Light-dependent reactions - AbsoluteAstronomy.com

The 'light-dependent reactions', or light reactions, are the first stage of photosynthesis

Photosynthesis

Photosynthesis is a chemical process that converts carbon dioxide into organic compounds, especially sugars, using the energy from sunlight. Photosynthesis occurs in plants, algae, and many species of bacteria, but not in archaea. Photosynthetic organisms are called photoautotrophs, since they can...

, the process by which plant

Plant

Plants are living organisms belonging to the kingdom Plantae. Precise definitions of the kingdom vary, but as the term is used here, plants include familiar organisms such as trees, flowers, herbs, bushes, grasses, vines, ferns, mosses, and green algae. The group is also called green plants or...

s capture and store energy

Energy

In physics, energy is an indirectly observed quantity. It is often understood as the ability a physical system has to do work on other physical systems...

from sunlight

Sunlight

Sunlight, in the broad sense, is the total frequency spectrum of electromagnetic radiation given off by the Sun. On Earth, sunlight is filtered through the Earth's atmosphere, and solar radiation is obvious as daylight when the Sun is above the horizon.When the direct solar radiation is not blocked...

. In this process, light energy is converted into chemical energy

Chemical energy

Chemical energy is the potential of a chemical substance to undergo a transformation through a chemical reaction or, to transform other chemical substances...

, in the form of the energy-carrying molecules ATP

Adenosine triphosphate

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

and NADPH. In the light-independent reactions, the formed NADPH and ATP

Adenosine triphosphate

drive the reduction of to more useful organic compounds, such as glucose

Glucose

Glucose is a simple sugar and an important carbohydrate in biology. Cells use it as the primary source of energy and a metabolic intermediate...

. However, although light-independent reactions are, by convention, also called dark reactions, they are not independent of the need of light, for they are driven by ATP and NADPH, products of light.

The light-dependent reactions take place on the thylakoid

Thylakoid

A thylakoid is a membrane-bound compartment inside chloroplasts and cyanobacteria. They are the site of the light-dependent reactions of photosynthesis. Thylakoids consist of a thylakoid membrane surrounding a thylakoid lumen. Chloroplast thylakoids frequently form stacks of disks referred to as...

membrane inside a chloroplast

Chloroplast

Chloroplasts are organelles found in plant cells and other eukaryotic organisms that conduct photosynthesis. Chloroplasts capture light energy to conserve free energy in the form of ATP and reduce NADP to NADPH through a complex set of processes called photosynthesis.Chloroplasts are green...

. The inside of the thylakoid membrane is called the lumen

Lumen (anatomy)

A lumen in biology is the inside space of a tubular structure, such as an artery or intestine...

, and outside the thylakoid membrane is the stroma

Stroma (fluid)

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

, where the light-independent reactions take place. The thylakoid membrane contains some integral membrane protein

Integral membrane protein

An integral membrane protein is a protein molecule that is permanently attached to the biological membrane. Proteins that cross the membrane are surrounded by "annular" lipids, which are defined as lipids that are in direct contact with a membrane protein...

complexes that catalyze the light reactions. There are four major protein complexes in the thylakoid membrane: Photosystem I

Photosystem I

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

(PSI), Photosystem II

Photosystem II

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

(PSII), Cytochrome b6f complex

Cytochrome b6f complex

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

, and ATP synthase

ATP synthase

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

. These four complexes work together to ultimately create the products ATP and NADPH.

The two photosystems absorb light energy through protein

Protein

Proteins are biochemical compounds consisting of one or more polypeptides typically folded into a globular or fibrous form, facilitating a biological function. A polypeptide is a single linear polymer chain of amino acids bonded together by peptide bonds between the carboxyl and amino groups of...

s containing pigment

Pigment

A pigment is a material that changes the color of reflected or transmitted light as the result of wavelength-selective absorption. This physical process differs from fluorescence, phosphorescence, and other forms of luminescence, in which a material emits light.Many materials selectively absorb...

s, such as chlorophyll

Chlorophyll

Chlorophyll is a green pigment found in almost all plants, algae, and cyanobacteria. Its name is derived from the Greek words χλωρος, chloros and φύλλον, phyllon . Chlorophyll is an extremely important biomolecule, critical in photosynthesis, which allows plants to obtain energy from light...

