in the rod cells of the eye, 11-cis-retinal is converted into rhodopsin, a light-sensitive substance formen from the protiein opsin and 11-cis-retinal. when light strikes the rod cells, isomerization of the c11-c12 double bond occurs and trans-rhodopsin, called metarhodopsin ii, is produced.
this cis-trans isomerization of rhodopsin is accompanied by a change in molecular geometry, which in turn causes a nerve impulse to be sent to the brain whrer it is perceived as vision. (in the absence of light, the cis-trans isomerization takes appoximately 1100 years; in the presence of light, it occurs within 2 x 10 -11 seconds!)