In the eye, specialized photoreceptor cells of the retina, called rod cells are primarily responsible for low-light vision, with relatively little color detection. Rod cell outer segments contain lamellar protein disks rich in the protein opsin (Figure 19.26). Oxidation and isomerization of all-trans-retinol yields an intermediate, 11-cis retinal, which is important in photoreception. The chemistry of photoreception is shown in Figure 19.27 (see here also) and summarized as follows.

1. 11-cis-retinal is linked to opsin via a Schiff's base to form rhodopsin.

2. Absorption of light by the retinal portion of the complex isomerizes the cis-bond in 11-cis retinal to a trans-bond, forming an all-trans compound called bathorhodopsin.

3. Release of a proton yields metarhodopsin II

4. Hydrolysis yields opsin and all-trans retinal.

5. Retinal isomerase converts all-trans retinal to 11-cis retinal.

At step 3 above, bathorhodopsin (activated form of rhodopsin) can activate transducin so that it binds GTP. The transducin-GTP complex can bind to a specific phosphodiesterase that cleaves cyclic GMP to GMP. This, in turn, stimulates a cascade of events that generates a visual signal to the brain.