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
chromoproteins, 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
Chlorophyll is any of several related green pigments found in cyanobacteria and in the chloroplasts of algae and plants. Its name is derived from the Greek words (, "pale green") and (, "leaf"). Chlorophyll allows plants to absorb energy ...
,
carotenoids and
phycobilins.
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
chloroplast
A chloroplast () is a type of membrane-bound organelle, organelle known as a plastid that conducts photosynthesis mostly in plant cell, plant and algae, algal cells. Chloroplasts have a high concentration of chlorophyll pigments which captur ...
s).
In
chloroplast
A chloroplast () is a type of membrane-bound organelle, organelle known as a plastid that conducts photosynthesis mostly in plant cell, plant and algae, algal cells. Chloroplasts have a high concentration of chlorophyll pigments which captur ...
s, 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
Redox ( , , reduction–oxidation or oxidation–reduction) is a type of chemical reaction in which the oxidation states of the reactants change. Oxidation is the loss of electrons or an increase in the oxidation state, while reduction is t ...
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)
Photopigments of the vertebrate retina
In medical terminology, the term photopigment is applied to
opsin
Animal opsins are G-protein-coupled receptors and a group of proteins made light-sensitive via a chromophore, typically retinal. When bound to retinal, opsins become retinylidene proteins, but are usually still called opsins regardless. Most pro ...
-type photoreceptor proteins, specifically
rhodopsin and
photopsins, the photoreceptor proteins in the retinal
rods and
cones of vertebrates that are responsible for
visual perception
Visual perception is the ability to detect light and use it to form an image of the surrounding Biophysical environment, environment. Photodetection without image formation is classified as ''light sensing''. In most vertebrates, visual percept ...
, but also
melanopsin and others.
[{{cite book , last1=Williams , title=Photoreceptor Cell Biology and Inherited Retinal Degenerations , date=2004 , publisher=World Scientific , pages=89–145]
See also
*
Biological pigment
References
Molecular biology
Pigments
Photosynthetic pigments
Sensory receptors