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Light-dependent reactions is jargon for certain
photochemical reaction Organic photochemistry encompasses organic reactions that are induced by the action of light. The absorption of ultraviolet light by organic molecules often leads to reactions. In the earliest days, sunlight was employed, while in more modern times ...
s that are involved in
photosynthesis Photosynthesis is a process used by plants and other organisms to convert light energy into chemical energy that, through cellular respiration, can later be released to fuel the organism's activities. Some of this chemical energy is stored in ...
, the main process by which plants acquire energy. There are two light dependent reactions, the first occurs at photosystem II (PSII) and the second occurs at photosystem I (PSI), PSII absorbs a photon to produce a so-called high energy electron which transfers via an
electron transport chain An electron transport chain (ETC) is a series of protein complexes and other molecules that transfer electrons from electron donors to electron acceptors via redox reactions (both reduction and oxidation occurring simultaneously) and couples ...
to cytochrome ''bf'' and then to PSI. The then-reduced PSI, absorbs another photon producing a more highly reducing electron, which converts NADP to NADPH. In
oxygenic photosynthesis Photosynthesis is a process used by plants and other organisms to convert light energy into chemical energy that, through cellular respiration, can later be released to fuel the organism's activities. Some of this chemical energy is stored in ...
, the first electron donor is
water Water (chemical formula ) is an inorganic, transparent, tasteless, odorless, and nearly colorless chemical substance, which is the main constituent of Earth's hydrosphere and the fluids of all known living organisms (in which it acts as ...
, creating
oxygen Oxygen is the chemical element with the symbol O and atomic number 8. It is a member of the chalcogen group in the periodic table, a highly reactive nonmetal, and an oxidizing agent that readily forms oxides with most elements ...
(O2) as a by-product. In anoxygenic photosynthesis various electron donors are used. Cytochrome ''b6f'' and ATP synthase work together to produce ATP ( photophosphorylation) in two distinct ways. In non-cyclic photophosphorylation, cytochrome ''b6f'' uses electrons from PSII and energy from PSI to pump
protons A proton is a stable subatomic particle, symbol , H+, or 1H+ with a positive electric charge of +1 ''e'' elementary charge. Its mass is slightly less than that of a neutron and 1,836 times the mass of an electron (the proton–electron m ...
from the stroma to the lumen. The resulting proton gradient across the thylakoid membrane creates a proton-motive force, used by ATP synthase to form ATP. In cyclic photophosphorylation, cytochrome ''b6f'' uses electrons and energy from PSI to create more ATP and to stop the production of NADPH. Cyclic phosphorylation is important to create ATP and maintain NADPH in the right proportion for the light-independent reactions. The net-reaction of all light-dependent reactions in oxygenic photosynthesis is: :2 + 2 + 3ADP + 3P → + 2 H + 2NADPH + 3ATP PSI and PSII are light-harvesting complexes. If a special pigment molecule in a photosynthetic reaction center absorbs a photon, an
electron The electron ( or ) is a subatomic particle with a negative one elementary electric charge. Electrons belong to the first generation of the lepton particle family, and are generally thought to be elementary particles because they have n ...
in this pigment attains the
excited state In quantum mechanics, an excited state of a system (such as an atom, molecule or nucleus) is any quantum state of the system that has a higher energy than the ground state (that is, more energy than the absolute minimum). Excitation refers to ...
and then is transferred to another
molecule A molecule is a group of two or more atoms held together by attractive forces known as chemical bonds; depending on context, the term may or may not include ions which satisfy this criterion. In quantum physics, organic chemistry, and b ...
in the reaction center. This reaction, called photoinduced charge separation, is the start of the electron flow and transforms light energy into chemical forms.


Light dependent reactions

In
chemistry Chemistry is the scientific study of the properties and behavior of matter. It is a natural science that covers the elements that make up matter to the compounds made of atoms, molecules and ions: their composition, structure, proper ...
, many reactions depend on the absorption of photons to provide the energy needed to overcome the
activation energy In chemistry and physics, activation energy is the minimum amount of energy that must be provided for compounds to result in a chemical reaction. The activation energy (''E''a) of a reaction is measured in joules per mole (J/mol), kilojoules p ...
barrier and hence can be labelled light-dependent. Such reactions range from the
silver halide A silver halide (or silver salt) is one of the chemical compounds that can form between the Chemical element, element silver (Ag) and one of the halogens. In particular, bromine (Br), chlorine (Cl), iodine (I) and fluorine (F) may each combine wi ...
reactions used in photographic film to the creation and destruction of
ozone Ozone (), or trioxygen, is an inorganic molecule with the chemical formula . It is a pale blue gas with a distinctively pungent smell. It is an allotrope of oxygen that is much less stable than the diatomic allotrope , breaking down in the l ...
in the
upper atmosphere Upper atmosphere is a collective term that refers to various layers of the atmosphere of the Earth above the troposphere and corresponding regions of the atmospheres of other planets, and includes: * The mesosphere, which on Earth lies between th ...
. This article discusses a specific subset of these, the series of light-dependent reactions related to
photosynthesis Photosynthesis is a process used by plants and other organisms to convert light energy into chemical energy that, through cellular respiration, can later be released to fuel the organism's activities. Some of this chemical energy is stored in ...
in living organisms.


