Photosynthetic reaction centre - AbsoluteAstronomy.com

A photosynthetic reaction center (or photosynthetic reaction centre) is a complex of several proteins, pigments and other co-factors assembled together to execute the primary energy conversion reactions of 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...

. Molecular excitations, either originating directly from sunlight or transferred as excitation energy

Fluorescence resonance energy transfer

Förster resonance energy transfer , also known as fluorescence resonance energy transfer, resonance energy transfer or electronic energy transfer , is a mechanism describing energy transfer between two chromophores.A donor chromophore, initially in its electronic excited state, may transfer energy...

via light-harvesting antenna systems

Light-harvesting complex

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 plants and photosynthetic bacteria to collect more of the incoming light than would be captured by the photosynthetic reaction...

, give rise to electron transfer

Electron transfer

Electron transfer is the process by which an electron moves from an atom or a chemical species to another atom or chemical species...

reactions along a series of protein-bound co-factors. These co-factors are light-absorbing molecules (also named 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...

s or pigments) such as 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...

and phaeophytin

Pheophytin

Pheophytin or phaeophytin is a chemical compound that serves as the first electron carrier intermediate in the electron transfer pathway of photosystem II in plants, and the photosynthetic reaction center found in purple bacteria...

, as well as quinones. The energy of the photon

Photon

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...

is used to promote an electron

Electron

The electron is a subatomic particle with a negative elementary electric charge. It has no known components or substructure; in other words, it is generally thought to be an elementary particle. An electron has a mass that is approximately 1/1836 that of the proton...

to a higher molecular energy level

Excited state

Excitation is an elevation in energy level above an arbitrary baseline energy state. In physics there is a specific technical definition for energy level which is often associated with an atom being excited to an excited state....

of a pigment. The free energy

Thermodynamic free energy

The thermodynamic free energy is the amount of work that a thermodynamic system can perform. The concept is useful in the thermodynamics of chemical or thermal processes in engineering and science. The free energy is the internal energy of a system less the amount of energy that cannot be used to...

created is then used to reduce a chain of nearby electron acceptor

Electron acceptor

An electron acceptor is a chemical entity that accepts electrons transferred to it from another compound. It is an oxidizing agent that, by virtue of its accepting electrons, is itself reduced in the process....

s, which have subsequently lowered redox-potentials. These electron transfer

Electron transfer

Electron transfer is the process by which an electron moves from an atom or a chemical species to another atom or chemical species...

steps are the initial phase of a series of energy conversion reactions, ultimately resulting in the production of chemical energy during photosynthesis

Photosynthesis

Transforming light energy into charge separation

Reaction centres are present in all green 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, algae

Algae

Algae are a large and diverse group of simple, typically autotrophic organisms, ranging from unicellular to multicellular forms, such as the giant kelps that grow to 65 meters in length. They are photosynthetic like plants, and "simple" because their tissues are not organized into the many...

, and many bacteria

Bacteria

Bacteria are a large domain of prokaryotic microorganisms. Typically a few micrometres in length, bacteria have a wide range of shapes, ranging from spheres to rods and spirals...

. Although these species are separated by billions of years of evolution

Evolution

Evolution is any change across successive generations in the heritable characteristics of biological populations. Evolutionary processes give rise to diversity at every level of biological organisation, including species, individual organisms and molecules such as DNA and proteins.Life on Earth...

, the reaction centres are homologous

Homology (biology)

Homology forms the basis of organization for comparative biology. In 1843, Richard Owen defined homology as "the same organ in different animals under every variety of form and function". Organs as different as a bat's wing, a seal's flipper, a cat's paw and a human hand have a common underlying...

for all photosynthetic species. In contrast, a large variety in light-harvesting complex

Light-harvesting complex

es exist between the photosynthetic species. Green plants and algae have two different types of reaction centres that are part of larger supercomplexes known as photosystem I

Photosystem I

Photosystem I is the second photosystem in the photosynthetic light reactions of algae, plants, and some bacteria. Photosystem I is so named because it was discovered before photosystem II. Aspects of PS I were discovered in the 1950s, but the significances of these discoveries was not yet known...

and photosystem II

Photosystem II

Photosystem II is the first protein complex in the Light-dependent reactions. It is located in the thylakoid membrane of plants, algae, and cyanobacteria. The enzyme uses photons of light to energize electrons that are then transferred through a variety of coenzymes and cofactors to reduce...

