THE PHOTORECEPTOR PROCESS IN VISION 



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• Cyanopsm absorption 



Spectral sensiiivity. 

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FIG. 12. The absorption spectrum of cyanopsin compared with Granit's electrophysiological 

 measurements of the spectral sensitivity of cone vision in a fresh-water fish, the tench, and in the 

 European tortoise, Testudo graeca. [From Wald et a/. (71).] 



the outer segment of a frog rod, or about 60 per cent 

 of the nonlipid dry weight. In cattle rods, it accounts 

 for about 14 per cent of the dry weight of the outer 

 segment, or about 22 per cent of the nonlipid dry 

 weight (30). The outer segments of the rods and 

 cones are layered structures composed of several 

 hundred to several thousand layers, apparently of 

 protein, each about 40 to 160 A thick (52). The 

 membranes of the rod must be made in large part of 

 rhodopsin (or porphyropsin). A cone has much the 

 same construction, though in some cones the visual 

 pigments may compose a smaller fraction of the 

 membranes (65). 



Two model systems have been described in which 

 the bleaching of rhodopsin in solution registers 

 directly as an electrical fluctuation (45, 68). Both are 

 based on the fact that light exposes ion-binding 

 groups on opsin, sulfhydryl groups in one case, an 

 acid-binding group with pK 6.6 in the other, which 

 aflTect the ion concentration in the medium. These 

 models show that rhodopsin has the capacity to 

 translate the absorption of a quantum of light into 

 an electrical event. The eflfective utilization of this 

 capacity depends entirely upon the structural frame- 

 work within which it occurs. A dark-adapted rod is 

 stimulated by the absorption of a single quantum of 

 light (6, 29, 44). The same probably is true of a 

 dark-adapted cone. One quantum of light is absorbed 



by one molecule of visual pigment, and a rod or cone 

 is so peculiarly constructed that so small a change 

 can excite it. 



PHYSIOLOGICAL CORREL.ATIONS 



Every physiological function, normal and path- 

 ological, has its roots in biochemistry; conversely 

 every facet of biochemistry finds expression in the 

 properties and behavior of the organism. In a sense 

 the organism is a macroscopic representation of 

 certain of its component molecules, and one of the 

 principal tasks of physiology is to learn to read its 

 features in their features. 



This is nowhere plainer than in \ision. The re- 

 actions initiated by light in the rods and cones in- 

 troduce a long train of nervous and s\naptic proc- 

 esses which end in visual sensations. The primary 

 events have been described in some detail. The 

 visual apparatus as a whole is largely concerned 

 with conducting the information they dictate. For 

 this reason many of the basic properties of vision 

 reflect simply and directly the properties of retinal 

 molecules. 



It is of the highest importance to explore lhe.se 

 relationships. Needless to say, there is much more in 

 vision than photochemistry, or indeed than any of 



