902 



LIOHT AND LIFE 



T^hodopsin 



//gM 



Vitamin Af + Opsin -^ 



n 



rei'inene reductase 



DP A/- Hz 



ViiaminAi from 



pigmerri epithelium 



and circulation 



L um i- r hod op s in 

 Mefa- rhodo ps i n 



Ret/nenef ■*- Opsin 



Fig. 13. Diagram of the rhodopsin system. 



The vitamin A molecule has four double bonds in its side-chain, 



and each of these might theoretically be cis or trans m configuration 



(Fig. 14). Actually, only the cis forms at positions 9 and 1.8 are 



CH, H CH, H CH3 



I I I ! I 



.C^ /C^ 8 /C^ 10 /C^ 12 /C^ 14 /CH2OH 



II2C* 6C 



H 



H 



H 



II2C 3 2 1 C-CH3 



CH3 



H 



II 



H2C 



IT2C 



H2 



CFI3 



1 



\cVS\o 



I 



H 



II 



I 



CII3 



I 



II 



/^\r/^\c/^\rA\r/ 



H 



II 



C— CH3 



II 



.C 



, CII3 

 H, 



CH3 



\r 



II 



CII2OII 



&\\-trans Vitamin .V 



Retinenc 



ll-r/,s {neo-h) 



Fig. II. Geometric isomers of \itaTnin .\ and retinenc. .\ similar series of struc- 

 tures, differing onlv in possessing an extra double bond in tiie '5,4 position, repre- 

 sents vitamin A, and retinencj. The upper structure represents the unhindered, 

 M-lrans configuration. The lowermost structure is the hindered cis isomer from 

 vvliith all known visual pigments are derived. 