The reaction center

The reaction center is in the thylakoid membrane. It transfers absorbed light energy to a
dimer Dimer may refer to: * Dimer (chemistry), a chemical structure formed from two similar sub-units ** Protein dimer, a protein quaternary structure ** d-dimer * Dimer model, an item in statistical mechanics, based on ''domino tiling'' * Julius Dimer ( ...
of
chlorophyll Chlorophyll (also chlorophyl) is any of several related green pigments found in cyanobacteria and in the chloroplasts of algae and plants. Its name is derived from the Greek words , ("pale green") and , ("leaf"). Chlorophyll allow plants to ...
pigment molecules near the periplasmic (or thylakoid lumen) side of the membrane. This dimer is called a special pair because of its fundamental role in photosynthesis. This special pair is slightly different in PSI and PSII reaction centers. In PSII, it absorbs photons with a wavelength of 680 nm, and is therefore called
P680 P680, or photosystem II primary donor, is the reaction-center chlorophyll ''a'' molecular dimer associated with photosystem II in plants, algae, and cyanobacteria, and central to oxygenic photosynthesis. Etymology Its name is derived from th ...
. In PSI, it absorbs photons at 700 nm and is called P700. In bacteria, the special pair is called P760, P840, P870, or P960. "P" here means pigment, and the number following it is the wavelength of light absorbed. Electrons in pigment molecules can exist at specific energy levels. Under normal circumstances, they are at the lowest possible energy level, the ground state. However, absorption of light of the right photon energy can lift them to a higher energy level. Any light that has too little or too much energy cannot be absorbed and is reflected. The electron in the higher energy level is unstable and will quickly return to its normal lower energy level. To do this, it must release the absorbed energy. This can happen in various ways. The extra energy can be converted into molecular motion and lost as heat, or re-emitted by the electron as light (
fluorescence Fluorescence is the emission of light by a substance that has absorbed light or other electromagnetic radiation. It is a form of luminescence. In most cases, the emitted light has a longer wavelength, and therefore a lower photon energy, tha ...
). The energy, but not the electron itself, may be passed onto another molecule; this is called
resonance energy transfer Resonance describes the phenomenon of increased amplitude that occurs when the frequency of an applied periodic force (or a Fourier component of it) is equal or close to a natural frequency of the system on which it acts. When an oscilla ...
. If an
electron The electron ( or ) is a subatomic particle with a negative one elementary electric charge. Electrons belong to the first generation of the lepton particle family, and are generally thought to be elementary particles because they have n ...
of the special pair in the reaction center becomes excited, it cannot transfer this energy to another pigment using resonance energy transfer. Under normal circumstances, the electron would return to the ground state, but because the reaction center is arranged so that a suitable electron acceptor is nearby, the excited electron is taken up by the acceptor. The loss of the electron gives the special pair a positive charge and, as an ionization process, further boosts its energy. The formation of a positive charge on the special pair and a negative charge on the acceptor is referred to as photoinduced charge separation. The electron can be transferred to another molecule. As the ionized pigment returns to the ground state, it takes up an electron and gives off energy to the oxygen evolving complex so it can split water into electrons, protons, and molecular oxygen (after receiving energy from the pigment four times). Plant pigments usually utilize the last two of these reactions to convert the sun's energy into their own. This initial charge separation occurs in less than 10
picosecond A picosecond (abbreviated as ps) is a unit of time in the International System of Units (SI) equal to 10−12 or (one trillionth) of a second. That is one trillionth, or one millionth of one millionth of a second, or 0.000 000 000&nbs ...
s (10 seconds). In their high-energy states, the special pigment and the acceptor could undergo charge recombination; that is, the
electron The electron ( or ) is a subatomic particle with a negative one elementary electric charge. Electrons belong to the first generation of the lepton particle family, and are generally thought to be elementary particles because they have n ...
on the acceptor could move back to neutralize the positive charge on the special pair. Its return to the special pair would waste a valuable high-energy electron and simply convert the absorbed light energy into heat. In the case of PSII, this backflow of electrons can produce reactive oxygen species leading to photoinhibition. Three factors in the structure of the reaction center work together to suppress charge recombination nearly completely: * Another
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. Electron acceptors are sometimes mista ...
is less than 1 nanometer away from the first acceptor, and so the electron is rapidly transferred farther away from the special pair. * An electron donor is less than 1 nm away from the special pair, and so the positive charge is neutralized by the transfer of another electron. * The electron transfer back from the electron acceptor to the positively charged special pair is especially slow. The rate of an electron transfer reaction increases with its thermodynamic favorability up to a point and then decreases. The back transfer is so favorable that it takes place in the inverted region where electron-transfer rates become slower. Thus, electron transfer proceeds efficiently from the first electron acceptor to the next, creating an electron transport chain that ends when it has reached
NADPH Nicotinamide adenine dinucleotide phosphate, abbreviated NADP or, in older notation, TPN (triphosphopyridine nucleotide), is a cofactor used in anabolic reactions, such as the Calvin cycle and lipid and nucleic acid syntheses, which require NA ...
.


