Iodopsin

Photoreceptors are sensory cells that are sensitive to light. In the human retina, they are called rods and cones according to their shapes. Both types of cells contain light-sensitive molecules called visual pigments whose primary function is to absorb light. The rods contain rhodopsin (visual purple), which is composed of chromophore (a variant of vitamin A) and a protein (opsin). The cones contain iodopsin, which is made up of the same chromophore as in rhodopsin but with a different protein. ³

Color vision, a synthesis of red (577 nm), green (540 nm), and blue (440 nm) light, is induced by changes within the pigments of different types of cones during a photochemical process in which coded nerve impulses are sent to the brain. This constitutes the structural basis for trichromatic vision. Birds have tetrachromatic vision with an extra retinal pigment in the ultraviolet range. ²

The differences in absorption spectra are due to differences in amino acid sequence of the opsin component

The color sensitive pigments of the cones are combinations of protopsins (cone proteins) and retinal. Three different types of photochemicals are present in different cones selectively. These photochemicals are called blue sensitive pigment, green sensitive pigment, and red sensitive pigment.

References:
1. Textbook of veterinary physiological chemistry. Larry Rex Engelking
2. Atlas of the sensory organs: functional and clinical anatomy. András Laszlo Csillag
3. Anatomy & Physiology Super Review

Primary Concepts

Iodopsin - a photopigment in the retinal cones of the eyes of most vertebrates; plays a role in daylight vision.

Broader Concepts

Photopigment - a special pigment found in the rods and cones of the retina