. The structures of these supercomplexes are large, involving multiple light-harvesting complex

Light-harvesting complex

es. The reaction centre found in Rhodopseudomonas
Bradyrhizobiaceae
The Bradyrhizobiaceae are a family of bacteria, with ten genera. They include plant-associated bacteria such as Bradyrhizobium, a genus of rhizobia associated with some legumes. It also contains animal-associated bacteria such as Afipia felis, formerly thought to cause cat-scratch disease...

bacteria is currently best understood, since it was the first reaction centre of known structure and has fewer polypeptide chains than the examples in green plants.

A reaction centre is laid out in such a way that it captures the energy of a photon using pigment molecules and turns it into a usable form. Once the light energy has been absorbed directly by the pigment molecules, or passed to them by resonance transfer

Fluorescence resonance energy transfer

from a surrounding light-harvesting complex

Light-harvesting complex

, they release two electron

Electron

s into an electron transport chain

Electron transport chain

An electron transport chain couples electron transfer between an electron donor and an electron acceptor with the transfer of H+ ions across a membrane. The resulting electrochemical proton gradient is used to generate chemical energy in the form of adenosine triphosphate...

.

Light is made up of small bundles of energy called photon

Photon

s. If a photon with the right amount of energy hits an electron, it will raise the electron to a higher energy level

Energy level

A quantum mechanical system or particle that is bound -- that is, confined spatially—can only take on certain discrete values of energy. This contrasts with classical particles, which can have any energy. These discrete values are called energy levels...

. Electrons are most stable at their lowest energy level, what is also called its ground state. In this state, the electron is in the orbit that has the least amount of energy. Electrons in higher energy levels can return to ground state in a manner analogous to a ball falling down a staircase. In doing so, the electrons release energy. This is the process that is exploited by a photosynthetic reaction centre.

When an electron rises to a higher energy level, increase in the reduction potential

Reduction potential

Reduction potential is a measure of the tendency of a chemical species to acquire electrons and thereby be reduced. Reduction potential is measured in volts , or millivolts...

of the molecule in which the electron resides occurs. This means that the molecule has a greater tendency to donate electrons, the key to the conversion of light energy to chemical energy. In green plants, the electron transport chain that follows has many electron acceptors including phaeophytin

Pheophytin

, quinone

Quinone

A quinone is a class of organic compounds that are formally "derived from aromatic compounds [such as benzene or naphthalene] by conversion of an even number of –CH= groups into –C– groups with any necessary rearrangement of double bonds," resulting in "a fully conjugated cyclic dione structure."...

, plastoquinone

Plastoquinone

Plastoquinone is a quinone molecule involved in the electron transport chain in the light-dependent reactions of photosynthesis. Plastoquinone is reduced , forming plastoquinol...

, cytochrome bf

Cytochrome b6f complex

The cytochrome b6f complex is an enzyme found in the thylakoid membrane in chloroplasts of plants, cyanobacteria, and green algae, catalyzing the transfer of electrons from plastoquinol to plastocyanin...

, and ferredoxin

Ferredoxin

Ferredoxins are iron-sulfur proteins that mediate electron transfer in a range of metabolic reactions. The term "ferredoxin" was coined by D.C. Wharton of the DuPont Co...

, which result in the reduced molecule NADPH. The passage of the electron through the electron transport chain also results in the pumping of protons (hydrogen ions) from the chloroplast's stroma

Stroma (fluid)

Stroma, in botany, refers to the colourless fluid surrounding the grana within the Plastid, chloroplast.Within the stroma are grana, stacks of thylakoids, the sub-organelles, where photosynthesis is commenced before the chemical changes are completed in the stroma.Photosynthesis occurs in two...

into the lumen, resulting in a proton gradient across the thylakoid membrane that can be used to synthesise ATP

Adenosine triphosphate

Adenosine-5'-triphosphate is a multifunctional nucleoside triphosphate used in cells as a coenzyme. It is often called the "molecular unit of currency" of intracellular energy transfer. ATP transports chemical energy within cells for metabolism...

using ATP synthase

ATP synthase

right|thumb|300px|Molecular model of ATP synthase by X-ray diffraction methodATP synthase is an important enzyme that provides energy for the cell to use through the synthesis of adenosine triphosphate . ATP is the most commonly used "energy currency" of cells from most organisms...

. Both the ATP and NADPH are used in the Calvin cycle

Calvin cycle

The Calvin cycle or Calvin–Benson-Bassham cycle or reductive pentose phosphate cycle or C3 cycle or CBB cycle is a series of biochemical redox reactions that take place in the stroma of chloroplasts in photosynthetic organisms...

to fix carbon dioxide into triose sugars.

