The Retinal molecule - rotatable in 3 dimensions
Retinal is a derivative of 
retinol - vitamin A.
The alcohol group ([CH]OH) is converted into an
aldehyde (CHO) group. Retinal exists in two forms called 11-cis and all-trans.
In the light-sensitive cells of the eye, 11-cis retinal becomes covalently attached to a protein section enbedded in the membrane, and this combination is called an opsin.
There are several sorts of these proteins which react in different ways. For example in a cone cell there will be one of three types of photopsin which broadly respond to red, green and blue light. In rod cells rhodopsin is found.
After this the all trans retinal detaches from the protein and is recycled into retinol. In the epithelium layer behind the retina, it is reactivated by a number of molecular changes, then passed back into the cells of the retina.
retinol - vitamin A.
The alcohol group ([CH]OH) is converted into an
aldehyde (CHO) group. Retinal exists in two forms called 11-cis and all-trans.
In the light-sensitive cells of the eye, 11-cis retinal becomes covalently attached to a protein section enbedded in the membrane, and this combination is called an opsin.
There are several sorts of these proteins which react in different ways. For example in a cone cell there will be one of three types of photopsin which broadly respond to red, green and blue light. In rod cells rhodopsin is found.
Photoisomerisation - click to see the molecule flip
When it absorbs energy from a photon of light, the retinal section changes its molecular structure from the 11-cis form to the all trans form. This generates an electrochemical impulse.After this the all trans retinal detaches from the protein and is recycled into retinol. In the epithelium layer behind the retina, it is reactivated by a number of molecular changes, then passed back into the cells of the retina.