. The light-dependent reactions begin in photosystem II. When a chlorophyll a

Chlorophyll a

Chlorophyll a is a specific form of chlorophyll used in oxygenic photosynthesis. It absorbs most energy from wavelengths of violet-blue and orange-red light. This photosynthetic pigment is essential for photosynthesis in eukaryotes, cyanobacteria and prochlorophytes because of its role as primary...

molecule within the reaction center of PSII absorbs a photon

Photon

In physics, a photon is an elementary particle, the quantum of the electromagnetic interaction and the basic unit of light and all other forms of electromagnetic radiation. It is also the force carrier for the electromagnetic force...

, an electron

Electron

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

in this molecule attains a higher energy level

Energy level

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

. Because this state of an electron is very unstable, the electron is transferred from one to another molecule creating a chain of redox reactions

Redox

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

, called an electron transport chain

Electron transport chain

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

(ETC). The electron flow goes from PSII to cytochrome b6f to PSI. In PSI, the electron gets the energy from another photon. The final electron acceptor is NADP. In oxygenic photosynthesis, the first electron donor is water

Water

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

, creating oxygen

Oxygen

Oxygen is the element with atomic number 8 and represented by the symbol O. Its name derives from the Greek roots ὀξύς and -γενής , because at the time of naming, it was mistakenly thought that all acids required oxygen in their composition...

as a waste product. In anoxygenic photosynthesis various electron donors are used.

Cytochrome b6f and ATP synthase work together to create ATP

Adenosine triphosphate

. This process is called photophosphorylation

Photophosphorylation

The production of ATP using the energy of sunlight is called photophosphorylation. Only two sources of energy are available to living organisms: sunlight and reduction-oxidation reactions...

, which occurs in two different ways. In non-cyclic photophosphorylation, cytochrome b6f uses the energy of electrons from PSII to pump protons from the stroma to the lumen. The proton gradient across the thylakoid membrane creates a proton-motive force, used by ATP synthase to form ATP. In cyclic photophosphorylation, cytochrome b6f uses the energy of electrons from not only PSII but also 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_i → + 2NADPH + 3ATP
The two photosystem

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...

s are protein

Protein

complexes that absorb photons and are able to use this energy to create an electron transport chain

Electron transport chain

. Photosystem I and II are very similar in structure and function. They use special proteins, called light-harvesting complex

Light-harvesting complex

A light-harvesting complex is a complex of subunit proteins that may be part of a larger supercomplex of a photosystem, the functional unit in photosynthesis. It is used by plants and photosynthetic bacteria to collect more of the incoming light than would be captured by the photosynthetic reaction...

es, to absorb the photons with very high effectiveness. If a special pigment molecule in a photosynthetic reaction center absorbs a photon, an electron

Electron

in this pigment attains the excited state

Excited state

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

and then is transferred to another molecule

Molecule

A molecule is an electrically neutral group of at least two atoms held together by covalent chemical bonds. Molecules are distinguished from ions by their electrical charge...

in the reaction center. This reaction, called photoinduced charge separation

Photoinduced charge separation

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

, is the start of the electron flow and is unique because it transforms light energy into chemical forms.

The light-harvesting system

A common misconception is that photosynthesis

Photosynthesis

relies only on chlorophyll

Chlorophyll

pigments. The truth is that
photosynthesis would be rather inefficient using only chlorophyll molecules. Chlorophyll molecules absorb light only at specific wavelengths (see image). A large gap is present in the middle of the visible regions between approximately 450 and 650 nm. This gap corresponds to the peak of the solar spectrum, so failure to collect this light would constitute a considerable opportunity. That's why photosynthesis organisms have developed a light-harvesting system, which bundles different pigments to create a much wider absorption spectrum.

The light harvesting-system is composed of numerous light-harvesting complex

Light-harvesting complex

es that completely surround the reaction center where the Photoinduced charge separation

Photoinduced charge separation

takes place. Chlorophyll

Chlorophyll

, carotene

Carotene

The term carotene is used for several related unsaturated hydrocarbon substances having the formula C40Hx, which are synthesized by plants but cannot be made by animals. Carotene is an orange photosynthetic pigment important for photosynthesis. Carotenes are all coloured to the human eye...

s, and xanthophylls are arranged in such light-harvesting complexes or LHC-proteins. These pigments are referred to as accessory pigments and funnel the energy to a special pigment in the reaction center of PSI or PSII.