Structure

The bacterial photosynthetic reaction centre has been an important model to understand the structure and chemistry of the biological process of capturing light energy. In the 1960s, Roderick Clayton was the first to purify the reaction centre complex from purple bacteria. However, the first crystal structure was determined in 1982 by Hartmut Michel

Hartmut Michel

Hartmut Michel is a German biochemist and Nobel Laureate.He was born 18 July 1948 in Ludwigsburg. After compulsory military service, he studied biochemistry at the University of Tübingen, working for his final year at Dieter Oesterhelt’s laboratory on ATPase activity of halobacteria.In 1986, he...

, Johann Deisenhofer

Johann Deisenhofer

Johann Deisenhofer is a German biochemist who, along with Hartmut Michel and Robert Huber, received the Nobel Prize for Chemistry in 1988 for their determination of the structure of a membrane-bound complex of proteins and co-factors that is essential to photosynthesis.Deisenhofer earned his...

and Robert Huber

Robert Huber

Robert Huber is a German biochemist and Nobel laureate.He was born 20 February 1937 in Munich where his father, Sebastian, was a bank cashier. He was educated at the Humanistisches Karls-Gymnasium from 1947 to 1956 and then studied chemistry at the Technische Hochschule, receiving his diploma in 1960...

for which they shared the Nobel Prize

Nobel Prize in Chemistry

The Nobel Prize in Chemistry is awarded annually by the Royal Swedish Academy of Sciences to scientists in the various fields of chemistry. It is one of the five Nobel Prizes established by the will of Alfred Nobel in 1895, awarded for outstanding contributions in chemistry, physics, literature,...

in 1988. This was also significant, since it was the first structure for any membrane protein complex.

Four different subunits were found to be important for the function of the photosynthetic reaction centre. The L and M subunits

Photosynthetic reaction center protein family

Photosynthetic reaction centre proteins are main protein components of photosynthetic reaction centres of bacteria and plants.-In bacteria:...

, shown in blue and purple in the image of the structure, both span the lipid bilayer

Lipid bilayer

The lipid bilayer is a thin membrane made of two layers of lipid molecules. These membranes are flat sheets that form a continuous barrier around cells. The cell membrane of almost all living organisms and many viruses are made of a lipid bilayer, as are the membranes surrounding the cell nucleus...

of the plasma membrane. They are structurally similar to one another, both having 5 transmembrane alpha helices. Four bacteriochlorophyll b

Bacteriochlorophyll

Bacteriochlorophylls are photosynthetic pigments that occur in various phototrophic bacteria. They were discovered by Von Neil in 1932 . They are related to chlorophylls, which are the primary pigments in plants, algae, and cyanobacteria. Groups that contain bacteriochlorophyll conduct...

(BChl-b) molecules, two bacteriophaeophytin b molecules (BPh) molecules, two quinone

Quinone

s (Q_A and Q_B), and a ferrous ion are associated with the L and M subunits. The H subunit, shown in gold, lies on the cytoplasmic side of the plasma membrane. A cytochrome subunit, here not shown, contains four c-type haems and is located on the periplasmic surface (outer) of the membrane. The latter sub-unit is not a general structural motif in photosynthetic bacteria. The L and M subunits bind the functional and light-interacting cofactors, shown here in green.

Reaction centres from different bacterial species may contain slightly altered bacterio-chlorophyll and bacterio-phaeophytin chromophores as functional co-factors. These alterations cause shifts in the colour of light that can be absorbed, thus creating specific niches for photosynthesis. The reaction centre contains two pigments that serve to collect and transfer the energy from photon absorption: BChl and Bph. BChl roughly resembles the chlorophyll molecule found in green plants, but, due to minor structural differences, its peak absorption wavelength is shifted into the infrared

Infrared

Infrared light is electromagnetic radiation with a wavelength longer than that of visible light, measured from the nominal edge of visible red light at 0.74 micrometres , and extending conventionally to 300 µm...

, with wavelengths as long as 1000 nm. Bph has the same structure as BChl, but the central magnesium ion is replaced by two protons. This alteration causes both an absorbance maximum shift and a lowered redox-potential.

