The pigments in photoreceptor proteins either change their conformation or undergo photoreduction when they absorb a photon. This change in the conformation or redox state of the chromophore then affects the protein conformation or activity and triggers a signal transduction cascade.
Examples of photoreceptor pigments include:
*retinal (in rhodopsin)
*flavin (in cryptochrome)
*bilin (in phytochrome)