Photopigments are unstable pigments that undergo a chemical change when they absorb light. The term is generally applied to the non-protein chromophore

Chromophore

A chromophore is the part of a molecule responsible for its color. The color arises when a molecule absorbs certain wavelengths of visible light and transmits or reflects others. The chromophore is a region in the molecule where the energy difference between two different molecular orbitals falls...

 moiety of photosensitive chromoprotein

Chromoprotein

Chromoprotein: A conjugated protein, sulch as hemoglobin, that contains a pigmented prosthetic group, such as hem in hemoglobin . Other examples of chromoprotein are: myoglobin, cytochromes and flavoproteins.In hemoglobin:...

s, such as the pigments involved in photosynthesis

Photosynthesis

Photosynthesis is a chemical process that converts carbon dioxide into organic compounds, especially sugars, using the energy from sunlight. Photosynthesis occurs in plants, algae, and many species of bacteria, but not in archaea. Photosynthetic organisms are called photoautotrophs, since they can...

 and photoreception. In medical terminology, "photopigment" commonly refers to the photoreceptor protein

Photoreceptor protein

Photoreceptors are light-sensitive proteins involved in the sensing and response to light in a variety of organisms. Some examples are rhodopsin in the photoreceptor cells of the vertebrate retina, phytochrome in plants, and bacteriorhodopsin and bacteriophytochromes in some bacteria...

s of the retina

Retina

The vertebrate retina is a light-sensitive tissue lining the inner surface of the eye. The optics of the eye create an image of the visual world on the retina, which serves much the same function as the film in a camera. Light striking the retina initiates a cascade of chemical and electrical...

.

Photosynthetic pigments

Photosynthetic pigment

Photosynthetic pigment

A photosynthetic pigment is a pigment that is present in chloroplasts or photosynthetic bacteria and captures the light energy necessary for photosynthesis.- Plants :...

 (converting light into biochemical energy). Examples for photosynthetic pigments are chlorophyll

Chlorophyll

Chlorophyll is a green pigment found in almost all plants, algae, and cyanobacteria. Its name is derived from the Greek words χλωρος, chloros and φύλλον, phyllon . Chlorophyll is an extremely important biomolecule, critical in photosynthesis, which allows plants to obtain energy from light...

, carotenoid

Carotenoid

Carotenoids are tetraterpenoid organic pigments that are naturally occurring in the chloroplasts and chromoplasts of plants and some other photosynthetic organisms like algae, some bacteria, and some types of fungus. Carotenoids can be synthesized fats and other basic organic metabolic building...

s and phycobilin

Phycobilin

Phycobilins are chromophores found in cyanobacteria and in the chloroplasts of red algae, glaucophytes and some cryptomonads...

s. 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

Biological membrane

A biological membrane or biomembrane is an enclosing or separatingmembrane that acts as a selective barrier, within or around a cell. It consists of a lipid bilayer with embedded proteins that may constitute close to 50% of membrane content...

 (e.g. in the case of the proton pump bacteriorhodopsin

Bacteriorhodopsin

Bacteriorhodopsin is a protein used by Archaea, the most notable one being Halobacteria. It acts as a proton pump; that is, it captures light energy and uses it to move protons across the membrane out of the cell...

) or via excitation and transfer of electrons released by photolysis (e.g. in the photosystem

Photosystem

Photosystems are functional and structural units of protein complexes involved in photosynthesis that together carry out the primary photochemistry of photosynthesis: the absorption of light and the transfer of energy and electrons...

s of the thylakoid membranes of plant chloroplast

Chloroplast

Chloroplasts are organelles found in plant cells and other eukaryotic organisms that conduct photosynthesis. Chloroplasts capture light energy to conserve free energy in the form of ATP and reduce NADP to NADPH through a complex set of processes called photosynthesis.Chloroplasts are green...

s). In chloroplast

Chloroplast

Chloroplasts are organelles found in plant cells and other eukaryotic organisms that conduct photosynthesis. Chloroplasts capture light energy to conserve free energy in the form of ATP and reduce NADP to NADPH through a complex set of processes called photosynthesis.Chloroplasts are green...

s, the light-driven electron transfer chain in turn drives the pumping of protons across the membrane.