Mechanism

The process starts when light is absorbed by two BChl molecules that lie near the periplasmic side of the membrane. This pair of chlorophyll molecules, often called the "special pair", absorbs photons between 870 nm and 960 nm, depending on the species and, thus, is called P870 (for the species rhodobacter sphaeroides) or P960 (for rhodopseudomonas viridis), with P standing for "pigment"). Once P absorbs a photon, it ejects an electron, which is transferred through another molecule of Bchl to the BPh in the L subunit. This initial charge separation yields a positive charge on P and a negative charge on the BPh. This process takes place in 10 picoseconds (10^-11 seconds).

The charges on the specialpair ⁺ and the BPh^- could undergo charge recombination in this state. This would waste the high-energy electron and convert the absorbed light energy in to heat

Heat

In physics and thermodynamics, heat is energy transferred from one body, region, or thermodynamic system to another due to thermal contact or thermal radiation when the systems are at different temperatures. It is often described as one of the fundamental processes of energy transfer between...

. Several factors of the reaction centre structure serve to prevent this. First, the transfer of an electron from BPh^- to P960⁺ is relatively slow compared to two other redox reactions in the reaction centre. The faster reactions involve the transfer of an electron from BPh^- (BPh^- is oxidised to BPh) to the electron acceptor quinone (Q_A), and the transfer of an electron to P960⁺ (P960⁺ is reduced to P960) from a haem in the cytochrome subunit above the reaction centre.

The high-energy electron that resides on the tightly bound quinone molecule Q_A is transferred to an exchangeable quinone molecule Q_B. This molecule is loosely associated with the protein and is fairly easy to detach. Two of the high-energy electrons are required to fully reduce Q_B to QH₂, taking up two protons from the cytoplasm in the process. The reduced quinone QH₂ diffuses through the membrane to another protein complex (cytochrome bc₁-complex

Coenzyme Q - cytochrome c reductase

In enzymology, a ubiquinol—cytochrome-c reductase is an enzyme that catalyzes the chemical reactionThus, the two substrates of this enzyme are dihydroquinone and ferri- cytochrome c, whereas its 3 products are quinone , ferro- cytochrome c, and H+.This enzyme belongs to the family of...

) where it is oxidised. In the process the reducing power of the QH₂ is used to pump protons across the membrane to the periplasmic space. The electrons from the cytochrome bc₁-complex are then transferred through a soluble cytochrome c intermediate, called cytochrome c₂, in the periplasm to the cytochrome subunit. Thus, the flow of electrons in this system is cyclical.

Oxygenic photosynthesis

In 1772, the chemist Joseph Priestley

Joseph Priestley

Joseph Priestley, FRS was an 18th-century English theologian, Dissenting clergyman, natural philosopher, chemist, educator, and political theorist who published over 150 works...

carried out a series of experiments relating to the gases involved in respiration and combustion. In his first experiment, he lit a candle and placed it under an upturned jar. After a short period of time, the candle burned out. He carried out a similar experiment with a mouse

Mouse

A mouse is a small mammal belonging to the order of rodents. The best known mouse species is the common house mouse . It is also a popular pet. In some places, certain kinds of field mice are also common. This rodent is eaten by large birds such as hawks and eagles...

in the confined space of the burning candle. He found that the mouse died a short time after the candle had been extinguished. However, he could revivify the foul air by placing green plants in the area and exposing them to light. Priestley's observations were some of the first experiments that demonstrated the activity of a photosynthetic reaction centre.

In 1779, Jan Ingenhousz

Jan Ingenhousz

Jan Ingenhousz or Ingen-Housz FRS was a Dutch physiologist, biologist and chemist. He is best known for showing that light is essential to photosynthesis and thus having discovered photosynthesis. He also discovered that plants, like animals, have cellular respiration...

carried out more than 500 experiments spread out over 4 months in an attempt to understand what was really going on. He wrote up his discoveries in a book entitled Experiments upon Vegetables. Ingenhousz took green plants and immersed them in water inside a transparent tank. He observed many bubbles rising from the surface of the leaves whenever the plants were exposed to light. Ingenhousz collected the gas that was given off by the plants and performed several different tests in attempt to determine what the gas was. The test that finally revealed the identity of the gas was placing a smouldering taper into the gas sample and having it relight. This test proved it was oxygen, or, as Joseph Priestley had called it, 'de-phlogisticated

Phlogiston theory

The phlogiston theory , first stated in 1667 by Johann Joachim Becher, is an obsolete scientific theory that postulated the existence of a fire-like element called "phlogiston", which was contained within combustible bodies and released during combustion...

air'.