In chloroplasts

The
photosynthesis Photosynthesis is a process used by plants and other organisms to convert light energy into chemical energy that, through cellular respiration, can later be released to fuel the organism's activities. Some of this chemical energy is stored in ...
process in chloroplasts begins when an
electron The electron ( or ) is a subatomic particle with a negative one elementary electric charge. Electrons belong to the first generation of the lepton particle family, and are generally thought to be elementary particles because they have n ...
of
P680 P680, or photosystem II primary donor, is the reaction-center chlorophyll ''a'' molecular dimer associated with photosystem II in plants, algae, and cyanobacteria, and central to oxygenic photosynthesis. Etymology Its name is derived from th ...
of PSII attains a higher-energy level. This energy is used to reduce a chain of
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. Electron acceptors are sometimes mista ...
s that have subsequently higher
redox Redox (reduction–oxidation, , ) is a type of chemical reaction in which the oxidation states of substrate change. Oxidation is the loss of electrons or an increase in the oxidation state, while reduction is the gain of electrons or ...
potentials. This chain of electron acceptors is known as an
electron transport chain An electron transport chain (ETC) is a series of protein complexes and other molecules that transfer electrons from electron donors to electron acceptors via redox reactions (both reduction and oxidation occurring simultaneously) and couples ...
. When this chain reaches PSI, an electron is again excited, creating a high redox-potential. The electron transport chain of photosynthesis is often put in a diagram called the
Z-scheme Light-dependent reactions is jargon for certain photochemical reactions that are involved in photosynthesis, the main process by which plants acquire energy. There are two light dependent reactions, the first occurs at photosystem II (PSII) and ...
, because the
redox Redox (reduction–oxidation, , ) is a type of chemical reaction in which the oxidation states of substrate change. Oxidation is the loss of electrons or an increase in the oxidation state, while reduction is the gain of electrons or ...
diagram from P680 to P700 resembles the letter Z. The final product of PSII is plastoquinol, a mobile electron carrier in the membrane. Plastoquinol transfers the electron from PSII to the proton pump, cytochrome b6f. The ultimate electron donor of PSII is water. Cytochrome b6f transfers the electron chain to PSI through plastocyanin molecules. PSI can continue the electron transfer in two different ways. It can transfer the electrons either to plastoquinol again, creating a cyclic electron flow, or to an enzyme called FNR ( Ferredoxin—NADP(+) reductase), creating a non-cyclic electron flow. PSI releases FNR into the stroma, where it reduces to
NADPH Nicotinamide adenine dinucleotide phosphate, abbreviated NADP or, in older notation, TPN (triphosphopyridine nucleotide), is a cofactor used in anabolic reactions, such as the Calvin cycle and lipid and nucleic acid syntheses, which require NA ...
. Activities of the electron transport chain, especially from cytochrome ''b6f'', lead to pumping of
proton A proton is a stable subatomic particle, symbol , H+, or 1H+ with a positive electric charge of +1 ''e'' elementary charge. Its mass is slightly less than that of a neutron and 1,836 times the mass of an electron (the proton–electron mass ...
s from the stroma to the lumen. The resulting transmembrane proton gradient is used to make ATP via
ATP synthase ATP synthase is a protein that catalyzes the formation of the energy storage molecule adenosine triphosphate (ATP) using adenosine diphosphate (ADP) and inorganic phosphate (Pi). It is classified under ligases as it changes ADP by the formation ...
. The overall process of the photosynthetic electron transport chain in chloroplasts is:


