Some examples are rhodopsin in the photoreceptor cells of the vertebrate retina, phytochrome in plants, and bacteriorhodopsin and bacteriophytochromes in some bacteria.
The chromophore reacts to light via photoisomerization or photoreduction, thus initiating a change of the receptor protein which triggers a signal transduction cascade.
Plants use light of different wavelengths as environmental cues to both alter their position and to trigger important developmental transitions.
When a seed germinates underground in the absence of light, its stem rapidly elongates upwards.
The activated photoreceptors cause a change in developmental program; the plant starts producing chlorophyll and switches to photosynthetic growth.