where do the electrons needed by photosystem two originate
from the splitting of water
4011708
Photosystem II Photosystem II (or water-plastoquinone oxidoreductase) is the first protein complex in the light-dependent reactions of oxygenic photosynthesis. It is located in the thylakoid membrane of plants, algae, and cyanobacteria. Within the photosystem, enzymes capture photons of light to energize electrons that are then transferred through a variety of coenzymes and cofactors to reduce plastoquinone to plastoquinol. The energized electrons are replaced by oxidizing water to form hydrogen ions and molecular oxygen. By replenishing lost electrons with electrons from the splitting of water, photosystem II provides the electrons for all of photosynthesis to occur. The hydrogen ions (protons) generated
Photosystem II
4011708
Photosystem II Photosystem II (or water-plastoquinone oxidoreductase) is the first protein complex in the light-dependent reactions of oxygenic photosynthesis. It is located in the thylakoid membrane of plants, algae, and cyanobacteria. Within the photosystem, enzymes capture photons of light to energize electrons that are then transferred through a variety of coenzymes and cofactors to reduce plastoquinone to plastoquinol. The energized electrons are replaced by oxidizing water to form hydrogen ions and molecular oxygen. By replenishing lost electrons with electrons from the splitting of water, photosystem II provides the electrons for all of photosynthesis to occur. The hydrogen ions (protons) generated
Photosystem II
79135
in position to be used in the dark reactions. Electrons are often removed from the electron transport chains to charge NADP with electrons, reducing it to NADPH. Like ATP synthase, ferredoxin-NADP reductase, the enzyme that reduces NADP, releases the NADPH it makes into the stroma, right where it is needed for the dark reactions. Because NADP reduction removes electrons from the electron transport chains, they must be replaced—the job of photosystem II, which splits water molecules (HO) to obtain the electrons from its hydrogen atoms. While photosystem II photolyzes water to obtain and energize new electrons, photosystem I simply reenergizes
Chloroplast
331495
the electron is passed down the electron acceptor molecules and returns to photosystem I, from where it was emitted, hence the name "cyclic reaction". Linear electron transport through a photosystem will leave the reaction center of that photosystem oxidized. Elevating another electron will first require re-reduction of the reaction center. The excited electrons lost from the reaction center (P700) of photosystem I are replaced by transfer from plastocyanin, whose electrons come from electron transport through photosystem II. Photosystem II, as the first step of the "Z-scheme", requires an external source of electrons to reduce its oxidized chlorophyll "a" reaction center,
Photosynthesis
4433322
chlorophyll photosystems. Being a light reaction, non-cyclic photophosphorylation occurs in the thylakoid membrane. First, a water molecule is broken down into 2H + 1/2 O + 2e by a process called photolysis (or "light-splitting"). The two electrons from the water molecule are kept in photosystem II, while the 2H and 1/2O are left out for further use. Then a photon is absorbed by chlorophyll pigments surrounding the reaction core center of the photosystem. The light excites the electrons of each pigment, causing a chain reaction that eventually transfers energy to the core of photosystem II, exciting the two electrons that
Photophosphorylation
4011704
believed to be the progenitors of the photosystem-containing chloroplasts of eukaryotes. Photosynthetic bacteria that cannot produce oxygen have a single photosystem called BRC, bacterial reaction center. The photosystem I was named "I" since it was discovered before photosystem II, but this does not represent the order of the electron flow. When photosystem II absorbs light, electrons in the reaction-center chlorophyll are excited to a higher energy level and are trapped by the primary electron acceptors. To replenish the deficit of electrons thus created, electrons are extracted from water by a cluster of four Manganese ions in photosystem II and supplied
Photosystem
4433324
This creates a gradient, making H ions flow back into the stroma of the chloroplast, providing the energy for the regeneration of ATP. The photosystem II complex replaced its lost electrons from an external source; however, the two other electrons are not returned to photosystem II as they would in the analogous cyclic pathway. Instead, the still-excited electrons are transferred to a photosystem I complex, which boosts their energy level to a higher level using a second solar photon. The highly excited electrons are transferred to the acceptor molecule, but this time are passed on to an enzyme called Ferredoxin-NADP
Photophosphorylation
19148233
of water provides the electrons needed by the antenna system of photosystem II where they are boosted in energy and then sent along the electron transport system (ETS). Those high energy electrons are able to move against a chemical potential gradient. Hence, solar energy is converted to chemical energy by the reduction of NADP to NADPH. Photosynthesis is the process in which light energy is absorbed and converted to chemical energy. This chemical energy is eventually used in the conversion of carbon dioxide (CO) to sugar (CHO) in plants. During the process of photosynthesis, a natural electron acceptor, nicotinamide adenine
Hill reaction
4872973
psilocybin's effect on the change in free energy is currently available. The first protein complex of the photosynthesis component light-dependent reactions is referred to as photosystem II. The complex utilizes an enzyme to capture photons of light, providing the greater photosynthesis process with all of the electrons needed to produce ATP. Photosystem II is particularly temperature sensitive, and desphosphorylation has been implicated as a driver of plasticity in responding to varied temperature. Accelerated protein dephosphorylation in photosynthetic thylakoid membranes occurs at elevated temperatures, directly impacting the desphosphorylation of key proteins within the photosystem II complex. Excessive dephosphorylation of the membrane
Dephosphorylation
10092239
an electron from a heme in cytochrome "c". This regenerates the substrates and completes the cycle, allowing for subsequent reactions to take place. In photosystem II, pheophytin plays a very similar role. It again acts as the first electron carrier intermediate in the photosystem. After P680 becomes excited to P680, it transfers an electron to pheophytin, which converts the molecule into a negatively charged radical. Two negatively charged pheophytin radicals quickly pass their extra electrons to two consecutive plastoquinone molecules. Eventually, the electrons pass through the cytochrome "bf" molecule and leaves photosystem II. The reactions outlined above in the section
Pheophytin
4011706
photosystem I, it fills the electron deficit of the reaction-center chlorophyll of photosystem I. The deficit is due to photo-excitation of electrons that are again trapped in an electron acceptor molecule, this time that of photosystem I. ATP is generated when the ATP synthase transports the protons present in the lumen to the stroma, through the membrane. The electrons may either continue to go through cyclic electron transport around PS I or pass, via ferredoxin, to the enzyme NADP reductase. Electrons and hydrogen ions are added to NADP to form NADPH. This reducing agent is transported to the Calvin cycle
Photosystem
4900397
reduction of a series of electron acceptors, eventually reducing NADP to NADPH and PS I is oxidized. The oxidized photosystem I captures electrons from photosystem II through a series of steps involving agents like plastoquinone, cytochromes and plastocyanine. The photosystem II then brings about water oxidation resulting in evolution of oxygen, the reaction being catalyzed by CaMnO clusters embedded in complex protein environment; the complex is known as oxygen evolving complex (OEC). In biological hydrogen production, the electrons produced by the photosystem are shunted not to a chemical synthesis apparatus but to hydrogenases, resulting in formation of H. This biohydrogen