Photosystem II

PSII is extremely complex, a highly organized transmembrane structure that contains a ''
water splitting Water splitting is the chemical reaction in which water is broken down into oxygen and hydrogen: :2 H2O → 2 H2 + O2 Efficient and economical water splitting would be a technological breakthrough that could underpin a hydrogen economy, base ...
complex'', chlorophylls and carotenoid pigments, a '' reaction center'' (P680), pheophytin (a pigment similar to chlorophyll), and two quinones. It uses the energy of sunlight to transfer electrons from water to a mobile electron carrier in the membrane called ''
plastoquinone Plastoquinone (PQ) is an isoprenoid quinone molecule involved in the electron transport chain in the light-dependent reactions of photosynthesis. The most common form of plastoquinone, known as PQ-A or PQ-9, is a 2,3-dimethyl-1,4-benzoquinone ...
'': Plastoquinol, in turn, transfers electrons to cyt ''bf'', which feeds them into PSI.


The water-splitting complex

The step → P680 is performed by an imperfectly understood structure embedded within PSII called the ''water-splitting complex'' or '' oxygen-evolving complex'' ''(OEC)''. It catalyzes a reaction that splits water into electrons, protons and oxygen, using energy from P680. The actual steps of the above reaction possibly occur in the following way (Kok's diagram of S-states): (I) 2 (monoxide) (II) OH. (hydroxide) (III) (peroxide) (IV) (super oxide)(V) (di-oxygen). (Dolai's mechanism) The electrons are transferred to special chlorophyll molecules (embedded in PSII) that are promoted to a higher-energy state by the energy of
photons A photon () is an elementary particle that is a quantum of the electromagnetic field, including electromagnetic radiation such as light and radio waves, and the force carrier for the electromagnetic force. Photons are Massless particle, massless ...
.


The reaction center

The excitation P680 → P680 of the reaction center pigment P680 occurs here. These special chlorophyll molecules embedded in PSII absorb the energy of photons, with maximal absorption at 680 nm. Electrons within these molecules are promoted to a higher-energy state. This is one of two core processes in photosynthesis, and it occurs with astonishing efficiency (greater than 90%) because, in addition to direct excitation by light at 680 nm, the energy of light first harvested by ''antenna proteins'' at other wavelengths in the light-harvesting system is also transferred to these special chlorophyll molecules. This is followed by the electron transfer P680→ pheophytin, and then on to plastoquinol, which occurs within the reaction center of PSII. The electrons are transferred to plastoquinone and two protons, generating plastoquinol, which released into the membrane as a mobile electron carrier. This is the second core process in photosynthesis. The initial stages occur within ''picoseconds'', with an efficiency of 100%. The seemingly impossible efficiency is due to the precise positioning of molecules within the reaction center. This is a
solid-state Solid state, or solid matter, is one of the four fundamental states of matter. Solid state may also refer to: Electronics * Solid-state electronics, circuits built of solid materials * Solid state ionics, study of ionic conductors and their ...
process, not a typical chemical reaction. It occurs within an essentially crystalline environment created by the macromolecular structure of PSII. The usual rules of chemistry (which involve random collisions and random energy distributions) do not apply in solid-state environments.


Link of water-splitting complex and chlorophyll excitation

When the excited chlorophyll P passes the electron to pheophytin, it converts to high-energy P, which can oxidize the tyrosineZ (or YZ) molecule by ripping off one of its hydrogen atoms. The high-energy oxidized tyrosine gives off its energy and returns to the ground state by taking up a proton and removing an electron from the oxygen-evolving complex and ultimately from water. Kok's S-state diagram shows the reactions of water splitting in the oxygen-evolving complex.


Summary

PSII is a transmembrane structure found in all chloroplasts. It splits water into electrons, protons and molecular oxygen. The electrons are transferred to plastoquinol, which carries them to a proton pump. The oxygen is released into the atmosphere. The emergence of such an incredibly complex structure, a macromolecule that converts the energy of sunlight into chemical energy and thus potentially useful work with efficiencies that are impossible in ordinary experience, seems almost magical at first glance. Thus, it is of considerable interest that, in essence, the same structure is found in
purple bacteria Purple bacteria or purple photosynthetic bacteria are Gram-negative proteobacteria that are phototrophic, capable of producing their own food via photosynthesis. They are pigmented with bacteriochlorophyll ''a'' or ''b'', together with variou ...
.