If a pigment

Pigment

molecule absorbs a photon

Photon

, an electron

Electron

in the molecule becomes excited. For most compounds that absorb light, the electron simply returns to the ground state and the absorbed energy is converted into heat and/or fluorescence. But, in a LHC-protein, the pigments are so arranged that the excitation energy can be transferred from one molecule to a nearby molecule. The rate of this process, called resonance energy transfer, depends strongly on the distance between the energy donor and energy acceptor molecules. For reasons of conservation of energy, energy transfer must be from a donor in the excited state to an acceptor of equal or lower energy. If the energy of a photon becomes lower, the wavelength

Wavelength

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

also becomes longer. When chlorophyll is isolated from the enzymes it is associated with, the second scenario can be seen to happen. The pigments in an LHC-protein are so arranged that pigments are very close to each other and a pigment is near another pigment that absorbs photons with a longer wavelength. As a consequence, the pigment in the reaction center has to absorb photons with the longest wavelength and cannot transfer this energy to another pigment. The function of LHC-proteins is to create a constant supply of excitation-energy to the reaction center pigment. Every reaction center has a couple of LHC-proteins.

The reaction center

(chlorophyll)

The reaction center is in the thylakoid membrane. It transfers light energy to a dimer of chlorophyll

Chlorophyll

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 center. In PSII, it absorbs photons with a wavelength of 680 nm, and it is therefore called P680

P680

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

. In PSI, it absorbs photons at 700 nm, and it is called P700

P700

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

. In bacteria, the special pair is called P760, P840, P870, or P960.

If an electron

Electron

of the special pair in the reaction center becomes excited, it cannot transfer this energy to another pigment using resonance energy transfer. In normal circumstances, the electron should return to the ground state, but, because the reaction center is so arranged that a suitable electron acceptor is nearby, the excited electron can move from the initial molecule to the acceptor. This process results in the formation of a positive charge on the special pair (due to the loss of an electron) and a negative charge on the acceptor and is, hence, referred to as photoinduced charge separation

Photoinduced charge separation

. In other words, electrons in pigment molecules can exist at specific energy levels. Under normal circumstances, they exist at the lowest possible energy level they can. However, if there is enough energy to move them into the next energy level, they can absorb that energy and occupy that higher energy level. The light they absorb contains the necessary amount of energy needed to push them into the next level. Any light that does not have enough or has too much energy cannot be absorbed and is reflected. The electron in the higher energy level, however, does not want to be there; the electron is unstable and must return to its normal lower energy level. To do this, it must release the energy that has put it into the higher energy state to begin with. This can happen various ways. The extra energy can be converted into molecular motion and lost as heat. Some of the extra energy can be lost as heat energy, while the rest is lost as light. This re-emission of light energy is called florescence. The energy, but not the e- itself, can be passed onto another molecule. This is called resonance. The energy and the e- can be transferred to another molecule. 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

Picosecond

A picosecond is 10−12 of a second. That is one trillionth, or one millionth of one millionth of a second, or 0.000 000 000 001 seconds. A picosecond is to one second as one second is to 31,700 years....

s (10⁻¹¹ seconds). In their high-energy states, the special pigment and the acceptor could undergo charge recombination; that is, the electron

Electron

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. Three factors in the structure of the reaction center work together to suppress charge recombination nearly completely.

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

is less than 10 Å
Ångström
The angstrom or ångström, is a unit of length equal to 1/10,000,000,000 of a meter . Its symbol is the Swedish letter Å....

away from the first acceptor, and so the electron is rapidly transferred farther away from the special pair.
An electron donor is less than 10 Å
Ångström
The angstrom or ångström, is a unit of length equal to 1/10,000,000,000 of a meter . Its symbol is the Swedish letter Å....

away from the special pair, and so the positive charge is neutralized by the transfer of another electron
The electron transfer from the electron acceptor to the positively charged special pair is especially slow: The transfer is so thermodynamically
Thermodynamics
Thermodynamics is a physical science that studies the effects on material bodies, and on radiation in regions of space, of transfer of heat and of work done on or by the bodies or radiation...

