Visual pigment contained in the outer segments of the rod cells of the retina and involved in scotopic vision. When light stimulates the retina, the chromophore of the pigment molecule '11-
cis' retinal (which is vitamin A aldehyde) isomerizes to 'all-
trans' retinal. This leads to other chemical transformations which carry on even in the absence of light. The first stage is
prelumirhodopsin, then
lumirhodopsin and finally
metarhodopsin (of which there are two types). This last transformation may lead to the breakdown of the molecule into
retinal and
opsin. The molecule is regenerated by recombining retinal and opsin with some enzymes. The absorption spectrum of rhodopsin has a maximum around 498 nm. The isomerization from '11-
cis' to 'all-
trans' also gives rise to the process of
transduction in which the membrane potential covering the pigment molecules in the outer segment changes towards a hyperpolarization of the cell. This is the first step in the nervous response to a light stimulation of the retina.
Syn. visual purple (not used any more); erythropsin.
See dark adaptation;
bleaching;
receptor potential;
absorption spectrum;
transduction.