Cytochrome ''bf''

PSII and PSI are connected by a transmembrane proton pump, cytochrome ''bf'' complex (plastoquinol—plastocyanin reductase; ). Electrons from PSII are carried by plastoquinol to cyt ''bf'', where they are removed in a stepwise fashion (re-forming plastoquinone) and transferred to a water-soluble electron carrier called '' plastocyanin''. This redox process is coupled to the pumping of four protons across the membrane. The resulting proton gradient (together with the proton gradient produced by the water-splitting complex in PSI) is used to make ATP via ATP synthase. The structure and function of cytochrome ''bf'' (in chloroplasts) is very similar to cytochrome ''bc1'' (''Complex III'' in mitochondria). Both are transmembrane structures that remove electrons from a mobile, lipid-soluble electron carrier (plastoquinone in chloroplasts; ubiquinone in mitochondria) and transfer them to a mobile, water-soluble electron carrier (plastocyanin in chloroplasts; cytochrome ''c'' in mitochondria). Both are proton pumps that produce a transmembrane proton gradient. In fact, cytochrome b6 and subunit IV are homologous to mitochondrial cytochrome b and the Rieske iron-sulfur proteins of the two complexes are homologous. However, cytochrome f and cytochrome c1 are not homologous.


Photosystem I

PSI accepts electrons from plastocyanin and transfers them either to NADPH ('' noncyclic electron transport'') or back to cytochrome ''bf'' ('' cyclic electron transport''): plastocyanin → P700 → P700* → FNR → NADPH ↑ ↓ ''bf'' ← phylloquinone PSI, like PSII, is a complex, highly organized transmembrane structure that contains antenna chlorophylls, a reaction center (P700), phylloquinone, and a number of iron-sulfur proteins that serve as intermediate redox carriers. The light-harvesting system of PSI uses multiple copies of the same transmembrane proteins used by PSII. The energy of absorbed light (in the form of delocalized, high-energy electrons) is funneled into the reaction center, where it excites special chlorophyll molecules (P700, with maximum light absorption at 700 nm) to a higher energy level. The process occurs with astonishingly high efficiency. Electrons are removed from excited chlorophyll molecules and transferred through a series of intermediate carriers to ''
ferredoxin Ferredoxins (from Latin ''ferrum'': iron + redox, often abbreviated "fd") 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. and applied t ...
'', a water-soluble electron carrier. As in PSII, this is a solid-state process that operates with 100% efficiency. There are two different pathways of electron transport in PSI. In ''noncyclic electron transport'', ferredoxin carries the electron to the enzyme ferredoxin reductase (FNR) that reduces to NADPH. In ''cyclic electron transport'', electrons from ferredoxin are transferred (via plastoquinol) to a proton pump, cytochrome ''bf''. They are then returned (via plastocyanin) to P700. NADPH and ATP are used to synthesize organic molecules from . The ratio of NADPH to ATP production can be adjusted by adjusting the balance between cyclic and noncyclic electron transport. It is noteworthy that PSI closely resembles photosynthetic structures found in green sulfur bacteria, just as PSII resembles structures found in purple bacteria.


In bacteria

PSII, PSI, and cytochrome ''b6f'' are found in chloroplasts. All plants and all photosynthetic algae contain chloroplasts, which produce NADPH and ATP by the mechanisms described above. In essence, the same transmembrane structures are also found in ''cyanobacteria''. Unlike plants and algae, cyanobacteria are prokaryotes. They do not contain chloroplasts; rather, they bear a striking resemblance to chloroplasts themselves. This suggests that organisms resembling cyanobacteria were the evolutionary precursors of chloroplasts. One imagines primitive eukaryotic cells taking up cyanobacteria as intracellular symbionts in a process known as endosymbiosis.


