In photosynthesis, the light-dependent reaction occurs in the thylakoid membrane. The inside of the thylakoid membrane is called the lumen, and outside of the thylakoid membrane is the stroma, where an independent light reaction occurs. The thylakoid membrane contains several integral membrane protein complexes that catalyze the reaction of light. There are four major protein complexes in the thylakoid membrane: Photosystem II (PSII), cytochrome b6f complex, Photosystem I (PSI), and ATP synthase. These four complexes work together to eventually create ATP and NADPH products.
Four photo systems absorb light energy through the pigment - especially chlorophyll, which is responsible for the green color of the leaves. Light dependent reactions begin in photosystem II. When the chlorophyll molecule a inside the PSII reaction center absorbs the photon, the electrons in this molecule reach a higher energy level. Because the state of this electron is so unstable, electrons transferred from one to another create a redox reaction chain, called the electron transport chain (ETC). Electrons flow from PSII to cytochrome b6f to PSI. In PSI, electrons get energy from other photons. The last electron acceptor is NADP. In oxygen photosynthesis, the first electron donor is water, creating oxygen as a waste product. In photosynthesis anoxygenic various electron donors are used.
Cytochrome b6f and ATP synthase work together to create ATP. This process is called photophosphorylation, which occurs in two different ways. In non-cyclic photophosphorylation, cytochrome b6f uses the electron energy of PSII to pump protons from the stroma to the lumen. The proton gradient across the thylakoid membrane creates a proton motive force, which is used by ATP synthase to form ATP. In cyclic photophosphorylation, cytochrome b6f uses electron energy from not only PSII but also PSI to create more ATP and stop production of NADPH. Cyclic phosphorylation is important for creating ATP and maintaining NADPH in the right proportion for light-independent reactions.
The net reaction of all light dependent reactions to oxygen photosynthesis is:
2 H
2 O 2 NADP
3ADP 3P i -> O
2 2NADPH 3ATP
Both photo systems are complex proteins that absorb photons and can use this energy to create an electron transport chain. Photosystem I and II are very similar in structure and function. They use a special protein, called a light harvesting complex, to absorb photons with very high effectiveness. If the special pigment molecule at the photosynthetic reaction center absorbs the photons, the electrons in this pigment reach the excited state and then transfer to another molecule at the reaction center. This reaction, called photo-induced photo splitting, is the beginning of electron flow and is unique in that it converts light energy into chemical forms.
Video Light-dependent reactions
Reaction center
The reaction center is in the thylakoid membrane. It transfers light energy to the chlorophyll pigment molecular dimer near the side of the periplasmic membrane (or the lumen tilakoid). This dimer is called a special couple because of its fundamental role in photosynthesis. These special pairs differ slightly in the PSI and PSII reaction centers. In PSII, it absorbs photons of 680 nm wavelength, and is therefore called P680. In PSI, it absorbs photons at 700 m, and it's called P700. In bacteria, a special pair is called P760, P840, P870, or P960. Where "P" means pigment, and the number following it is the wavelength of the absorbed light.
If an electron from a special pair in the center of the reaction becomes excited, it can not transfer this energy to another pigment using the transfer of resonance energy. Under normal circumstances, the electrons must return to their ground state, but, since the reaction center is adjusted so that the corresponding electron acceptor is nearby, the excited electrons can move from the initial molecule to the acceptor. This process results in the formation of a positive charge on a particular pair (due to the loss of electrons) and a negative charge on the acceptor and, hence, referred to as photo-induced charge separation. In other words, electrons in pigment molecules can exist at specific energy levels. Under normal circumstances, they are at the lowest possible energy level they can. However, if there is enough energy to move it to the next energy level, they can absorb that energy and occupy a higher energy level. The light they absorb contains the amount of energy needed to push it to the next level. Any light that is not enough or has too much energy can not be absorbed and reflected. However, electrons in higher energy levels do not want to be there; electrons are unstable and must return to normal lower energy levels. To do this, he must release the energy that has put him into a higher energy state to begin with. This can happen in many ways. Extra energy can be transformed into molecular motion and lost as heat. Some extra energy can be lost as heat energy, while the rest is lost as light. (This re-emission of light energy is called fluorescence.) Energy, but not e-itself, can be passed on to other molecules. (This is called resonance.) Energy and e-can be transferred to other molecules. Plant pigments typically use the last two reactions to convert solar energy into their own energy.
This initial charge separation occurs in less than 10 picoseconds (10 -11 sec). In high-energy states, special pigments and acceptors may undergo recombination of the load; ie, the electrons in the acceptor can move back to neutralize the positive charge on the particular pair. The return to a particular partner will waste valuable high-energy electrons and only convert the energy of light absorbed into heat. In the case of PSII, these return electrons can produce reactive oxygen species leading to photographs. Three factors in the structure of the reaction center work together to reduce the cost of recombination almost completely.