In 1932, Professor Robert Emerson and an undergraduate student, William Arnold, used a repetitive flash technique to precisely measure small quantities of oxygen evolved by chlorophyll in the algae Chlorella. Their experiment proved the existence of a photosynthetic unit. Gaffron and Wohl later interpreted the experiment and realized that the light absorbed by the photosynthetic unit was transferred. This reaction occurs at the reaction centre of photosystem II and takes place in cyanobacteria, algae and green plants.

Photosystem II

is the photosystem that generates the two electrons that will eventually reduce NADP⁺ in Ferredoxin-NADP-reduktase. Photosystem II is present on the thylakoid membranes inside chloroplasts, the site of photosynthesis in green plants. The structure of Photosystem II is remarkably similar to the bacterial reaction centre, and it is theorized that they share a common ancestor.

The core of photosystem II consists of two subunits referred to as D1 and D2

Photosynthetic reaction center protein family

Photosynthetic reaction centre proteins are main protein components of photosynthetic reaction centres of bacteria and plants.-In bacteria:...

. These two subunits are similar to the L and M subunits present in the bacterial reaction centre. Photosystem II differs from the bacterial reaction centre in that it has many additional subunits that bind additional chlorophylls to increase efficiency. The overall reaction catalysed by photosystem II is:

Q represents plastoquinone, the oxidized form of Q. QH₂ represents plastoquinol, the reduced form of Q. This process of reducing quinone is comparable to that which takes place in the bacterial reaction centre. Photosystem II obtains electrons by oxidizing water in a process called photolysis. Molecular oxygen is a byproduct of this process, and it is this reaction that supplies the atmosphere

Atmosphere

An atmosphere is a layer of gases that may surround a material body of sufficient mass, and that is held in place by the gravity of the body. An atmosphere may be retained for a longer duration, if the gravity is high and the atmosphere's temperature is low...

with oxygen. The fact that the oxygen from green plants originated from water was first deduced by the Canadian-born American biochemist Martin David Kamen. He used a natural, stable isotope

Stable isotope

Stable isotopes are chemical isotopes that may or may not be radioactive, but if radioactive, have half-lives too long to be measured.Only 90 nuclides from the first 40 elements are energetically stable to any kind of decay save proton decay, in theory...

of oxygen, O₁₈ to trace the path of the oxygen, from water to gaseous molecular oxygen. This reaction is catalysed by a reactive centre in photosystem II containing four manganese

Manganese

Manganese is a chemical element, designated by the symbol Mn. It has the atomic number 25. It is found as a free element in nature , and in many minerals...

ions.

The reaction begins with the excitation of a pair of chlorophyll molecules similar to those in the bacterial reaction centre. Due to the presence of chlorophyll a, as opposed to bacteriochlorophyll

Bacteriochlorophyll

, photosystem II absorbs light at a shorter wavelength. The pair of chlorophyll molecules at the reaction centre are often referred to as P680

P680

P680, or Photosystem II primary donor, refers to any of the 2 special chlorophyll dimers , PD1 or PD2. These 2 special pairs form an excitonic dimer, which means that they behave in function as a single entity; i.e., they are excited as if they were a single molecule...

. When the photon has been absorbed, the resulting high-energy electron is transferred to a nearby phaeophytin molecule. This is above and to the right of the pair on the diagram and is coloured grey. The electron travels from the phaeophytin molecule through two plastoquinone molecules, the first tightly bound, the second loosely bound. The tightly bound molecule is shown above the phaeophytin molecule and is coloured red. The loosely bound molecule is to the left of this and is also coloured red. This flow of electrons is similar to that of the bacterial reaction centre. Two electrons are required to fully reduce the loosely bound plastoquinone molecule to QH₂ as well as the uptake of two protons.

The difference between photosystem II and the bacterial reaction centre is the source of the electron that neutralizes the pair of chlorophyll a molecules. In the bacterial reaction centre, the electron is obtained from a reduced compound haem group in a cytochrome subunit or from a water-soluble cytochrome-c protein.

Once photoinduced charge separation

Photoinduced charge separation

Photoinduced charge separation is the process of an electron in an atom being excited to a higher energy level by the absorption of a photon and then leaving the atom to a nearby electron acceptor.-Rutherford model:...

has taken place, the P680 molecule carries a positive charge

Electric charge

Electric charge is a physical property of matter that causes it to experience a force when near other electrically charged matter. Electric charge comes in two types, called positive and negative. Two positively charged substances, or objects, experience a mutual repulsive force, as do two...