Water splitting
4011701
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. Photosystems are found in the thylakoid membranes of plants, algae and cyanobacteria. They are located in the chloroplasts of plants and algae, and in the cytoplasmic membrane of photosynthetic bacteria. There are two kinds of photosystems: II and I. At the heart of a photosystem lies the reaction center, which is an enzyme that uses light to reduce molecules (provide with electrons). This reaction center is surrounded
Photosystem
7084094
photosystem that generates the two electrons that will eventually reduce NADP in ferredoxin-NADP-reductase. 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 center, 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. These two subunits are similar to the L and M subunits present in the bacterial reaction center. Photosystem II differs from the bacterial reaction center in that it has many additional subunits
Photosynthetic reaction centre
17697655
absent, biological photovoltaic systems based on isolated photosystems have relatively short lifetimes (a few hours) and often require low temperatures to improve stability. Sub-cellular fractions of photosynthetic organisms, such as purified thylakoid membranes, can also be used in biological photovoltaic systems. A benefit of using material that contains both photosystem II and photosystem I is that electrons extracted from water by photosystem II can be donated to the anode at a more negative redox potential (from the reductive end of photosystem I). A redox mediator (e.g. ferricyanide) is required to transfer electrons between the photosynthetic components and the anode. Biological
Biological photovoltaics
331496
called P680. The source of electrons for photosynthesis in green plants and cyanobacteria is water. Two water molecules are oxidized by four successive charge-separation reactions by photosystem II to yield a molecule of diatomic oxygen and four hydrogen ions. The electrons yielded are transferred to a redox-active tyrosine residue that then reduces the oxidized P680. This resets the ability of P680 to absorb another photon and release another photo-dissociated electron. The oxidation of water is catalyzed in photosystem II by a redox-active structure that contains four manganese ions and a calcium ion; this oxygen-evolving complex binds two water molecules and
Photosynthesis
3511840
the initial light reaction reach Photosystem I, then are raised to a higher energy level by light energy and then received by an electron acceptor and reduce NADP+ to NADPH. The electrons lost from Photosystem II get replaced by the oxidation of water, which is "split" into protons and oxygen by the oxygen-evolving complex (OEC, also known as WOC, or the water-oxidizing complex). To generate one molecule of diatomic oxygen, 10 photons must be absorbed by photosystems I and II, four electrons must move through the two photosystems, and 2 NAPDH are generated (later used for carbon dioxide fixation in
Chemiosmosis
278167
bacteria only having one type, while plants and cyanobacteria have two. In plants, algae, and cyanobacteria, photosystem II uses light energy to remove electrons from water, releasing oxygen as a waste product. The electrons then flow to the cytochrome b6f complex, which uses their energy to pump protons across the thylakoid membrane in the chloroplast. These protons move back through the membrane as they drive the ATP synthase, as before. The electrons then flow through photosystem I and can then either be used to reduce the coenzyme NADP, for use in the Calvin cycle, which is discussed below, or recycled
Metabolism
1673583
to the separate location of the two photosystems in the thylakoid membrane system, mobile electron carriers are required to shuttle electrons between them. These carriers are plastoquinone and plastocyanin. Plastoquinone shuttles electrons from photosystem II to the cytochrome b6f complex, whereas plastocyanin carries electrons from the cytochrome b6f complex to photosystem I. Together, these proteins make use of light energy to drive electron transport chains that generate a chemiosmotic potential across the thylakoid membrane and NADPH, a product of the terminal redox reaction. The ATP synthase uses the chemiosmotic potential to make ATP during photophosphorylation. These photosystems are light-driven redox
Thylakoid
1673586
a CF1FO-ATP synthase similar to the mitochondrial ATPase. It is integrated into the thylakoid membrane with the CF1-part sticking into stroma. Thus, ATP synthesis occurs on the stromal side of the thylakoids where the ATP is needed for the light-independent reactions of photosynthesis. The electron transport protein plastocyanin is present in the lumen and shuttles electrons from the cytochrome b6f protein complex to photosystem I. While plastoquinones are lipid-soluble and therefore move within the thylakoid membrane, plastocyanin moves through the thylakoid lumen. The lumen of the thylakoids is also the site of water oxidation by the oxygen evolving complex associated
Thylakoid
3902955
with electron transfer, a process that is common to all life. Addition of one electron gives the radical cation: The radical cation is also susceptible to further reduction to the neutral [MV]: As an herbicide, paraquat acts by inhibiting photosynthesis. In light-exposed plants, it accepts electrons from photosystem I (more specifically ferredoxin, which is presented with electrons from PS I) and transfers them to molecular oxygen. In this manner, destructive reactive oxygen species are produced. In forming these reactive oxygen species, the oxidized form of paraquat is regenerated, and is again available to shunt electrons from photosystem I to restart
Paraquat
7084100
molecules that are oxidized in the manganese center are the source of the electrons that reduce the two molecules of Q to QH. To date, this water-splitting catalytic center cannot be reproduced by any man-made catalyst. After the electron has left photosystem II it is transferred to a cytochrome b6f complex and then to plastocyanin, a blue copper protein and electron carrier. The plastocyanin complex carries the electron that will neutralize the pair in the next reaction center, photosystem I. As with photosystem II and the bacterial reaction center, a pair of chlorophyll "a" molecules initiates photoinduced charge separation. This
Photosynthetic reaction centre
1673585
maximally absorbs 700 nm light. Photosystem II contains P680 chlorophyll that absorbs 680 nm light best (note that these wavelengths correspond to deep red – see the visible spectrum). The P is short for pigment and the number is the specific absorption peak in nanometers for the chlorophyll molecules in each reaction center. The cytochrome b6f complex is part of the thylakoid electron transport chain and couples electron transfer to the pumping of protons into the thylakoid lumen. Energetically, it is situated between the two photosystems and transfers electrons from photosystem II-plastoquinone to plastocyanin-photosystem I. The thylakoid ATP synthase is
Thylakoid
10092240
concerning purple bacteria give a general illustration of the actual movement of the electrons through pheophytin and the photosystem. The overall scheme is: Pheophytin Pheophytin or phaeophytin (abbreviated Pheo) is a chemical compound that serves as the first electron carrier intermediate in the electron transfer pathway of Photosystem II (PS II) in plants, and the photosynthetic reaction center (RC P870) found in purple bacteria. In both PS II and RC P870, light drives electrons from the reaction center through pheophytin, which then passes the electrons to a quinone (Q) in RC P870 and RC P680. The overall mechanisms, roles, and
Pheophytin
5258732
Q cycle and is not found in Complex III, it has been proposed that it is used for cyclic photophosphorylation by the following mechanism: Cytochrome b6f complex The cytochrome "bf" complex (plastoquinol—plastocyanin reductase; ) is an enzyme found in the thylakoid membrane in chloroplasts of plants, cyanobacteria, and green algae, that catalyzes the transfer of electrons from plastoquinol to plastocyanin. The reaction is analogous to the reaction catalyzed by cytochrome bc (Complex III) of the mitochondrial electron transport chain. During photosynthesis, the cytochrome bf complex is one step along the chain that transfers electrons from Photosystem II to Photosystem I,
