light-harvesting complexes
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A light-harvesting complex consists of a number of chromophores which are complex
subunit protein In structural biology, a protein subunit is a polypeptide chain or single protein molecule that assembles (or "''coassembles''") with others to form a protein complex. Large assemblies of proteins such as viruses often use a small number of ty ...
s that may be part of a larger super complex of a photosystem, the functional unit in
photosynthesis Photosynthesis is a process used by plants and other organisms to convert light energy into chemical energy that, through cellular respiration, can later be released to fuel the organism's activities. Some of this chemical energy is stored i ...
. It is used by
plant Plants are predominantly photosynthetic eukaryotes of the kingdom Plantae. Historically, the plant kingdom encompassed all living things that were not animals, and included algae and fungi; however, all current definitions of Plantae exclu ...
s and photosynthetic bacteria to collect more of the incoming light than would be captured by the photosynthetic reaction center alone. The light which is captured by the chromophores is capable of exciting molecules from their ground state to a higher energy state, known as the excited state. This excited state does not last very long and is known to be short-lived. Light-harvesting complexes are found in a wide variety among the different photosynthetic species, with no homology among the major groups. The complexes consist of proteins and
photosynthetic pigment A photosynthetic pigment (accessory pigment; chloroplast pigment; antenna pigment) is a pigment that is present in chloroplasts or photosynthetic bacteria and captures the light energy necessary for photosynthesis. List of photosynthetic pigme ...
s and surround a photosynthetic reaction center to focus energy, attained from photons absorbed by the
pigment A pigment is a colored material that is completely or nearly insoluble in water. In contrast, dyes are typically soluble, at least at some stage in their use. Generally dyes are often organic compounds whereas pigments are often inorganic compou ...
, toward the reaction center using
Förster resonance energy transfer Förster resonance energy transfer (FRET), fluorescence resonance energy transfer, resonance energy transfer (RET) or electronic energy transfer (EET) is a mechanism describing energy transfer between two light-sensitive molecules (chromophores). ...
.


Function

Photosynthesis is a process where light is absorbed or harvested by pigment protein complexes which are able to turn sunlight into energy. Absorption of a photon by a molecule takes place when pigment protein complexes harvest sunlight leading to electronic excitation delivered to the reaction centre where the process of charge separation can take place. when the energy of the captured
photon A photon () is an elementary particle that is a quantum of the electromagnetic field, including electromagnetic radiation such as light and radio waves, and the force carrier for the electromagnetic force. Photons are massless, so they a ...
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. Before an excited photon can transition back to ground state, the energy needs to be harvested. This excitation is transferred among chromophores where it is delivered to the reaction centre. Light-harvesting complexes have their pigments specifically positioned to optimize these rates.


In purple bacteria

Purple bacteria is a type of photosynthetic organism with a light harvesting complex consisting of two pigment protein complexes referred to as LH1 and LH2. Within the photosynthetic membrane, these two complexes differ in terms of their arrangement. The LH1 complexes surrounds the reaction centre, while the LH2 complexes are arranged around the LH1 complexes and the reaction centre in a peripheral fashion. Purple bacteria use
bacteriochlorophyll Bacteriochlorophylls (BChl) are photosynthetic pigments that occur in various phototrophic bacteria. They were discovered by C. B. van Niel in 1932. They are related to chlorophylls, which are the primary pigments in plants, algae, and cyanoba ...
and carotenoids to gather light energy. These proteins are arranged in a ring-like fashion creating a cylinder that spans the membrane.


In green bacteria

The main light harvesting complex in Green bacteria is known as the chlorosome. The chlorosome is equipped with rod-like BChl c aggregates with protein embedded lipids surrounding it. Chlorosomes are found outside of the membrane which covers the reaction centre. Green sulphur bacteria and some
Chloroflexia The Chloroflexia are a class of bacteria in the phylum Chloroflexota, known as filamentous green non-sulfur bacteria. They use light for energy and are named for their green pigment, usually found in photosynthetic bodies called chlorosomes. Chl ...
use ellipsoidal complexes known as the chlorosome to capture light. Their form of bacteriochlorophyll is green.