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 if it has reached NADPH.

Photosynthetic electron transport chains in chloroplasts

The photosynthesis

Photosynthesis

process in chloroplasts begins when an electron

Electron

of P680

P680

of PSII attains an higher-energy level. This energy is used to reduce a chain of electron acceptor

Electron acceptor

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

s that have subsequently lowered redox

Redox

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

-potentials. This chain of electron acceptors is known as an electron transport chain

Electron transport chain

. When this chain reaches PS I, 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, because the redox

Redox

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

diagram from P680 to P700 resembles the letter z.

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 proceeds the electron chain to PSI through plastocyanin

Plastocyanin

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

molecules. PSI is able to 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, creating a non-cyclic electron flow. PSI releases FNR into the stroma

Stroma (fluid)

, where it reduces to NADPH.

Activities of the electron transport chain, especially from cytochrome b6f, lead to pumping of protons from the stroma to the lumen. The resulting transmembrane proton

Proton

The proton is a subatomic particle with the symbol or and a positive electric charge of 1 elementary charge. One or more protons are present in the nucleus of each atom, along with neutrons. The number of protons in each atom is its atomic number....

gradient is used to make ATP via ATP synthase

ATP synthase

.

The overall process of the photosynthetic electron transport chain in chloroplasts is:

→ PS II → plastoquinone → cyt → plastocyanin → PS I → NADPH

Photosystem II

PS II is an extremely complex, highly organized transmembrane structure that contains a water-splitting 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

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

:

' → P680 → P680^* → plastoquinone

Plastoquinone, in turn, transfers electrons to ', which feeds them into PS I.

The water-splitting complex
The step → P680 is performed by a poorly-understood structure embedded within PS II called the water-splitting complex or the oxygen-evolving complex. It catalyzes a reaction that splits water into electrons, protons and oxygen:

2 → 4H⁺ + 4e^- + '

The electrons are transferred to special chlorophyll molecules (embedded in PS II) that are promoted to a higher-energy state by the energy of photons.

The reaction center
The excitation P680 → P680^*of the reaction center pigment P680 occurs here. These special chlorophyll molecules embedded in PS II 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 step P680^*→ pheophytin, and then on to plastoquinone, which occurs within the reaction center of PS II. High-energy electrons are transferred to plastoquinone. Plastoquinone is then 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 physics

Solid-state physics is the study of rigid matter, or solids, through methods such as quantum mechanics, crystallography, electromagnetism, and metallurgy. It is the largest branch of condensed matter physics. Solid-state physics studies how the large-scale properties of solid materials result from...

process, not a chemical reaction. It occurs within an essentially crystalline environment created by the macromolecular structure of PS II. 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 chlorophyll passes the electron to pheophytin, it obtains an electron from P₆₈₀^*. In turn, P₆₈₀^* can oxidize the Z (or Y_Z) molecule. Once oxidized, the Z molecule can derive electrons from the water-splitting complex.

Summary

PS II is a transmembrane structure found in all chloroplasts. It splits water into electrons, protons and molecular oxygen. The electrons are transferred to plastoquinone, which carries them to a proton pump. Molecular oxygen is released into the atmosphere.

The emergence of such an incredibly complex structure, a macromolecule that converts the energy of sunlight into 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.

Cytochrome '

PS II and PS I are connected by a transmembrane proton pump, cytochrome ' complexCytochrome b6f complex
The cytochrome b6f complex is an enzyme found in the thylakoid membrane in chloroplasts of plants, cyanobacteria, and green algae, catalyzing the transfer of electrons from plastoquinol to plastocyanin...

(plastoquinol—plastocyanin reductase; . Electrons from PS II are carried by plastoquinone to ', where they are removed in a stepwise fashion and transferred to a water-soluble electron carrier called plastocyanin

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 PS II) is used to make ATP via ATP synthase.

The similarity in structure and function between cytochrome ' (in chloroplasts) and cytochrome ' (Complex III in mitochondria) is striking. 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.