Cytochrome b6f complex
4011718
interchange of water molecules in S-states. The arrangement of hydrogen bonding is main criteria of the diagrams. The effect of photon(hv), emission of electron(e) and production of hydrogen ion(H+) from water molecules can be shown by the diagrams in a more specific way of experimental detection. The diagrams almost completely analyze the pathway of water-splitting in photosynthesis. Photosystem II Photosystem II (or water-plastoquinone oxidoreductase) is the first protein complex in the light-dependent reactions of oxygenic photosynthesis. It is located in the thylakoid membrane of plants, algae, and cyanobacteria. Within the photosystem, enzymes capture photons of light to energize electrons that
Photosystem II
2775588
where the atom is fully ionised. During internal conversion, the atomic number does not change, and thus (as is the case with gamma decay) no transmutation of one element to another takes place. Since an electron is lost from the atom, a hole appears in an electron shell which is subsequently filled by other electrons that descend to that empty, lower energy level, and in the process emit characteristic X-ray(s), Auger electron(s), or both. The atom thus emits high-energy electrons and X-ray photons, none of which originate in that nucleus. The atom supplied the energy needed to eject the electron,
Internal conversion
4011705
to the chlorophyll via a redox-active tyrosine. Photoexcited electrons travel through the cytochrome b6f complex to photosystem I via an electron transport chain set in the thylakoid membrane. This energy fall is harnessed, (the whole process termed chemiosmosis), to transport hydrogen (H) through the membrane, into the thylakoid lumen, to provide a potential energy difference between the thylakoid lumen space and the chloroplast stroma, which amounts to a proton-motive force that can be used to generate ATP. The protons are transported by the plastoquinone. If electrons only pass through once, the process is termed noncyclic photophosphorylation. When the electron reaches
Photosystem
5258725
Cytochrome b6f complex The cytochrome "bf" complex (plastoquinol—plastocyanin reductase; ) is an enzyme found in the thylakoid membrane in chloroplasts of plants, cyanobacteria, and green algae, that catalyzes the transfer of electrons from plastoquinol to plastocyanin. The reaction is analogous to the reaction catalyzed by cytochrome bc (Complex III) of the mitochondrial electron transport chain. During photosynthesis, the cytochrome bf complex is one step along the chain that transfers electrons from Photosystem II to Photosystem I, and at the same time pumps protons into the thylakoid space that contribute to create an electrochemical (energy) gradient which is later used to
Cytochrome b6f complex
15135185
a slow decline. First observed by "Kautsky et al., 1960", this is called the Kautsky Effect. This variable rise in chlorophyll fluorescence rise is due to photosystem II. Fluorescence from photosystem I is not variable, but constant. The increase in fluorescence is due to PSII reaction centers being in a "closed" or chemically reduced state. Reaction centers are "closed" when unable to accept further electrons. This occurs when electron acceptors downstream of PSII have not yet passed their electrons to a subsequent electron carrier, so are unable to accept another electron. Closed reaction centres reduce the overall photochemical efficiency, and
Chlorophyll fluorescence
1722762
contains bacteriochlorophyll "a", and is taken place in chlorosomes. Type 1 reaction centre is equivalent to photosystem I found in plants and cyanobacteria. Green sulfur bacteria use sulfide ions, hydrogen or ferrous iron as electron donors and the process is mediated by the Type I reaction centre and Fenna-Matthews-Olson complex. Reaction centre contains bacteriochlorophylls, P840, which donates electrons to cytochrome c-551 when it is excited by light. Cytochrome c-551 then passes the electrons down the electron chain. P840 is returned to its reduced state by the oxidation of sulfide. Sulfide donates two electrons to yield elemental sulfur. Elemental sulfur is
Green sulfur bacteria
4011838
and excites their electrons. Since the chlorophyll "a" molecules only capture certain wavelengths, organisms may use accessory pigments to capture a wider range of light energy shown as the yellow circles. It then transfers captured light from one pigment to the next as resonance energy, passing energy one pigment to the other until reaching the special chlorophyll "a" molecules in the reaction center. These special chlorophyll "a" molecules are located in both photosystem II and photosystem I. They are known as P680 for Photosystem II and P700 for Photosystem I. P680 and P700 are the primary electron donors to the
Chlorophyll a
6258826
pigment molecules and the energy they absorb is shuttled by resonance energy transfer to a specialized chlorophyll-protein complex known as the reaction center of each photosystem. The reaction center initiates a complex series of chemical reactions that capture energy in the form of chemical bonds. For photosystem II, when either of the two chlorophyll "a" molecules at the reaction center absorb energy, an electron is excited and transferred to an electron acceptor molecule, pheophytin, leaving the chlorophyll "a" in an oxidized state. The oxidised chlorophyll "a" replaces the electrons by photolysis that involves the oxidation of water molecules to oxygen,
Light-harvesting complexes of green plants
5258728
known as heme x). The inter-monomer space within the core of the cytochrome b6f complex dimer is occupied by lipids, which provides directionality to heme-heme electron transfer through modulation of the intra-protein dielectric environment. In photosynthesis, the cytochrome bf complex functions to mediate the transfer of electrons between the two photosynthetic reaction center complexes, from Photosystem II to Photosystem I, while transferring protons from the chloroplast stroma across the thylakoid membrane into the lumen. Electron transport via cytochrome bf is responsible for creating the proton gradient that drives the synthesis of ATP in chloroplasts. In a separate reaction, the cytochrome
Cytochrome b6f complex
4900396
the initial heat energy into chemical energy (hydrogen), potentially doubling efficiency to about 50%. Because some of the energy in HTE is supplied in the form of heat, less of the energy must be converted twice (from heat to electricity, and then to chemical form), and so the process is more efficient. A version of water splitting occurs in photosynthesis, but the electrons are shunted, not to protons, but to the electron transport chain in photosystem II. The electrons are used to convert carbon dioxide into sugars. When photosystem I gets photo-excited, electron transfer reactions gets initiated, which results in
Water splitting
3980900
the cell membrane. One example of how it does this is by reacting with superoxides to form hydrogen peroxide and plastosemiquinone. The prefix "plasto-" means either plastid or chloroplast, alluding to its location within the cell. The role that plastoquinone plays in photosynthesis, more specifically in the light-dependent reactions of photosynthesis, is that of a mobile electron carrier through the membrane of the thylakoid. Plastoquinone is reduced when it accepts two electrons from photosystem II and two hydrogen cations (H) from the stroma of the chloroplast, thereby forming plastoquinol (PQH). It transfers the electrons further down the electron transport chain
Plastoquinone
4011707
to react with glycerate 3-phosphate, along with ATP to form glyceraldehyde 3-phosphate, the basic building-block from which plants can make a variety of substances. 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. Photosystems are found in the thylakoid membranes of plants, algae and cyanobacteria. They are located in the chloroplasts of plants and algae, and in the cytoplasmic membrane of photosynthetic bacteria. There are two kinds of photosystems: II and I. At the heart of
Photosystem
13691847
