In
vertebrates,
rhodopsin, sometimes also known as
visual purple, is a pigment of the
retina, which is responsible for the first events in the perception of light. A so-called "7TM"
G-protein coupled receptor, each protein binds as a
cofactor a molecule of
retinaldehyde[?], which derives from
Vitamin A and is made in the retina.
Isomerization[?] of retinaldehyde by
light induces a conformational change in rhodopsin and a
biochemical cascade[?] by way of the associated
G protein. The rhodopsins encoded by different genes differ in the wavelengths of light which they absorb most strongly. This variation causes the
cones of the retina to differ in their response and is the basis for
color vision[?]. The rhodopsin of the
rods most strongly absorbs purple light, which is why it is some times called "visual purple", even though it appears reddish. Some bacteria express a related protein called
bacteriorhodopsin[?] to carry out
bacterial photosynthesis[?].