Photopigments are unstable pigments that undergo a chemical change when they absorb light. The term is generally applied to the non-protein chromophore moiety of photosensitive , such as the pigments involved in photosynthesis and photoreception. In medical terminology, "photopigment" commonly refers to the photoreceptor proteins of the retina.
Photosynthetic pigments
Photosynthetic pigments convert light into biochemical energy. Examples for photosynthetic pigments are
chlorophyll,
and
.
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
of the thylakoid membranes of plant
).
In
, the light-driven electron transfer chain in turn drives the pumping of protons across the membrane.
Photoreceptor pigments
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:
Photopigments of the vertebrate retina
In medical terminology, the term photopigment is applied to
opsin-type photoreceptor proteins, specifically
rhodopsin and
, the photoreceptor proteins in the retinal
Rod cell and
Cone cell of vertebrates that are responsible for visual perception, but also
melanopsin and others.
See also