(hydrogen, hydrogen sulfide, sulfur, sulfite, or organic molecules such as succinate and lactate) to feed electrons into a reverse electron transport chain. Green sulfur bacteria contain a photosystem that is analogous to PS I in chloroplasts: There are two pathways of electron transfer. In "cyclic electron transfer", electrons are removed from an excited chlorophyll molecule, passed through an electron transport chain to a proton pump, and then returned to the chlorophyll. The mobile electron carriers are, as usual, a lipid-soluble quinone and a water-soluble cytochrome. The resulting proton gradient is used to make ATP. In "noncyclic electron transfer", electrons are
Light-dependent reactions
19980398
to attach to photosystem I during the night period, adding a further layer of protection to its nitrogen fixing machinery. Another method of temporarily disabling photosystem II that "Crocosphaera watsonii" employs is the deactivation of the D1 protein. The D1 protein forms the reaction center of photosystem II and is important as it binds chlorophyll. By mutating and thus deactivating D1, photosystem II is also shut down, allowing nitrogen fixation to occur. Regulating photosystem II in "Crocosphaera watsonii" is a fast process, and it can be inactivated or activated immediately when needed. The recycling of the same iron molecules in
Crocosphaera watsonii
6258827
protons and electrons. Under changing light conditions, the reversible phosphorylation of light harvesting chlorophyll a/b binding proteins (LHCII) represents a system for balancing the excitation energy between the two photosystems. The N-terminus of the chlorophyll A-B binding protein extends into the stroma where it is involved with adhesion of granal membranes and photo-regulated by reversible phosphorylation of its threonine residues. Both these processes are believed to mediate the distribution of excitation energy between photosystems I and II. This family also includes the photosystem II protein PsbS, which plays a role in energy-dependent quenching that increases thermal dissipation of excess absorbed
Light-harvesting complexes of green plants
13691845
can also synthesize ATP by oxidative phosphorylation, in the manner of other bacteria. The electron transport chain is where the mobile electron carriers are plastoquinone and cytochrome ', while the proton pumps are NADH dehydrogenase, ' and cytochrome ". Cyanobacteria are the only bacteria that produce oxygen during photosynthesis. Earth's primordial atmosphere was anoxic. Organisms like cyanobacteria produced our present-day oxygen-containing atmosphere. The other two major groups of photosynthetic bacteria, purple bacteria and green sulfur bacteria, contain only a single photosystem and do not produce oxygen. Purple bacteria contain a single photosystem that is structurally related to PS II in
Light-dependent reactions
4489791
porphyrin-derived ones) were prepared during the early 2000s. Both types of assembly are under development to improve their stability and increase their turnover numbers, both necessary for constructing a sturdy, long-lived solar fuel cell. As with water oxidation catalysis, not only fully artificial systems have been idealized: hydrogenase enzymes themselves have been engineered for photoproduction of hydrogen, by coupling the enzyme to an artificial photosensitizer, such as [Ru(bipy)] or even photosystem I. In natural photosynthesis, the NADP coenzyme is reducible to NADPH through binding of a proton and two electrons. This reduced form can then deliver the proton and electrons,
Artificial photosynthesis
3511839
oxidative phosphorylation because it uses energy released by the oxidation of NADH and FADH2 to phospolyrize ADP into ATP. The light reactions of photosynthesis generate ATP by the action of chemiosmosis. The photons in sunlight are received by the antenna complex of Photosystem II, which excites electrons to a higher energy level. These electrons travel down an electron transport chain, causing protons to be actively pumped across the thylakoid membrane into the thylakoid lumen. These protons then flow down their electrochemical potential gradient through an enzyme called ATP-synthase, creating ATP by the phosphorylation of ADP to ATP. The electrons from
Chemiosmosis
331493
electron-acceptor molecule, pheophytin. As the electrons are shuttled through an electron transport chain (the so-called Z-scheme shown in the diagram), it initially functions to generate a chemiosmotic potential by pumping proton cations (H) across the membrane and into the thylakoid space. An ATP synthase enzyme uses that chemiosmotic potential to make ATP during photophosphorylation, whereas NADPH is a product of the terminal redox reaction in the "Z-scheme". The electron enters a chlorophyll molecule in Photosystem I. There it is further excited by the light absorbed by that photosystem. The electron is then passed along a chain of electron acceptors to
Photosynthesis
7084095
that bind additional chlorophylls to increase efficiency. The overall reaction catalysed by photosystem II is: formula_1 Q represents plastoquinone, the oxidized form of Q. QH represents plastoquinol, the reduced form of Q. This process of reducing quinone is comparable to that which takes place in the bacterial reaction center. 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 with oxygen. The fact that the oxygen from green plants originated from water was first deduced by the Canadian-born American biochemist Martin
Photosynthetic reaction centre
8143057
Photosystem II produces in direct conversion of sunlight 80% of the electrons that end up in the hydrogen gas. Light-harvesting complex photosystem II light-harvesting protein LHCBM9 promotes efficient light energy dissipation. The Fe-Fe-hydrogenases need an anaerobic environment as they are inactivated by oxygen. Fourier transform infrared spectroscopy is used to examine metabolic pathways. The chlorophyll (Chl) antenna size in green algae is minimized, or truncated, to maximize photobiological solar conversion efficiency and H production. The truncated Chl antenna size minimizes absorption and wasteful dissipation of sunlight by individual cells, resulting in better light utilization efficiency and greater photosynthetic productivity by
Biohydrogen
10092235
Pheophytin Pheophytin or phaeophytin (abbreviated Pheo) is a chemical compound that serves as the first electron carrier intermediate in the electron transfer pathway of Photosystem II (PS II) in plants, and the photosynthetic reaction center (RC P870) found in purple bacteria. In both PS II and RC P870, light drives electrons from the reaction center through pheophytin, which then passes the electrons to a quinone (Q) in RC P870 and RC P680. The overall mechanisms, roles, and purposes of the pheophytin molecules in the two transport chains are analogous to each other. In biochemical terms, pheophytin is a chlorophyll molecule
Pheophytin
4011711
3BZ2 are monomeric structures of the Photosystem II dimer), , , , , , . The oxygen-evolving complex is the site of water oxidation. It is a metallo-oxo cluster comprising four manganese ions (in oxidation states ranging from +2 to +4) and one divalent calcium ion. When it oxidizes water, producing oxygen gas and protons, it sequentially delivers the four electrons from water to a tyrosine (D1-Y161) sidechain and then to P680 itself. The first structural model of the oxygen-evolving complex are solved by X-ray crystallography from frozen protein crystals with a resolution of 3.8Å in 2001. Over the next
Photosystem II
11273970
PSII, a redox-active residue in the D1 protein reduces P680, the oxidised tyrosine then withdrawing electrons from a manganese cluster, which in turn withdraw electrons from water, leading to the splitting of water and the formation of molecular oxygen. PSII thus provides a source of electrons that can be used by photosystem I to produce the reducing power (NADPH) required to convert CO to glucose. Photosynthetic reaction centre protein family Photosynthetic reaction centre proteins are main protein components of photosynthetic reaction centres of bacteria and plants. The photosynthetic apparatus in non-oxygenic bacteria consists of light-harvesting protein-pigment complexes LH1 and LH2,
Photosynthetic reaction centre protein family
12627
