The term is generally applied to the non-protein chromophore moiety of photosensitive chromoproteins, such as the pigments involved in photosynthesis and photoreception.
[2] These pigments enter a high-energy state upon absorbing a photon which they can release in the form of chemical energy.
This can occur via light-driven pumping of ions across a biological membrane (e.g. in the case of the proton pump bacteriorhodopsin) or via excitation and transfer of electrons released by photolysis (e.g. in the photosystems of the thylakoid membranes of plant chloroplasts).
[2] In chloroplasts, the light-driven electron transfer chain in turn drives the pumping of protons across the membrane.
[2] The pigments in photoreceptor proteins either change their conformation or undergo photoreduction when they absorb a photon.