Two molecules in the retina—vitamin A and a protein named "opsin" that together make "rhodopsin"—capture single light photons. When light strikes the vitamin, it changes shape and becomes the molecule "11-cis-retinal." This in turn changes the rhodopsin's shape. When light activates enough of these molecular switches within the light-sensitive cell, they cause downstream biochemical systems to amplify and send the signal from the retina, through the optic nerve, and to the brain. This complex photochemical reaction is at the heart of what allows eyes to detect light and send signals that the brain can form into meaningful images.
When light strikes vitamin A, the molecule bends at the 11th carbon bond. In other versions, or "isomers," of this chemical, the bend could occur at the 9th, 10th, or 13th carbon atoms. Curious to find out why vertebrate and squid eyes use 11-cis-retinal instead of another isomer, researchers tested the various isomers for light receptivity. They constructed digital molecular models and "analyzed the structure, stability, energetics, and spectroscopy to try to find out what makes 11-cis-retinal nature's preferred isomer," according to a report by PhysOrg.
But did nature really "prefer" this particular vitamin, and did it integrate the vitamin with opsin in order to generate an electrical impulse from light?