In cyanobacteria and plants

Chlorophylls and
carotenoids Carotenoids (), also called tetraterpenoids, are yellow, orange, and red organic pigments that are produced by plants and algae, as well as several bacteria, and fungi. Carotenoids give the characteristic color to pumpkins, carrots, parsnips, co ...
are important in light-harvesting complexes present in plants. Chlorophyll b is almost identical to chlorophyll a, except it has a
formyl group In organic chemistry, an aldehyde () is an organic compound containing a functional group with the structure . The functional group itself (without the "R" side chain) can be referred to as an aldehyde but can also be classified as a formyl group ...
in place of a
methyl group In organic chemistry, a methyl group is an alkyl derived from methane, containing one carbon atom bonded to three hydrogen atoms, having chemical formula . In formulas, the group is often abbreviated as Me. This hydrocarbon group occurs in ma ...
. This small difference makes chlorophyll b absorb
light Light or visible light is electromagnetic radiation that can be perceived by the human eye. Visible light is usually defined as having wavelengths in the range of 400–700 nanometres (nm), corresponding to frequencies of 750–420 te ...
with
wavelengths In physics, the wavelength is the spatial period of a periodic wave—the distance over which the wave's shape repeats. It is the distance between consecutive corresponding points of the same phase on the wave, such as two adjacent crests, tr ...
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 In organic chemistry, polyenes are poly- unsaturated, organic compounds that contain at least three alternating double () and single () carbon–carbon bonds. These carbon–carbon double bonds interact in a process known as conjugation, result ...
. Two examples of carotenoids are
lycopene Lycopene is an organic compound classified as a tetraterpene and a carotene. Lycopene (from the neo-Latin '' Lycopersicum'', the tomato species) is a bright red carotenoid hydrocarbon found in tomatoes and other red fruits and vegetables. Occu ...
and
β-carotene β-Carotene is an organic, strongly coloured red-orange pigment abundant in fungi, plants, and fruits. It is a member of the carotenes, which are terpenoids (isoprenoids), synthesized biochemically from eight isoprene units and thus having 40 ...
. 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 A flower, sometimes known as a bloom or blossom, is the reproductive structure found in flowering plants (plants of the division Angiospermae). The biological function of a flower is to facilitate reproduction, usually by providing a mechanism ...
. The carotenoid molecules also serve a safeguarding function. Carotenoid molecules suppress damaging photochemical reactions, in particular those including
oxygen Oxygen is the chemical element with the symbol O and atomic number 8. It is a member of the chalcogen group in the periodic table, a highly reactive nonmetal, and an oxidizing agent that readily forms oxides with most elements as ...
, which exposure to sunlight can cause. Plants that lack carotenoid molecules quickly die upon exposure to oxygen and light.


Phycobilisome

The antenna-shaped light harvesting complex of cyanobacteria, glaucocystophyta, and red algae is known as the phycobilisome which is composed of linear tetrapyrrole pigments. Pigment-protein complexes referred to as R-phycoerythrin are rod-like in shape and make up the rods and core of the phycobilisome. Little light reaches algae that reside at a depth of one meter or more in seawater, as light is absorbed by seawater. The pigments, such as phycocyanobilin and phycoerythrobilin, are the chromophores that bind through a covalent thioether bond to their apoproteins at cysteins residues. The apoprotein with its chromophore is called phycocyanin, phycoerythrin, and allophycocyanin, respectively. They often occur as hexamers of α and β subunits (α3β3)2. They enhance the amount and spectral window of light absorption and fill the "green gap", which occur in higher Plants. 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 ("other algal blue protein"; from Greek: '' (allos)'' meaning "other", '' (phykos)'' meaning “alga”, and '' (kyanos)'' meaning "blue") is a protein from the light-harvesting phycobiliprotein family, along with phycocyanin, phyco ...
, which sits above a photosynthetic reaction center. There are
phycocyanin Phycocyanin is a pigment-protein complex from the light-harvesting phycobiliprotein family, along with allophycocyanin and phycoerythrin. It is an accessory pigment to chlorophyll. All phycobiliproteins are water-soluble, so they cannot exist w ...
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 to a Chlorophyll. The energy transfer from excited electrons absorbed by pigments in the phycoerythrin subunits at the periphery of these antennas appears at the reaction center in less than 100 ps.Light Harvesting by Phycobilisomes Annual Review of Biophysics and Biophysical Chemistry Vol. 14: 47-77 (Volume publication date June 1985)


See also

*
Photosynthesis Photosynthesis is a process used by plants and other organisms to convert light energy into chemical energy that, through cellular respiration, can later be released to fuel the organism's activities. Some of this chemical energy is stored i ...
* Photosynthetic reaction center *
Photosystem II light-harvesting protein Photosystems are functional and structural units of protein complexes involved in photosynthesis. Together they carry out the primary photochemistry of photosynthesis: the absorption of light and the transfer of energy and electrons. Photosystems ...
* Light harvesting pigment


References


Further reading

* Caffarri (2009)Functional architecture of higher plantphotosystem II supercomplexes. ''The EMBO Journal'' 28: 3052–3063 * Govindjee & Shevela (2011) Adventures with cyanobacteria: a personal perspective. ''Frontiers in Plant Science''. * Liu et al. (2004) Crystal structure of spinach major light-harvesting complex at 2.72A° resolution. ''Nature'' 428: 287–292. * Lokstein (1994)The role of light-harvesting complex II energy dissipation: an in-vivo fluorescence in excess excitation study on the origin of high-energy quenching. Journal of Photochemistry and Photobiology 26: 175-184 * MacColl (1998) Cyanobacterial Phycobilisomes. JOURNAL OF STRUCTURAL BIOLOGY 124(2-3): 311-34.


External links

*
Photosynthesis and all sub categories
{{Carrier proteins Photosynthesis Transmembrane proteins