ABSTRACT
The fact that all-trans-retinal is liberated as a consequence of bleaching requires that a physiological mechanism be in place for the regeneration of 11-cis-retinal. Otherwise vision would have the character of a once-in-a-lifetime event. The fact that vertebrate vision operates via a bleaching mechanism is presumably related to the fact that bleaching plays a role in visual adaptation (11). Visual adaptation refers to the ability of the visual system to alter its sensitivity to match the strength of the ambient signal, and this ability is importantly dependent on the amount of rhodopsin present in the rods at low light intensities. The immense sensitivity of human vision, which can operate at the single-photon level (12), is partly a reflection of the exceedingly high concentration of rhodopsin in the rod outer segments (which can be as high as 109 rhodopsin molecules/rod) (13). To decrease the inherent sensitivity of this system at high light intensities requires that the amount of pigment be modulated by the bleachingregeneration cycle.
