When a photon is absorbed by a retinal photopigment, part of the molecule in the outer segment of a receptor, called the chromophore, undergoes a morphological (shape) change called isomerization.The chromophore unbends at the point shown by the short arrows and goes from the 11-cis to the all-trans state. On the left is a simplified version of what actually transpires.
In darkness the molecule returns to the 11-cis state.
A more picturesque representation of the 11-cis retinal molecule
A more picturesque representation of the all-trans retinal molecule.
These molecules are related to vitamin A. On the left is a picture of what happens to the rod photopigment call rhodopsin.
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