development of quantum mechanics and experimental findings (such as the two slit diffraction of electrons), it was found that the orbiting electrons around a nucleus could not be fully described as particles, but needed to be explained by the wave-particle duality. In this sense, the electrons have the following properties: Wave-like properties: Particle-like properties: Thus, despite the popular analogy to planets revolving around the Sun, electrons cannot be described simply as solid particles. In addition, atomic orbitals do not closely resemble a planet's elliptical path in ordinary atoms. A more accurate analogy might be that of a large and often
Atomic orbital
4011709
by the oxidation of water help to create a proton gradient that is used by ATP synthase to generate ATP. The energized electrons transferred to plastoquinone are ultimately used to reduce to NADPH or are used in Non cyclic photophosphorylation. The core of PSII consists of a pseudo-symmetric heterodimer of two homologous proteins D1 and D2. Unlike the reaction centers of all other photosystems in which the positive charge sitting on the chlorophyll dimer that undergoes the initial photoinduced charge separation is equally shared by the two monomers, in intact PSII the charge is mostly localized on one chlorophyll molecule
Photosystem II
87964
via many intermediates in the thylakoid membrane, by electrons coming, ultimately, from Photosystem II. Electron transfer reactions in the thylakoid membranes are complex, however, and the source of electrons used to reduce P700 can vary. The electron flow produced by the reaction center chlorophyll pigments is used to pump H ions across the thylakoid membrane, setting up a chemiosmotic potential used mainly in the production of ATP (stored chemical energy) or to reduce NADP to NADPH. NADPH is a universal agent used to reduce CO into sugars as well as other biosynthetic reactions. Reaction center chlorophyll–protein complexes are capable of
Chlorophyll
2385443
Plastocyanin Plastocyanin is a copper-containing protein involved in electron-transfer. The protein is a monomer, with a molecular weight around 10,500 Daltons, and 99 amino acids in most vascular plants. It is a member of the plastocyanin family of copper-binding proteins. In photosynthesis, plastocyanin functions as an electron transfer agent between cytochrome f of the cytochrome "b""f" complex from photosystem II and P700+ from photosystem I. Cytochrome "b""f" complex and P700 are both membrane-bound proteins with exposed residues on the lumen-side of the thylakoid membrane of chloroplasts. Cytochrome f acts as an electron donor while P700+ accepts electrons from reduced plastocyanin.
Plastocyanin
6516569
water molecules has been converted to four protons released into the thylakoid lumen (Dolai's S-state diagrams). These protons, as well as additional protons pumped across the thylakoid membrane coupled with the electron transfer chain, form a proton gradient across the membrane that drives photophosphorylation and thus the generation of chemical energy in the form of adenosine triphosphate (ATP). The electrons reach the P700 reaction center of photosystem I where they are energized again by light. They are passed down another electron transfer chain and finally combine with the coenzyme NADP and protons outside the thylakoids to form NADPH. Thus, the
Photodissociation
79123
they lack thylakoids organized into grana stacks—though bundle sheath chloroplasts still have free-floating thylakoids in the stroma where they still carry out cyclic electron flow, a light-driven method of synthesizing ATP to power the Calvin cycle without generating oxygen. They lack photosystem II, and only have photosystem I—the only protein complex needed for cyclic electron flow. Because the job of bundle sheath chloroplasts is to carry out the Calvin cycle and make sugar, they often contain large starch grains. Both types of chloroplast contain large amounts of chloroplast peripheral reticulum, which they use to get more surface area to transport
Chloroplast
11447752
almost all oxygen produced in tropical forests is consumed by organisms living there. A simplified overall formula for photosynthesis is: Photolytic oxygen evolution during photosynthesis occurs via the light-dependent oxidation of water to molecular oxygen and can be written as the following simplified chemical reaction: 2HO → 4e + 4H + O The reaction occurs in the thylakoid membranes of cyanobacteria, and algal and plant chloroplasts and requires the energy of four photons. The electrons from the oxidized water molecules replace electrons in the P component of photosystem II, which have been removed into an electron transport chain via light-dependent
Dioxygen in biological reactions
4011703
as terminal electron acceptors, while type II photosystems ultimately shuttle electrons to a quinone terminal electron acceptor. Both reaction center types are present in chloroplasts and cyanobacteria, and work together to form a unique photosynthetic chain able to extract electrons from water, creating oxygen as a byproduct. A reaction center comprises several (>10 or >11) protein subunits, that provide a scaffold for a series of cofactors. The cofactors can be pigments (like chlorophyll, pheophytin, carotenoids), quinones, or iron-sulfur clusters. For oxygenic photosynthesis, both photosystems I and II are required. Oxygenic photosynthesis can be performed by plants and cyanobacteria; cyanobacteria are
Photosystem
11230474
resonance (EPR) and optically detected magnetic resonance (ODMR), which can be viewed as a hybrid of PL and EPR; most details of the structure originate from EPR. A nitrogen atom has five valence electrons. Three of them covalently bond to the carbon atoms and two remain non-bonded and are called a lone pair. The vacancy has three unpaired electrons. Two of them make a quasi covalent bond and one remains unpaired. The overall symmetry, however, is axial (trigonal C); one can visualize this by imagining the three unpaired vacancy electrons continuously exchanging their roles. The N-V thus has one unpaired
Nitrogen-vacancy center
10240836
contrast aperture to pass only those electrons that contribute to that particular spot. In the image planes after the contrast aperture it is then possible to observe where the electrons originate from in real space. This technique allows scientists to study on which areas of a specimen a structure with a certain lattice vector (periodicity) exists. In photoemission electron microscopy (PEEM), upon exposure to electromagnetic radiation (photons), secondary electrons are excited from the surface and imaged. PEEM was first developed in the early 1930s, using ultraviolet (UV) light to induce photoemission of (secondary) electrons. However, since then, this technique has
Low-energy electron microscopy
331507
C plants. Electrons may also flow to other electron sinks. For this reason, it is not uncommon for authors to differentiate between work done under non-photorespiratory conditions and under photorespiratory conditions. Chlorophyll fluorescence of photosystem II can measure the light reaction, and Infrared gas analyzers can measure the dark reaction. It is also possible to investigate both at the same time using an integrated chlorophyll fluorometer and gas exchange system, or by using two separate systems together. Infrared gas analyzers and some moisture sensors are sensitive enough to measure the photosynthetic assimilation of CO, and of ΔHO using reliable methods
Photosynthesis
2575532
in the dimension perpendicular to the sample surface. Due to the lack of a third diffracting condition, the reciprocal lattice of a crystal surface is a series of infinite rods extending perpendicular to the sample’s surface. These rods originate at the conventional 2D reciprocal lattice points of the sample’s surface. The Ewald's sphere is centered on the sample surface with a radius equal to the reciprocal of the wavelength of the incident electrons. The relationship is given by formula_1, where λ is the wavelength of incident electrons. Diffraction conditions are satisfied where the rods of reciprocal lattice intersect the Ewald's
Reflection high-energy electron diffraction
3980902
