Light-harvesting complex
Encyclopedia
A light-harvesting complex is a complex of subunit proteins that may be part of a larger supercomplex of a photosystem
, the functional unit in photosynthesis
. It is used by plant
s and photosynthetic bacteria
to collect more of the incoming light than would be captured by the photosynthetic reaction center alone. Light-harvesting complexes are found in a wide variety among the different photosynthetic species. The complexes consist of proteins and photosynthetic pigment
s and surround a photosynthetic reaction center to focus energy, attained from photons absorbed by the pigment
, toward the reaction center using resonance energy transfer.
matches that of an electronic transition. The fate of such excitation can be a return to the ground state or another electronic state of the same molecule. When the excited molecule has a nearby neighbour molecule, the excitation energy may also be transferred, through electromagnetic interactions, from one molecule to another. This process is called resonance energy transfer, and the rate depends strongly on the distance between the energy donor and energy acceptor molecules. Light-harvesting complexes have their pigments specifically positioned to optimize these rates.
Light-harvesting complexes are located around the reaction center and add additional pigments, carotenoids, and chlorophyll
s to funnel absorbed energy to the special pair via resonance energy transfer. Carotenoid
s serve a secondary function, suppressing damaging photochemical reactions, in particular those including oxygen, which can be induced by bright sunlight. Laboratory tests have shown that plants lacking carotenoids quickly die on exposure to light and oxygen.
s and carotenoids are important in light-harvesting complexes present in plants. Chlorophyll b is almost identical to chlorophyll a except it has a formyl group in place of a methyl group
. This small difference makes chlorophyll b absorb light
with wavelengths between 400 and 500 nm more efficiently. Carotenoids are long linear organic molecules
that have alternating single and double bonds along their length. Such molecules are called polyenes. Two examples of carotenoids are lycopene
and β-carotene
. These molecules also absorb light most efficiently in the 400 – 500 nm range.
Due to their absorption region, carotenoids appear red and yellow and provide most of the red and yellow colours present in fruits and flowers.
The carotenoid molecules also serve a safeguarding function. Carotenoid molecules suppress damaging photochemical reactions, in particular those including oxygen
, which exposure to sunlight can cause. Plants that lack carotenoid molecules quickly die upon exposure to oxygen and light.
Little light reaches algae that reside at a depth of 1 meter or more in seawater, as light is absorbed by seawater. A phycobilisome
is a light-harvesting protein complex present in cyanobacteria, glaucocystophyta, and red algae
and is structured like a real antenna. The pigments that are present in the phycobilisome, such as phycocyanobilin
and phycoerythrobilin
, enhance the amount and spectral window of light absorption.
The geometrical arrangement of a phycobilisome is very elegant and results in 95% efficiency of energy transfer. There is a central core of allophycocyanin
, which sits above a photosynthetic reaction center. There are phycocyanin
and phycoerythrin
subunits that radiate out from this center like thin tubes. This increases the surface area of the absorbing section and helps focus and concentrate light energy down into the reaction center. The energy transfer from exited electrons absorbed by pigments in the phycoercythrin subunits at the periphery of these antennas appears at the reaction center in less than 100 ps.
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...
, the functional unit 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...
. It is used by plant
Plant
Plants are living organisms belonging to the kingdom Plantae. Precise definitions of the kingdom vary, but as the term is used here, plants include familiar organisms such as trees, flowers, herbs, bushes, grasses, vines, ferns, mosses, and green algae. The group is also called green plants or...
s and photosynthetic bacteria
Anoxygenic photosynthesis
Phototrophy is the process by which organisms trap light energy and store it as chemical energy in the form of ATP. There are three major types of phototrophy: Oxygenic photosynthesis, Anoxygenic photosynthesis, and Rhodopsin-based phototrophy...
to collect more of the incoming light than would be captured by the photosynthetic reaction center alone. Light-harvesting complexes are found in a wide variety among the different photosynthetic species. The complexes consist of proteins and 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 :...
s and surround a photosynthetic reaction center to focus energy, attained from photons absorbed by the pigment
Pigment
A pigment is a material that changes the color of reflected or transmitted light as the result of wavelength-selective absorption. This physical process differs from fluorescence, phosphorescence, and other forms of luminescence, in which a material emits light.Many materials selectively absorb...
, toward the reaction center using resonance energy transfer.
The function of a light-harvesting complex
Absorption of a photon by a molecule can lead to electronic excitation when the energy of the captured photonPhoton
In physics, a photon is an elementary particle, the quantum of the electromagnetic interaction and the basic unit of light and all other forms of electromagnetic radiation. It is also the force carrier for the electromagnetic force...
matches that of an electronic transition. The fate of such excitation can be a return to the ground state or another electronic state of the same molecule. When the excited molecule has a nearby neighbour molecule, the excitation energy may also be transferred, through electromagnetic interactions, from one molecule to another. This process is called resonance energy transfer, and the rate depends strongly on the distance between the energy donor and energy acceptor molecules. Light-harvesting complexes have their pigments specifically positioned to optimize these rates.
Light-harvesting complexes are located around the reaction center and add additional pigments, carotenoids, and 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...
s to funnel absorbed energy to the special pair via resonance energy transfer. 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 serve a secondary function, suppressing damaging photochemical reactions, in particular those including oxygen, which can be induced by bright sunlight. Laboratory tests have shown that plants lacking carotenoids quickly die on exposure to light and oxygen.