Photosystem I

PS I accepts electrons from plastocyanin and transfers them either to NADPH (noncyclic electron transport) or back to cytochrome ' (cyclic electron transport):

plastocyanin → P700 → P700^* → FNR → NADPH
↑ ↓
' ← plastoquinone

PS I, like PS II, is a complex, highly organized transmembrane structure that contains antenna chlorophylls, a reaction center (P700), phylloquinine, and a number of iron-sulfur protein

Iron-sulfur protein

Iron-sulfur proteins are proteins characterized by the presence of iron-sulfur clusters containing sulfide-linked di-, tri-, and tetrairon centers in variable oxidation states...

s that serve as intermediate redox carriers.

The light-harvesting system of PS I uses multiple copies of the same transmembrane proteins used by PS II. 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, 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

Ferredoxin

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

, a water-soluble electron carrier. As in PS II, this is a solid-state process that operates with 100% efficiency.

There are two different pathways of electron transport in PS I. In noncyclic electron transport, ferredoxin carries the electron to the enzyme ferredoxin oxidoreductase that reduces to NADPH. In cyclic electron transport, electrons from ferredoxin are transferred (via plastoquinone) to a proton pump, cytochrome '. 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 PS I closely resembles photosynthetic structures found in green sulfur bacteria, just as PS II resembles structures found in purple bacteria.

Photosynthetic electron transport chains in bacteria

PS II, PS I, and cytochrome' 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.

Cyanobacteria

Cyanobacteria contain structures similar to PS II and PS I in chloroplasts. Their light-harvesting system is different from that found in plants (they use phycobilins, rather than chlorophylls, as antenna pigments), but their electron transport chain

' → PS II → plastoquinone → ' → cytochrome ' → PS I → ferredoxin → NADPH
↑ ↓
' ← plastoquinone

is, in essence, the same as the electron transport chain in chloroplasts. The mobile water-soluble electron carrier is cytochrome ' in cyanobacteria, plastocyanin in plants.

Cyanobacteria can also synthesize ATP by oxidative phosphorylation, in the manner of other bacteria. The electron transport chain is

NADH dehydrogenase → plastoquinone → ' → cytochrome ' → cytochrome ' → '

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

Purple bacteria contain a single photosystem that is structurally related to PS II in cyanobacteria and chloroplasts:

P870 → P870^* → ubiquinone → ' → cytochrome → 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 ' complex, similar but not identical to cytochrome ' in chloroplasts), and then returned to the cholorophyll molecule. The result is a proton gradient, which 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 PS I in chloroplasts:

P840 → P840^* → ferredoxin → NADH
↑ ↓
cyt c₅₅₃ ← ' ← menaquinone

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 and green sulfur bacteria occupy relatively minor ecological niches in the present day biosphere. They are of interest because of their importance in precambrian

Precambrian

The Precambrian is the name which describes the large span of time in Earth's history before the current Phanerozoic Eon, and is a Supereon divided into several eons of the geologic time scale...

ecologies, and because their methods of photosynthesis were the likely evolutionary precursors of those in modern plants.

History

The first ideas about light's being used in photosynthesis were proposed by Colin Flannery in 1779 who recognized it was sunlight falling on plants that was required, although Joseph Priestly had noted the production of oxygen without the association with light in 1772.
Cornelius Van Niel

Cornelius Van Niel

Cornelis Bernardus van Niel was a Dutch-American microbiologist. He introduced the study of general microbiology to the United States and made key discoveries explaining the chemistry of photosynthesis.In 1923, Cornelis van Niel married Christina van Hemert, graduated in chemical engineering at...

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

Robin Hill (biochemist)

Robert Hill FRS , known as Robin Hill, was a British plant biochemist who, in 1939, demonstrated the 'Hill reaction' of photosynthesis, proving that oxygen is evolved during the light requiring steps of photosynthesis...

showed that isolated chloroplasts would make oxygen, but not fix showing the light and dark reactions occurred in different places. This led later to the discovery of photosystem

Photosystem

1 and 2.

The light-harvesting system

The reaction center

Photosynthetic electron transport chains in chloroplasts

Photosystem II

The water-splitting complex

The reaction center

Link of water-splitting complex and chlorophyll excitation

Summary

Cytochrome '

Photosystem I

Photosynthetic electron transport chains in bacteria

Cyanobacteria

Purple bacteria

Green sulfur bacteria

History

See also