736 



LIGHT AND LIFE 



scribed, by the light reaction. Similarly, it is sometimes stated on 

 the basis of the observations just described, that the quantimi effi- 

 ciency of bleaching is only about i/g. In fact, the efficiency of the 

 first quanta absorbed, which convert the visual pigment to the lumi- 

 pigment — may be 1. When the over-all efficiency of bleaching is less 

 than 1, it is largely because the absorption of subsequent quanta 

 undoes to a degree the work of the first quanta. 



All these processes are summarized in the following diagram: 



REACTIONS INVOLVED IN BLEACHING RHODOPSIN: 



REACTION 



1. photochemical 



stereolsomerizatlon 

 of chromophore 

 (reversible) 



2, thermal re- 

 arrangement of 

 opsin 



3. hydrolysis of 

 chromophoru 

 from opsin 



PRODUCT 



RHODOPSIN (ll-cis) 



30L0R 

 red 



LUKIRHODOPSIN (all-trans) 



METARHODOPSIN (all-trans) 



RETINENE (all-trans) + 

 OPSIN 



red to violet 



orange to red 



yellow 



It seems clear that — so far as we yet know — the only action of light 

 on visual pigments or in vision is to isomerize retinene. It is not 

 quite right to say that light bleaches visual pigments: what it does 

 is to isomerize retinene. Bleaching is only a secondary consequence 

 of the isomerization, and occurs only to the degree that the irradiation 

 produces isomers that do not serve as the chromophores of photo- 

 sensitive pigments. 



Carotenoids and Photoreception 



Some years ago I attempted to trace the widespread use of caro- 

 tenoids in photoreceptor systems, beginning with the systems that 

 govern phototropism in molds and higher plants, and ending with 

 the role of the haplo-carotenoids (vitamins A, retinenes) in vision 

 (41). 



We have reached a point at which one may ask why the carotenoids 

 have been selected for this role. I would suggest that it is because of 