. P680 is a very strong oxidant and extracts electrons from two water

Water

Water is a chemical substance with the chemical formula H2O. A water molecule contains one oxygen and two hydrogen atoms connected by covalent bonds. Water is a liquid at ambient conditions, but it often co-exists on Earth with its solid state, ice, and gaseous state . Water also exists in a...

molecules that are bound at the manganese centre directly below the pair. This centre, below and to the left of the pair in the diagram, contains four manganese ions, a calcium

Calcium

Calcium is the chemical element with the symbol Ca and atomic number 20. It has an atomic mass of 40.078 amu. Calcium is a soft gray alkaline earth metal, and is the fifth-most-abundant element by mass in the Earth's crust...

ion, a chloride

Chlorine

Chlorine is the chemical element with atomic number 17 and symbol Cl. It is the second lightest halogen, found in the periodic table in group 17. The element forms diatomic molecules under standard conditions, called dichlorine...

ion, and a tyrosine

Tyrosine

Tyrosine or 4-hydroxyphenylalanine, is one of the 22 amino acids that are used by cells to synthesize proteins. Its codons are UAC and UAU. It is a non-essential amino acid with a polar side group...

residue. Manganese is used because it is capable of existing in four oxidation states: Mn²⁺, Mn³⁺, Mn⁴⁺ and Mn⁵⁺. Manganese also forms strong bonds with oxygen-containing molecules such as water.

Every time the P680 absorbs a photon, it emits an electron, gaining a positive charge. This charge is neutralized by the extraction of an electron from the manganese centre, which sits directly below it. The process of oxidizing two molecules of water requires four electrons. The water molecules that are oxidized in the manganese centre are the source of the electrons that reduce the two molecules of Q to QH₂. To date, this water-splitting catalytic centre cannot be reproduced by any man-made catalyst.

Photosystem I

After the electron has left photosystem II it is transferred to a cytochrome b6f complex

Cytochrome b6f complex

and then to plastocyanin

Plastocyanin

Plastocyanin is an important copper-containing protein involved in electron-transfer. The protein is monomeric, with a molecular weight around 10,500 Daltons, and 99 amino acids in most vascular plants...

, a blue copper

Copper

Copper is a chemical element with the symbol Cu and atomic number 29. It is a ductile metal with very high thermal and electrical conductivity. Pure copper is soft and malleable; an exposed surface has a reddish-orange tarnish...

protein and electron carrier. The plastocyanin complex carries the electron that will neutralize the pair in the next reaction centre, photosystem I

Photosystem I

.

As with photosystem II and the bacterial reaction centre, a pair of chlorophyll a molecules initiates photoinduced charge separation. This pair is referred to as P700

P700

P700, or Photosystem I primary donor, is the reaction-center chlorophyll a molecule in association with photosystem I. Its absorption spectrum peaks at 700 nm. When photosystem I absorbs light, an electron is excited to a higher energy level in the P700 chlorophyll...

. 700 Is a reference to the wavelength

Wavelength

In physics, the wavelength of a sinusoidal wave is the spatial period of the wave—the distance over which the wave's shape repeats.It is usually determined by considering the distance between consecutive corresponding points of the same phase, such as crests, troughs, or zero crossings, and is a...

at which the chlorophyll molecules absorb light maximally. The P700 lies in the centre of the protein. Once photoinduced charge separation has been initiated, the electron travels down a pathway through a chlorophyll α molecule situated directly above the P700, through a quinone molecule situated directly above that, through three 4Fe-4S clusters, and finally to an interchangeable ferredoxin complex. Ferredoxin is a soluble protein containing a 2Fe-2S cluster coordinated by four cysteine residues. The positive charge left on the P700 is neutralized by the transfer of an electron from plastocyanin

Plastocyanin

. Thus the overall reaction catalysed by photosystem I is:

The cooperation between photosystems I and II creates an electron flow from H₂O to NADP⁺. This pathway is called the 'Z-scheme' because the redox

Redox

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

diagram from P680 to P700 resembles the letter z.

External links

- Calculated spatial positions of photosynthetic reaction centres and photosystems in membrane

http://www.life.illinois.edu/govindjee/photoweb - Photosynthesis and all sub categories

The source of this article is wikipedia, the free encyclopedia. The text of this article is licensed under the GFDL.

Transforming light energy into charge separation

Structure

Mechanism

Oxygenic photosynthesis

Photosystem II

Photosystem I

See also

External links