Rhodopsin Information

Rhodopsin is the major protein of the rod outer segment (ROS), a cellular domain that captures light and initiates the visual response. Rhodopsin consists of two building blocks, an opsin protein called scotopsin and a reversibly covalently bound cofactor , retinal (retinaldehyde). Rhodopsin is a transmembrane protein consisting of 7 membrane-spanning helices, that are interconnected by extracellular and cytoplasmic loops. Humans cannot make Rhodopsin, instead they use and external source, b -carotene, that is found in food in order to synthesis it. Light falling on the retina brings about certain chemical changes in the rhodopsin and other substances present in the rods and cones.

When the rhodopsin in the outer segment of the rod is exposed to light and begins to decompose, this decreases the outer segment conductance of sodium to the interior of the rod, even though the sodium ions continue to be pumped out of the inner segment. Rhodopsin of the rods most strongly absorbs green-blue light and therefore appears reddish-purple, which is why it is also called "visual purple". It is responsible for the monochromatic vision in the dark.