at Q, the secondary binding site, which is much more easily removed. Q is only transferred a single electron, so it has to transfer an electron to Q twice before Q is able to pick up two protons from the stroma and be replaced by another plastoquinone molecule. The protonated Q then joins a pool of free plastoquinone molecules in the membrane of the thylakoid. The free plastoquinone molecules eventually transfer electrons to the water-soluble plastocyanin so as to continue the light-dependent reactions. There are additional plastoquinone binding sites within photosystem II (Q and possibly Q), but their function and/or
Plastoquinone
4011715
we know many details about this process. The oxidation of water to molecular oxygen requires extraction of four electrons and four protons from two molecules of water. The experimental evidence that oxygen is released through cyclic reaction of oxygen evolving complex (OEC) within one PSII was provided by Pierre Joliot et al. They have shown that, if dark-adapted photosynthetic material (higher plants, algae, and cyanobacteria) is exposed to a series of single turnover flashes, oxygen evolution is detected with typical period-four damped oscillation with maxima on the third and the seventh flash and with minima on the first and the
Photosystem II
15395041
0.22 + 0.76 = 0.98 V Heterogeneous water oxidation Heterogeneous Water Oxidation Water oxidation is one of the half reactions of water splitting: 2HO → O + 4H + 4e Oxidation (generation of dioxygen) 4H + 4e → 2HReduction (generation of dihydrogen) 2HO → 2H + OTotal Reaction Of the two half reactions, the oxidation step is the most demanding because it requires the coupling of 4 electron and proton transfers and the formation of an oxygen-oxygen bond. This process occurs naturally in plants photosystem II to provide protons and electrons for the photosynthesis process and release oxygen to the
Heterogeneous water oxidation
7084098
to QH as well as the uptake of two protons. The difference between photosystem II and the bacterial reaction center is the source of the electron that neutralizes the pair of chlorophyll "a" molecules. In the bacterial reaction center, 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 has taken place, the P680 molecule carries a positive charge. P680 is a very strong oxidant and extracts electrons from two water molecules that are bound at the manganese center directly below the pair. This center, below
Photosynthetic reaction centre
4433321
chain produces a proton-motive force, pumping H ions across the membrane; this produces a concentration gradient that can be used to power ATP synthase during chemiosmosis. This pathway is known as cyclic photophosphorylation, and it produces neither O nor NADPH. Unlike non-cyclic photophosphorylation, NADP+ does not accept the electrons; they are instead sent back to cytochrome bf complex. In bacterial photosynthesis, a single photosystem is used, and therefore is involved in cyclic photophosphorylation. It is favored in anaerobic conditions and conditions of high irradiance and CO compensation points. The other pathway, non-cyclic photophosphorylation, is a two-stage process involving two different
Photophosphorylation
15395031
Heterogeneous water oxidation Heterogeneous Water Oxidation Water oxidation is one of the half reactions of water splitting: 2HO → O + 4H + 4e Oxidation (generation of dioxygen) 4H + 4e → 2HReduction (generation of dihydrogen) 2HO → 2H + OTotal Reaction Of the two half reactions, the oxidation step is the most demanding because it requires the coupling of 4 electron and proton transfers and the formation of an oxygen-oxygen bond. This process occurs naturally in plants photosystem II to provide protons and electrons for the photosynthesis process and release oxygen to the atmosphere, as well as in some
Heterogeneous water oxidation
4011723
is composed of modified chlorophyll a that best absorbs light at a wavelength of 700 nm, with higher wavelengths causing bleaching. P700 receives energy from antenna molecules and uses the energy from each photon to raise an electron to a higher energy level. These electrons are moved in pairs in an oxidation/reduction process from P700 to electron acceptors. P700 has an electric potential of about −1.2 volts. The reaction center is made of two chlorophyll molecules and is therefore referred to as a dimer. The dimer is thought to be composed of one chlorophyll "a" molecule and one chlorophyll "a"′
Photosystem I
379590
Magnification is therefore controlled by the current supplied to the x, y scanning coils, or the voltage supplied to the x, y deflector plates, and not by objective lens power. The most common imaging mode collects low-energy (<50 eV) secondary electrons that are ejected from the k-shell of the specimen atoms by inelastic scattering interactions with beam electrons. Due to their low energy, these electrons originate within a few nanometers from the sample surface. The electrons are detected by an Everhart-Thornley detector, which is a type of scintillator-photomultiplier system. The secondary electrons are first collected by attracting them towards an
Scanning electron microscope
11140487
monomers and several proline rich integral membrane proteins. As of late 2007, 54 structures had been solved for this class of enzymes, with PDB accession codes , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , and . Ferredoxin: NADP reductase is the last enzyme in the transfer of electrons during photosynthesis from photosystem I to NADPH.
Ferredoxin—NADP(+) reductase
311803
in its closed electron shell, helium has only two electrons in the outermost electron orbital, while the rest of the noble gases have eight. Although scandium and yttrium are always the first two elements in group 3, the identity of the next two elements is not completely settled. They are commonly lanthanum and actinium, and less often lutetium and lawrencium. The two variants originate from historical difficulties in placing the lanthanides in the periodic table, and arguments as to where the "f" block elements start and end. It has been claimed that such arguments are proof that, "it is a
Periodic table
7942929
cluster proteins as terminal electron acceptors. Photosystem I is more complex than photosystem II (P680), as it has a more complex antenna system, has more subunits in its overall structure, and may exhibit cyclic or non-cyclic electron transfer from the excited P700* to the electron acceptor. P700 P700, or photosystem I primary donor, (where P stands for pigment) 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. The resulting P700 with excited electron
P700
17818978
material, pseudocapacitance can originate when specifically adsorbed ions pervade the double-layer, proceeding in several one-electron stages. The electrons involved in the faradaic processes are transferred to or from the electrode's valence-electron states (orbitals) and flow through the external circuit to the opposite electrode where a second double-layer with an equal number of opposite-charged ions forms. The electrons remain in the strongly ionized and electrode surface's "electron hungry" transition-metal ions and are not transferred to the adsorbed ions. This kind of pseudocapacitance has a linear function within narrow limits and is determined by the potential-dependent degree of surface coverage of the
Pseudocapacitance
4011710
(70−80%). Because of this, P680 is highly oxidizing and can take part in the splitting of water. Photosystem II (of cyanobacteria and green plants) is composed of around 20 subunits (depending on the organism) as well as other accessory, light-harvesting proteins. Each photosystem II contains at least 99 cofactors: 35 chlorophyll a, 12 beta-carotene, two pheophytin, two plastoquinone, two heme, one bicarbonate, 20 lipid molecules, the cluster (including two chloride ions), one non heme and two putative ions per monomer. There are several crystal structures of photosystem II. The PDB accession codes for this protein are , , (3BZ1 and
Photosystem II
7942928
P700 P700, or photosystem I primary donor, (where P stands for pigment) 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. The resulting P700 with excited electron is designated P700*, and is the strongest biological reducing agent (in contrast to P680 of photosystem II, the strongest biological oxidizing agent). It has an estimated -1.2V of redox potential. The electron is subsequently captured by the primary electron acceptor. Type I photosystems use ferredoxin-like iron-sulfur
P700
2530731
