Word: rhodopsin
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Scientists have known for a long time that the photochemical action in the rods is connected in some way with a red substance, rhodopsin, which forms in the rods when the light gets dim. This is how eyes become "dark-adapted." Only when their rods are well fortified with rhodopsin can they respond to faint glimmers of light...
...find out how rhodopsin works, Biologist Wald extracted a protein called opsin from the eyes of freshly killed animals and mixed it with vitamin A and two enzymes (organic catalysts): alcohol oxidase (from horse livers) and cozymase (from yeast). When this mixture is placed in the dark, the enzymes convert the vitamin A to retinene, a yellow pigment. Then the retinene combines with the opsin to form bright red rhodopsin...
When the mixture is exposed to light, even very faint light, all the chemical processes go into reverse. The rhodopsin divides into retinene and opsin. The retinene reverts to vitamin A. This is just what happens when light shines into a dark-adapted eye: the rhodopsin in the rods is suddenly decomposed...
...Wald is not quite sure how the destruction of rhodopsin by light produces nerve impulses that the brain interprets as vision. But since the impulses are electrical, he suspects that they may be started by some electrical consequence of the destruction of the rhodopsin. One possibility: that sulfhydryl (sulphur and hydrogen) groups, are exposed when the rhodopsin molecules disintegrate. These are lively chemicals with the necessary punch to start an electrical commotion...
...second paper explains how the rhodopsin transfers the chemical reaction to the optic nerve which carries the impression to the brain. The response of the nerve is known to lead not only to sensations of sight, but also to electrical changes...