Light-harvesting complexes in plants
ChlorophyllChlorophyll
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...
s and carotenoids are important in light-harvesting complexes present in plants. Chlorophyll b is almost identical to chlorophyll a except it has a formyl group in place of a methyl group
Methyl group
Methyl group is a functional group derived from methane, containing one carbon atom bonded to three hydrogen atoms —CH3. The group is often abbreviated Me. Such hydrocarbon groups occur in many organic compounds. The methyl group can be found in three forms: anion, cation and radical. The anion...
. This small difference makes chlorophyll b absorb light
Light
Light or visible light is electromagnetic radiation that is visible to the human eye, and is responsible for the sense of sight. Visible light has wavelength in a range from about 380 nanometres to about 740 nm, with a frequency range of about 405 THz to 790 THz...
with wavelengths between 400 and 500 nm more efficiently. Carotenoids are long linear organic molecules
Organic chemistry
Organic chemistry is a subdiscipline within chemistry involving the scientific study of the structure, properties, composition, reactions, and preparation of carbon-based compounds, hydrocarbons, and their derivatives...
that have alternating single and double bonds along their length. Such molecules are called polyenes. Two examples of carotenoids are lycopene
Lycopene
Lycopene is a bright red carotene and carotenoid pigment and phytochemical found in tomatoes and other red fruits and vegetables, such as red carrots, watermelons and papayas...
and β-carotene
Carotene
The term carotene is used for several related unsaturated hydrocarbon substances having the formula C40Hx, which are synthesized by plants but cannot be made by animals. Carotene is an orange photosynthetic pigment important for photosynthesis. Carotenes are all coloured to the human eye...
. These molecules also absorb light most efficiently in the 400 – 500 nm range.
Due to their absorption region, carotenoids appear red and yellow and provide most of the red and yellow colours present in fruits and flowers.
The carotenoid molecules also serve a safeguarding function. Carotenoid molecules suppress damaging photochemical reactions, in particular those including oxygen
Oxygen
Oxygen is the element with atomic number 8 and represented by the symbol O. Its name derives from the Greek roots ὀξύς and -γενής , because at the time of naming, it was mistakenly thought that all acids required oxygen in their composition...
, which exposure to sunlight can cause. Plants that lack carotenoid molecules quickly die upon exposure to oxygen and light.
Phycobilisome
Phycobilisome
Phycobilisomes are light harvesting antennae of photosystem II in cyanobacteria, red algae and glaucophytes.- General structure :Phycobilisomes are protein complexes anchored to thylakoid membranes. They are made of stacks of chromophorylated proteins, the phycobiliproteins, and their associated...
is a light-harvesting protein complex present in cyanobacteria, glaucocystophyta, and red algae
Red algae
The red algae are one of the oldest groups of eukaryotic algae, and also one of the largest, with about 5,000–6,000 species of mostly multicellular, marine algae, including many notable seaweeds...
and is structured like a real antenna. The pigments that are present in the phycobilisome, such as phycocyanobilin
Phycocyanobilin
Phycocyanobilin is a blue phycobilin, i.e., a tetrapyrrole chromophore found in cyanobacteria and in the chloroplasts of red algae, glaucophytes, and some cryptomonads. Phycocyanobilin is present only in the phycobiliproteins allophycocyanin and phycocyanin, of which it is the terminal acceptor of...
and phycoerythrobilin
Phycoerythrobilin
Phycoerythrobilin is a red phycobilin, i.e. an open tetrapyrrole chromophore found in cyanobacteria and in the chloroplasts of red algae, glaucophytes and some cryptomonads. Phycoerythrobilin is present in the phycobiliprotein phycoerythrin, of which it is the terminal acceptor of energy. The...
, enhance the amount and spectral window of light absorption.
The geometrical arrangement of a phycobilisome is very elegant and results in 95% efficiency of energy transfer. There is a central core of allophycocyanin
Allophycocyanin
Allophycocyanin is a protein from the light-harvesting phycobiliprotein family, along with phycocyanin, phycoerythrin and phycoerythrocyanin. It is an accessory pigment to chlorophyll...
, which sits above a photosynthetic reaction center. There are phycocyanin
Phycocyanin
Phycocyanin is a pigment from the light-harvesting phycobiliprotein family, along with allophycocyanin and phycoerythrin. It is an accessory pigment to chlorophyll. All phycobiliproteins are water-soluble and therefore cannot exist within the membrane as do carotenoids, but aggregate forming...
and phycoerythrin
Phycoerythrin
Phycoerythrin is a red protein from the light-harvesting phycobiliprotein family, present in cyanobacteria, red algae and cryptomonads.Like all phycobiliproteins, phycoerythrin is composed of a protein part, organised in a hexameric structure of alpha and beta chains, covalently binding...
subunits that radiate out from this center like thin tubes. This increases the surface area of the absorbing section and helps focus and concentrate light energy down into the reaction center. The energy transfer from exited electrons absorbed by pigments in the phycoercythrin subunits at the periphery of these antennas appears at the reaction center in less than 100 ps.
See also
- PhotosynthesisPhotosynthesisPhotosynthesis 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...
- Photosynthetic reaction center
- Photosystem II light-harvesting protein
External links
- Calculated spatial positions of light harvesting complexes in membrane- http://www.life.illinois.edu/govindjee/photoweb - Photosynthesis and all sub categories