534 LEO E. LIPETZ 



ized and must contain local concentrations of potential energy for such a 

 sequence of events to occur. 



High energy radiation can act on the visual system to produce the same 

 sensation as light (Lipetz, 1955b), and the evidence is consistent with the 

 idea that radiation does so by the same mechanisms as does light (Lipetz, 

 1953, 1955a) . X-rays can evoke a sensation of light with a dose as low as 

 0.5 mr (Bornschein et al., 1953; Pape and Zakovsky, 1954). 



At each structural level of the visual system there are examples of energy 

 transfers which would be most improbable were it not for the organization 

 at that level. At the molecular level, there is strong evidence (Granit, 1947) 

 that scotopic (dim light ) vision is initiated by the absorption of photons by 

 a single kind of molecule, rhodopsin (visual purple). This molecule consists 

 of a lipoprotein, opsin, to which is attached a pigment group (chromophore) 

 called retinene, which is the aldehyde of vitamin A. Only one stereoisomer 

 of retinene, the 11-cis, a bent chain form, can unite with the opsin to form 

 rhodopsin. When a photon of sufficient energy is absorbed by a rhodopsin 

 molecule, the retinene changes to a different stereoisomer, the all-trans, or 

 straight chain form (Kropf and Hubbard, 1958). This is the primary action 

 of light on the visual system. 



It appears that this action occurs only if the photon is absorbed in the 

 retinene, not in the opsin, and only if the retinene receives at least 1.8 ev 

 energy. If the photon absorbed by the retinene has energy less than 1.8 ev, 

 it is still possible that thermal energy sufficient to bring the total to 1.8 

 ev would be transferred from the opsin to the retinene and cause the 

 retinene to change its shape. In this instance, an irreversible intramolecular 

 energy transfer has resulted from the particular organization of the rhodop- 

 sin molecule (St. George, 1952). 



A mechanism has been suggested (Kropf and Hubbard, 1958) by which 

 the primary action of light, this stereoisomeric change in the retinene, can 

 lead to further changes in the visual system. It is postulated that the 11-cis 

 isomer of retinene is the only one that can combine with opsin to form 

 rhodopsin because that isomer has a shape that permits its entire length to 

 fit closely to a surface of the opsin group, close enough so that interatomic 

 bonds can join the retinene to the opsin at several points. When the ab- 

 sorbed photon causes the retinene to change its shape, sections of the 

 retinene lift away from the opsin, breaking the bonds to opsin along that 

 section. This frees broken bond sites on both the retinene and opsin, allowing 

 them to react with other molecules. 



In support of this mechanism are the findings that the stereoisomeric 

 change of the rhodopsin chromophore by light will occur even at low tem- 

 peratures and without water, but that further changes in the rhodopsin 

 (such as bleaching) and further activation of the visual system occur only 