Photoreceptor pigments

The pigments in photoreceptor protein

Photoreceptor protein

Photoreceptors are light-sensitive proteins involved in the sensing and response to light in a variety of organisms. Some examples are rhodopsin in the photoreceptor cells of the vertebrate retina, phytochrome in plants, and bacteriorhodopsin and bacteriophytochromes in some bacteria...

s either change their conformation

Conformational isomerism

In chemistry, conformational isomerism is a form of stereoisomerism in which the isomers can be interconverted exclusively by rotations about formally single bonds...

 or undergo photoreduction when they absorb a photon. This change in the conformation or redox

Redox

Redox reactions describe all chemical reactions in which atoms have their oxidation state changed....

 state of the chromophore then affects the protein conformation or activity and triggers a signal transduction

Signal transduction

Signal transduction occurs when an extracellular signaling molecule activates a cell surface receptor. In turn, this receptor alters intracellular molecules creating a response...

 cascade. Examples for photoreceptor pigments include retinal

Retinal

Retinal, also called retinaldehyde or vitamin A aldehyde, is one of the many forms of vitamin A . Retinal is a polyene chromophore, and bound to proteins called opsins, is the chemical basis of animal vision...

 (for example in rhodopsin

Rhodopsin

Rhodopsin, also known as visual purple, is a biological pigment of the retina that is responsible for both the formation of the photoreceptor cells and the first events in the perception of light. Rhodopsins belong to the G-protein coupled receptor family and are extremely sensitive to light,...

), flavin (for example in cryptochrome

Cryptochrome

Cryptochromes are a class of blue light-sensitive flavoproteins found in plants and animals. Cryptochromes are involved in the circadian rhythms of plants and animals, and in the sensing of magnetic fields in a number of species...

), and bilin

Bilin (biochemistry)

Bilins, bilanes or bile pigments are biological pigments formed in many organisms as a metabolic product of certain porphyrins. Bilin was named as a bile pigment of mammals, but can also be found in lower vertebrates, invertebrates, as well as red algae, green plants and cyanobacteria...

 (for example in phytochrome

Phytochrome

Phytochrome is a photoreceptor, a pigment that plants use to detect light. It is sensitive to light in the red and far-red region of the visible spectrum. Many flowering plants use it to regulate the time of flowering based on the length of day and night and to set circadian rhythms...

).

Photopigments of the vertebrate retina

In medical terminology, the term photopigment is applied to opsin

Opsin

Opsins are a group of light-sensitive 35–55 kDa membrane-bound G protein-coupled receptors of the retinylidene protein family found in photoreceptor cells of the retina. Five classical groups of opsins are involved in vision, mediating the conversion of a photon of light into an electrochemical...

-type photoreceptor proteins, specifically rhodopsin

Rhodopsin

Rhodopsin, also known as visual purple, is a biological pigment of the retina that is responsible for both the formation of the photoreceptor cells and the first events in the perception of light. Rhodopsins belong to the G-protein coupled receptor family and are extremely sensitive to light,...

 and photopsin

Photopsin

Photopsins are the photoreceptor proteins found in the cone cells of the retina that are the basis of color vision. Photopsins are very close analogs of the visual purple rhodopsin that is used in night vision...

s, the photoreceptor proteins in the retinal rods

Rod cell

Rod cells, or rods, are photoreceptor cells in the retina of the eye that can function in less intense light than can the other type of visual photoreceptor, cone cells. Named for their cylindrical shape, rods are concentrated at the outer edges of the retina and are used in peripheral vision. On...

 and cones

Cone cell

Cone cells, or cones, are photoreceptor cells in the retina of the eye that are responsible for color vision; they function best in relatively bright light, as opposed to rod cells that work better in dim light. If the retina is exposed to an intense visual stimulus, a negative afterimage will be...

 of vertebrates that are responsible for visual perception

Visual perception

Visual perception is the ability to interpret information and surroundings from the effects of visible light reaching the eye. The resulting perception is also known as eyesight, sight, or vision...

, but also melanopsin

Melanopsin

Melanopsin is a photopigment found in specialized photosensitive ganglion cells of the retina that are involved in the regulation of circadian rhythms, pupillary light reflex, and other non-visual responses to light. In structure, melanopsin is an opsin, a retinylidene protein variety of...

and others.