for constructing Lewis structures, eliminating the need for electron counting: the atoms are drawn showing the valence electrons; bonds are then formed by pairing up valence electrons of the atoms involved in the bond-making process, and anions and cations are formed by adding or removing electrons to/from the appropriate atoms. A trick is to count up valence electrons, then count up the number of electrons needed to complete the octet rule (or with hydrogen just 2 electrons), then take the difference of these two numbers and the answer is the number of electrons that make up the bonds. The rest
Lewis structure
2343309
of acceleration and thus the electrons emit radiation at GeV energies. At a synchrotron facility, electrons are usually accelerated by a synchrotron, and then injected into a storage ring, in which they circulate, producing synchrotron radiation, but without gaining further energy. The radiation is projected at a tangent to the electron storage ring and captured by beamlines. These beamlines may originate at bending magnets, which mark the corners of the storage ring; or insertion devices, which are located in the straight sections of the storage ring. The spectrum and energy of X-rays differ between the two types. The beamline includes
Synchrotron light source
4011702
by light-harvesting complexes that enhance the absorption of light. Two families of reaction centers in photosystems exist: type I reaction centers (such as photosystem I (P700) in chloroplasts and in green-sulphur bacteria and type II reaction centers (such as photosystem II (P680) in chloroplasts and in non-sulphur purple bacteria. Each photosystem can be identified by the wavelength of light to which it is most reactive (700 and 680 nanometers, respectively for PSI and PSII in chloroplasts), the amount and type of light-harvesting complexes present and the type of terminal electron acceptor used. Type I photosystems use ferredoxin-like iron-sulfur cluster proteins
Photosystem
4928165
P680 P680, or Photosystem II primary donor, (where P stands for pigment) refers to either of the two special chlorophyll dimers (also named special pairs), P or P. 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. The 680 number is its absorption maximum in the red part of the visible spectrum (680 nm). The primary donor receives excitation energy either by absorbing a photon of suitable frequency (colour) or by excitation energy transfer from other chlorophylls within photosystem
P680
4928166
II. During excitation, an electron is excited to a higher energy level. This electron is subsequently captured by the primary electron acceptor, a pheophytin molecule located within photosystem II near P680. The oxidized P680 (P680) is subsequently reduced by an electron originating from water (via Oxygen evolving complex). Oxidized P680 (P680) is the strongest biological oxidizing agent known. It has an estimated redox potential of ~1.3 V. This makes it possible to oxidize water during oxygenic photosynthesis. P680 P680, or Photosystem II primary donor, (where P stands for pigment) refers to either of the two special chlorophyll dimers (also named
P680
87961
hundred molecules per photosystem) is to absorb light. Having done so, these same centers execute their second function: the transfer of that light energy by resonance energy transfer to a specific chlorophyll pair in the reaction center of the photosystems. This pair effects the final function of chlorophylls, charge separation, leading to biosynthesis. The two currently accepted photosystem units are photosystem II and photosystem I, which have their own distinct reaction centres, named P680 and P700, respectively. These centres are named after the wavelength (in nanometers) of their red-peak absorption maximum. The identity, function and spectral properties of the types
Chlorophyll
1615173
Auger electrons emissions, although the internal conversion process involves neither beta nor gamma decay. A neutrino is not emitted, and none of the electron(s) and photon(s) emitted originate in the nucleus, even though the energy to emit all of them does originate there. Internal conversion decay, like isomeric transition gamma decay and neutron emission, involves the release of energy by an excited nuclide, without the transmutation of one element into another. Rare events that involve a combination of two beta-decay type events happening simultaneously are known (see below). Any decay process that does not violate the conservation of energy or
Radioactive decay
17557107
to form oxygen, as well as the Mehler Reaction, in which oxygen is reduced to form and, finally, the scavenging of this by antioxidants to form water. Beginning in the 1970s, Professor Kozi Asada elucidated that oxygen can be reduced by electrons emerging from ferredoxin of photosystem I, to form superoxide, which is then reduced by superoxide dismutase to form . This photochemical is then reduced by the action of ascorbate peroxidase to form water and oxidized ascorbate. Asada argued that oxygen presents an important sink for excess excitation energy acquired during plant exposure to bright light. He would often
Mehler reaction
11273966
to produce viruses or for replication of the viral genome. These can include reverse transcriptases, integrases, nucleases or other enzymes. Photosystem components do not fit this mould either. The production of an active photosystem during viral infection provides active photosynthesis to dying cells. This is not viral altruism towards the host, however. The problem with viral infections tends to be that they disable the host relatively rapidly. As protein expression is shunted from the host genome to the viral genome, the photosystem degrades relatively rapidly (due in part to the interaction with light, which is highly corrosive), cutting off the
Photosynthetic reaction centre protein family
331492
plants, light-dependent reactions occur in the thylakoid membranes of the chloroplasts where they drive the synthesis of ATP and NADPH. The light-dependent reactions are of two forms: cyclic and non-cyclic. In the non-cyclic reaction, the photons are captured in the light-harvesting antenna complexes of photosystem II by chlorophyll and other accessory pigments (see diagram at right). The absorption of a photon by the antenna complex frees an electron by a process called photoinduced charge separation. The antenna system is at the core of the chlorophyll molecule of the photosystem II reaction center. That freed electron is transferred to the primary
Photosynthesis
12717803
in "Nature" the finding of a surplus of high energy electrons. During a 5-week observatory period in 2000 and 2003, ATIC counted 70 electrons with energies in the range 300-800 GeV; these electrons were in excess of those expected from the galactic background. The source of these electrons is unknown, but it is assumed to be relatively close, no more than about 3000 lightyears away, since high energy electrons rapidly lose energy as they travel through the galactic magnetic field and collide with photons. The electrons could originate from a nearby pulsar or other astrophysical object, but the researchers were
Advanced Thin Ionization Calorimeter
11273965
imply that the gene would not be transferred among hosts - which the photosystem within viruses is - but instead that there is a viral function for the gene, that it is not merely hitchhiking with the virus). However, free viruses lack the machinery needed to support metabolism, let alone photosynthesis. As a result, photosystem genes are not likely to be a functional component of the virus like a capsid protein or tail fibre. Instead, it is expressed within an infected host cell. Most virus genes that are expressed in the host context are useful for hijacking the host machinery
Photosynthetic reaction centre protein family
751967
are also formed within and between protamines in the sperm chromatin of many mammalian species. As disulfide bonds can be reversibly reduced and re-oxidized, the redox state of these bonds has evolved into a signaling element. In chloroplasts, for example, the enzymatic reduction of disulfide bonds has been linked to the control of numerous metabolic pathways as well as gene expression. The reductive signaling activity has been shown, thus far, to be carried by the ferredoxin thioredoxin system, channeling electrons from the light reactions of photosystem I to catalytically reduce disulfides in regulated proteins in a light dependent manner. In