Cyanobacteria

Cyanobacteria Cyanobacteria (), also known as Cyanophyta, are a phylum of gram-negative bacteria that obtain energy via photosynthesis. The name ''cyanobacteria'' refers to their color (), which similarly forms the basis of cyanobacteria's common name, bl ...
contain both PSI and PSII. Their light-harvesting system is different from that found in plants (they use ''
phycobilin Phycobilins (from Greek: '' (phykos)'' meaning "alga", and from Latin: ''bilis'' meaning "bile") are light-capturing bilins found in cyanobacteria and in the chloroplasts of red algae, glaucophytes and some cryptomonads (though not in green ...
s'', rather than chlorophylls, as antenna pigments), but their electron transport chain → PSII → plastoquinol → ''b6f'' → cytochrome ''c6'' → PSI → ferredoxin → NADPH ↑ ↓ ''b6f'' ← plastoquinol is, in essence, the same as the electron transport chain in chloroplasts. The mobile water-soluble electron carrier is cytochrome ''c6'' in cyanobacteria, having been replaced by plastocyanin in plants. Cyanobacteria can also synthesize ATP by oxidative phosphorylation, in the manner of other bacteria. The electron transport chain is NADH dehydrogenase → plastoquinol → ''b6f'' → cyt ''c6'' → cyt ''aa3'' → where the mobile electron carriers are plastoquinol and cytochrome ''c6'', while the proton pumps are NADH dehydrogenase, cyt ''b6f'' and cytochrome ''aa3'' (member of the COX3 family). 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

Purple bacteria Purple bacteria or purple photosynthetic bacteria are Gram-negative proteobacteria that are phototrophic, capable of producing their own food via photosynthesis. They are pigmented with bacteriochlorophyll ''a'' or ''b'', together with variou ...
contain a single photosystem that is structurally related to PSII in cyanobacteria and chloroplasts: : P870 → P870* → ubiquinone → cyt ''bc1'' → cyt ''c2'' → P870 This is a ''cyclic'' process in which electrons are removed from an excited chlorophyll molecule (''bacteriochlorophyll''; P870), passed through an electron transport chain to a proton pump (cytochrome ''bc1'' complex; similar to the chloroplastic one), and then returned to the chlorophyll molecule. The result is a proton gradient that is used to make ATP via ATP synthase. As in cyanobacteria and chloroplasts, this is a solid-state process that depends on the precise orientation of various functional groups within a complex transmembrane macromolecular structure. To make NADPH, purple bacteria use an external electron donor (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

Green sulfur bacteria contain a photosystem that is analogous to PSI in chloroplasts: P840 → P840* → ferredoxin → NADH ↑ ↓ cyt ''c553'' ← ''bc1'' ← menaquinol 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 removed from an excited chlorophyll molecule and used to reduce NAD+ to NADH. The electrons removed from P840 must be replaced. This is accomplished by removing electrons from , which is oxidized to sulfur (hence the name "green ''sulfur'' bacteria").
Purple bacteria Purple bacteria or purple photosynthetic bacteria are Gram-negative proteobacteria that are phototrophic, capable of producing their own food via photosynthesis. They are pigmented with bacteriochlorophyll ''a'' or ''b'', together with variou ...
and green sulfur bacteria occupy relatively minor ecological niches in the present day biosphere. They are of interest because of their importance in
precambrian The Precambrian (or Pre-Cambrian, sometimes abbreviated pꞒ, or Cryptozoic) is the earliest part of Earth's history, set before the current Phanerozoic Eon. The Precambrian is so named because it preceded the Cambrian, the first period of th ...
ecologies, and because their methods of photosynthesis were the likely evolutionary precursors of those in modern plants.


History

The first ideas about light being used in photosynthesis were proposed by
Colin Flannery Colin may refer to: * Colin (given name) * Colin (surname) * ''Colin'' (film), a 2008 Cannes film festival zombie movie * Colin (horse) (1905–1932), thoroughbred racehorse * Colin (humpback whale), a humpback whale calf abandoned north of Sydney, ...
in 1779 who recognized it was sunlight falling on plants that was required, although
Joseph Priestley Joseph Priestley (; 24 March 1733 – 6 February 1804) was an English chemist, natural philosopher, separatist theologian, grammarian, multi-subject educator, and liberal political theorist. He published over 150 works, and conducted ...
had noted the production of oxygen without the association with light in 1772. Cornelis Van Niel proposed in 1931 that photosynthesis is a case of general mechanism where a photon of light is used to photo decompose a hydrogen donor and the hydrogen being used to reduce . Then in 1939, Robin Hill demonstrated that isolated chloroplasts would make oxygen, but not fix , showing the light and dark reactions occurred in different places. Although they are referred to as light and dark reactions, both of them take place only in the presence of light. This led later to the discovery of photosystems I and II.


See also

* Light-independent reaction * Photosynthetic reaction centre *
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 photosyst ...
* Compensation point


References

{{DEFAULTSORT:Light-Dependent Reactions Photosynthesis