CHLOROPLASTS 



249 



XH-CH3 



CH2 



\h-CHj 



CH2 



CH, 



CH2 

 CH2 



o 



Cm 



CH-CH^ 



CH: 



CH2 

 II ^ 



CH 



I 



CH2 



O 

 I 



I CH2 



-CH I 



-C< I II '/^CH3 



)C-N^ N-Cs^ 



HCfi Mg rCH 



C=N N—C. 



/ I \ 



ICH3 



H3C. 



N,/ 



H3C 



HC 



H2 H2 



c-c^ 

 j:h2 



-CH, 



>c^c. 



■ C^ 

 II 

 ■CH 



CH3 



o=c- 



A 



HC^ 



M 

 HC^f^y 



II 

 HC^CH 



CH, 



green colour which distinguishes chlorophyll a by increased absorption 

 in the blue, to a yellowish green shade. 



The head of the chlorophyll molecule has a hydrophilic character owing 

 to the nitrogen atoms of the four pyrrole rings and the co-ordinately bound 

 magnesium. Its long phytol tail, on the other hand, is lipophilic; there is, 

 therefore, in this pigment a clavate molecule with a pronounced lipophilic 

 pole and a lipophobic pole. 



By contrast, the carotenes, 

 which are unsaturated hydro- 

 carbons of the empiric formula 

 C40H56, are completely lipo- 

 phiHc. The xanthophylls, on 

 the contrary, may contain as 

 many as six hydroxyl groups 

 and are therefore not so decid- 

 edly hydrophobic. Whereas it 

 was formerly held that the 

 carotenoids are dissolved in the 

 chloroplastic lipids, Menke 

 (1940c) is of opinion that, like 

 chlorophyll, they combine 

 with protein molecules to 

 form chromoproteins. As far 

 as carotene is concerned, an 

 argument against this hypo- 

 thesis is provided by the fact 

 that, unUke xanthophyll and 

 chlorophyll, this pigment can ch3Ch.^ 

 be extracted by benzene and 

 other organic solvents from 

 dry leaf powder without any 

 preliminary chemical action. 



The structural formula of 

 the ^-carotene contained in the 

 leaves is represented in Fig. 

 127. It is a chain of conjugated 

 double bonds which cause the 



blue absorption and, therefore, the yellow-to-orange colour; it has me- 

 thylic lateral groups and two terminal rings of six members. The consti- 

 tution of /3-carotene is a matter of paramount importance to vitamin 

 research, for the break-down of the double bond occupying a middle 

 position in the chain (shown by -> in Fig. 127) and addition of water 

 produce two molecules of vitamin A (Karrer, 193 5). ^-carotene is optically 

 inactive. In the a-carotene in carrot roots and in palm oil the double bond 

 between the C atoms marked in Fig. 127 5 and 6 in one of the six-membered 



WCv 



,H 



■CH 

 II 



o 



HC 

 II 



HC^QI^ 

 II 



CH3 



"\/y/^V 



H3CS 



CH3 

 AH3 



CH3] 



CH=CH2 " 



HC^ 



hI 



\ yC=C 



H3C^ ^c-c^ 



Chlorophyll a 



fi- 



H2 H2 



Carotene 



Fig. 127. Molecular structure of the pigments in 

 the chloroplast (it should be noted that it is not 

 certain whether the carotenes possess the trans- 

 configuration or the cis-configuration drawn here) . 



