A light-harvesting complex consists of a number of chromophores which are complex subunit proteins that may be part of a larger super complex of a

photosystem 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. Photosy ...

, 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 in ...

. 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 excl ...

s and photosynthetic bacteria to collect more of the incoming light than would be captured by the

photosynthetic reaction center A photosynthetic reaction center is a complex of several proteins, pigments and other co-factors that together execute the primary energy conversion reactions of photosynthesis. Molecular excitations, either originating directly from sunlight or t ...

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 pi ...

s and surround a

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 alwa ...

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 Resonance describes the phenomenon of increased amplitude that occurs when the frequency of an applied periodic force (or a Fourier component of it) is equal or close to a natural frequency of the system on which it acts. When an oscilla ...

, 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 Purple bacteria or purple photosynthetic bacteria are Gram-negative proteobacteria that are phototrophic, capable of producing their own food via photosynthesis. They are pigmented with bacteriochlorophyll ''a'' or ''b'', together with variou ...

use bacteriochlorophyll 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 use ellipsoidal complexes known as the chlorosome to capture light. Their form of bacteriochlorophyll is green.

In cyanobacteria and plants

Chlorophyll Chlorophyll (also chlorophyl) 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 allow plants to ...

s 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 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 t ...

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 In chemistry, organic compounds are generally any chemical compounds that contain carbon-hydrogen or carbon-carbon bonds. Due to carbon's ability to catenate (form chains with other carbon atoms), millions of organic compounds are known. The s ...

that have alternating single and double bonds along their length. Such molecules are called polyenes. 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. 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 In botany, a fruit is the seed-bearing structure in flowering plants that is formed from the ovary after flowering. Fruits are the means by which flowering plants (also known as angiosperms) disseminate their seeds. Edible fruits in partic ...

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 ...

, 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 Cyanobacteria (), also known as Cyanophyta, are a phylum of gram-negative bacteria that obtain energy via photosynthesis. The name ''cyanobacteria'' refers to their color (), which similarly forms the basis of cyanobacteria's common name, bl ...

glaucocystophyta The glaucophytes, also known as glaucocystophytes or glaucocystids, are a small group of unicellular algae found in freshwater and moist terrestrial environments, less common today than they were during the Proterozoic. The stated number of speci ...

, and

red algae Red algae, or Rhodophyta (, ; ), are one of the oldest groups of eukaryotic algae. The Rhodophyta also comprises one of the largest phyla of algae, containing over 7,000 currently recognized species with taxonomic revisions ongoing. The majority ...

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 Phycocyanobilin is a blue phycobilin, i.e., a tetrapyrrole chromophore found in cyanobacteria and in the chloroplasts of red algae Red algae, or Rhodophyta (, ; ), are one of the oldest groups of eukaryotic algae. The Rhodophyta also comprise ...

and

phycoerythrobilin Phycoerythrobilin is a red phycobilin, i.e. an open tetrapyrrole chromophore found in cyanobacteria and in the chloroplasts of red algae Red algae, or Rhodophyta (, ; ), are one of the oldest groups of eukaryotic algae. The Rhodophyta also co ...

, 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, which sits above a photosynthetic reaction center. There are phycocyanin and

phycoerythrin Phycoerythrin (PE) is a red protein-pigment complex from the light-harvesting phycobiliprotein family, present in cyanobacteria, red algae and cryptophytes, accessory to the main chlorophyll pigments responsible for photosynthesis.The red pigment ...

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

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)

References

External links

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