in the 

 purple photosynthetic bacteria. It is similar to chlorophyll a 

 but has the vinyl group — CH^CHg oxidized to — CO — CH3 

 in position 2 and double bonds in two of the pyrrole rings 

 hydrogenated, not one as in chlorophyll. The main absorp- 

 tion band is in the infra-red ^7,700 A in methanol) with 

 another band at about 6,000 A. The green photosynthetic 

 bacteria contain the pigment bacterioviridin, whose struc- 

 ture may be intermediate between that of chlorophyll a 

 and bacteriochlorophyll. The absorption spectrum is similar 

 to that of chlorophyll a, 



THE CAROTENOIDS 



Carotenoids occur in all green plants in the plastids, but 

 additional members of this group of compounds are also 

 found in the non-green parts of plants, such as the perianth 

 and brightly coloured fruits, and also widely throughout the 

 animal kingdom. A carotenoid of much importance in 

 photochemical reactions in animals is visual purple. The 

 original identification of two carotenoid compounds in green 

 leaves, xanthophyll and carotene, has now been extended to 

 the chromatographic recognition of a number of different 

 isomeric forms. A brief list of carotenoids with their distri- 

 bution is given in Table 3.3. It is by no means complete. 

 Chromatographic analysis has revealed the presence, in, for 

 example, the higher plants, of a wide variety of members of 

 the class of xanthophylls, in some cases the relative amount 

 of some minor constituent being shown to be controlled by 

 a genetic factor. In the algae taxonomic groups may be de- 

 fined by the presence of a particular carotenoid pigment. 

 Carotenoid pigments have been found in mitochondria in 

 both plants and animals, and it is possible that a minute 

 quantity of carotenoid pigment' is universal in the cytoplasm 

 of aerobic organisms. 