Disulfide
17697654
light harvesting material that they employ, and the mode of electron transfer from the biological material to the anode. The light harvesting materials employed in biological photovoltaic devices can be categorised by their complexity; more complex materials are typically less efficient but more robust. Isolated photosystems offer the most direct connection between water photolysis and anode reduction. Typically, photosystems are isolated and adsorbed to a conductive surface. A soluble redox mediator (a small molecule capable of accepting and donating electrons) may be required to improve the electrical communication between photosystem and anode. Because other cellular components required for repair are
Biological photovoltaics
3874151
what most people refer to as an "SOFC stack". The ceramics used in SOFCs do not become electrically and ionically active until they reach very high temperature and as a consequence, the stacks have to run at temperatures ranging from 500 to 1,000 °C. Reduction of oxygen into oxygen ions occurs at the cathode. These ions can then diffuse through the solid oxide electrolyte to the anode where they can electrochemically oxidize the fuel. In this reaction, a water byproduct is given off as well as two electrons. These electrons then flow through an external circuit where they can do
Solid oxide fuel cell
8584208
transmission of pathogens from parent to offspring; it is uncommon for pathogens to be transmitted from the plant to its seeds (in sexual reproduction or in apomixis), though there are occasions when it occurs. Seeds generated by apomixis are a means of asexual reproduction, involving the formation and dispersal of seeds that do not originate from the fertilization of the embryos. Hawkweed ("Hieracium"), dandelion ("Taraxacum"), some Citrus ("Citrus") and Kentucky blue grass ("Poa pratensis") all use this form of asexual reproduction. Pseudogamy occurs in some plants that have apomictic seeds, where pollination is often needed to initiate embryo growth, though
Plant reproduction
9938605
split a water molecule into its protons and electrons for photosynthesis. Free oxygen, generated as a by-product of this reaction, is released into the atmosphere. Water oxidation is catalyzed by a manganese-containing cofactor contained in photosystem II known as the oxygen-evolving complex (OEC) or water-splitting complex. Manganese is an important cofactor, and calcium and chloride are also required for the reaction to occur.The stoichiometry this reaction follows: The protons are released into the thylakoid lumen, thus contributing to the generation of a proton gradient across the thylakoid membrane. This proton gradient is the driving force for ATP synthesis via photophosphorylation
Oxygen evolution
7084299
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 complex A light-harvesting complex has 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
Light-harvesting complex
13300990
and electrons (or rather, electron density) do not move around so neatly and discretely in reality. Recently, arrow pushing has been extended to inorganic chemistry, especially to the chemistry of s- and p-block elements. It has been shown to work well for hypervalent compounds. Organic chemists use two types of arrows within molecular structures to describe electron movements. Single electrons' trajectories are designated with single barbed arrows, whereas double-barbed arrows show movement of electron pairs. When a bond is broken, electrons leave where the bond was and this is represented by a curved arrow pointing away from the bond and
Arrow pushing
561393
in organic chemistry for compounds such as amine oxides for which the electronic structure can be described in terms of the basic amine donating two electrons to an oxygen atom. The arrow → indicates that both electrons in the bond originate from the amine moiety. In a standard covalent bond each atom contributes one electron. Therefore, an alternative description is that the amine gives away one electron to the oxygen atom, which is then used, with the remaining unpaired electron on the nitrogen atom, to form a standard covalent bond. The process of transferring the electron from nitrogen to oxygen
Coordinate covalent bond
14640068
wavelength of 700 nm in the photosystem I, corresponding to an energy of each photon of ≈ 2 eV ≈ 3 x 10 J ≈ 75 kT, where kT denotes the thermal energy. A minimum of 48 photons is needed for the synthesis of a single glucose molecule from CO and water (chemical potential difference 5 x 10 J) with a maximal energy conversion efficiency of 35%. Photon energy Photon energy is the energy carried by a single photon. The amount of energy is directly proportional to the photon's electromagnetic frequency and inversely proportional to the wavelength. The higher the
Photon energy
7994074
DCMU DCMU (3-(3,4-dichlorophenyl)-1,1-dimethylurea) is an algicide and herbicide of the arylurea class that inhibits photosynthesis. It was introduced by Bayer in 1954 under the trade name of Diuron. DCMU is a very specific and sensitive inhibitor of photosynthesis. It blocks the plastoquinone binding site of photosystem II, disallowing the electron flow from where it is generated, in photosystem II, to plastoquinone. This interrupts the photosynthetic electron transport chain in photosynthesis and thus reduces the ability of the plant to turn light energy into chemical energy (ATP and reductant potential). DCMU only blocks electron flow from photosystem II, it has no
DCMU
4011728
photosystem of green sulfur bacteria. Photosystem I Photosystem I (PSI, or plastocyanin-ferredoxin oxidoreductase) is the second photosystem in the photosynthetic light reactions of algae, plants, and some bacteria. Photosystem I is an integral membrane protein complex that uses light energy to produce the high energy carriers ATP and NADPH. PSI comprises more than 110 cofactors, significantly more than photosystem II. This photosystem is known as PSI because it was discovered before photosystem II. Aspects of PSI were discovered in the 1950s, but the significances of these discoveries was not yet known. Louis Duysens first proposed the concepts of photosystems I
Photosystem I
11279972
NADPH. The ATP and NADPH produced by PSII and PSI, respectively, are used in the light-independent reactions for the formation of organic compounds. This process is non-cyclic, because the electron from PSII is lost and is only replenished through the oxidation of water. Hence, there is a constant flow of electrons and associated hydrogens from water for the formation of organic compounds. It is this stripping of hydrogens from water that produces the oxygen we breathe. IsiA has an inverse relationship with the Iron stress repressed RNA (IsrR). IsrR is an antisense RNA that acts as a reversible switch that
Photosystem II light-harvesting protein
7994075
effect on photosystem I or other reactions in photosynthesis, such as light absorption or carbon fixation in the Calvin cycle. However, because it absorbs electrons oxidized from water in PS II, the electron "hole" of PS I cannot be satisfied, effectively shutting down "linear" photosynthesis by blocking the reduction of NADP to NADPH. At the same time, it was found that DCMU not only does not inhibit the cyclic photosynthetic pathway, but, under certain circumstances, actually stimulates it. Because of these effects, DCMU is often used to study energy flow in photosynthesis. DCMU DCMU (3-(3,4-dichlorophenyl)-1,1-dimethylurea) is an algicide and herbicide
DCMU
4489779
include structural H-cluster models, a dirhodium photocatalyst, and cobalt catalysts. Water oxidation is a more complex chemical reaction than proton reduction. In nature, the oxygen-evolving complex performs this reaction by accumulating reducing equivalents (electrons) in a manganese-calcium cluster within photosystem II (PS II), then delivering them to water molecules, with the resulting production of molecular oxygen and protons: Without a catalyst (natural or artificial), this reaction is very endothermic, requiring high temperatures (at least 2500 K). The exact structure of the oxygen-evolving complex has been hard to determine experimentally. As of 2011, the most detailed model was from a 1.9
Artificial photosynthesis