- The other electron acceptor is less than 10 ÃÆ'... away from the first acceptor, so the electrons are quickly moved further away from the special pair.
- The electron donor is less than 10 ÃÆ'â € | away from the special pair, so the positive charge is neutralized by another electron transfer
- Transfer electrons back from the electron acceptor to the positively charged positive pair is very slow. The rate of electron transfer reaction increases with thermodynamic preferences up to a point and then decreases. The reverse transfers are very advantageous that occur in the inverse region where the electron transfer rate becomes slower.
Thus, the electron transfer takes place efficiently from the first electron acceptor to the next, creating an electron transport chain that ends when it reaches NADPH.
Maps Light-dependent reactions
The electron transport chain of photosynthesis in chloroplast
The process of photosynthesis in chloroplasts begins when the P680 electron from PSII reaches a higher energy level. This energy is used to reduce the electron acceptor chain which then decreases the redox potential. This electron acceptor chain is known as the electron transport chain. When this chain reaches PS I, the electrons re-energize, creating a high redox potential. The electron transport chain of photosynthesis is often incorporated into a diagram called the z-scheme, since the redox diagrams of P680 to P700 resemble the letter z.
The final product of PSII is plastoquinol, the moving electron carrier in the membrane. Plastoquinol transfers electrons from PSII to a proton pump, cytochrome b6f. The last electron donor from PSII is water. The cytochrome b6f produces an electron chain to the PSI via a plastocyanine molecule. PSI is able to resume electron transfer in two different ways. It can either transfer electrons to plastoquinol again, creating a cyclic electron stream, or to an enzyme called FNR (Ferredoxin - NADP () reductase), creating a non-cyclic electron stream. PSI releases FNR into the stroma, where it reduces NADP
to NADPH.
The activity of the electron transport chain, especially of the b6f cytochrome, causes the pumping of protons from the stroma to the lumen. The transmembrane generated emission gradient is used to make ATP through ATP synthase.
The overall process of the electron transport chain of photosynthesis in chloroplasts is:
2 O -> PS II -> plastoquinone -> cyt b
6 plastocyanin -> PS I -> NADPH
Photosystem II
PS II is a highly complex, highly organized transmembrane structure containing a water-solubling complex, chlorophyll and carotenoid pigments, reaction centers (P680), pheophytins (similar pigments) for chlorophyll) and two quinones. Uses solar energy to transfer electrons from water to moving electron carriers in a membrane called plastoquinone :
2 O -> P680 -> P680 * -> plastoquinone
Plastoquinone, in turn, transfers the electrons to cyt b 6 , which put it in PS I.
Water-splitting complex
Step H
2 O -> P680 is done by a poorly understood structure embedded in a PS II called the water dividing complex or an oxygen evolving complex . It catalyzes the reaction that divides water into electrons, protons and oxygen:
2 O -> 4H 4e - O
2
The actual steps of the above reaction proceed in the following way (Dolai diagram of the S-state): (I) 2H2O (monoxide) (II) OH. H2O (hydroxide) (III) H2O2 (peroxide) (IV) HO2 (super oxide) (V) O2 (di-oxygen).
The electrons are transferred to a special chlorophyll molecule (embedded in PS II) that is promoted to a higher energy state by the photon energy.
Reaction center
Excitation P680 -> P680 * reaction center pigment P680 occurs here. This special chlorophyll molecule embedded in PS II absorbs photon energy, with maximum absorption at 680 nm. Electrons in these molecules are promoted to a higher energy state. This is one of two core processes in photosynthesis, and it occurs with remarkable efficiency (over 90%) because, in addition to direct excitation by light at 680 nm, light energy is first harvested by antenna protein at other wavelengths in the light harvesting system are also transferred to this particular chlorophyll molecule.
This is followed by the P680 * -> pheophytin step, and then to plastoquinone , which occurs inside the PS II reaction center. High-energy electrons are transferred to plastoquinone before then taking two protons to become plastoquinol. Plastoquinol is then released into the membrane as a moving electron carrier.
This is the second core process in photosynthesis. Initial stages occur within picoseconds , with 100% efficiency. The seemingly impossible efficiency is due to the exact position of the molecule inside the reaction center. It is a solid-state process, not a chemical reaction. This occurs in the basic crystal environment created by the PS II macromolecular structure. Ordinary chemical rules (involving random collisions and random energy distributions) do not apply in solid-state environments.
Complex separation link and water chlorophyll
When chlorophyll passes electrons to pheophytin, it obtains an electron from P 680 * . In turn, P 680 * can oxidize the Z molecule (or Y Z ). Once oxidized, the Z molecule can decrease the electrons from the oxygen evolving complex (OEC). The Dolai S-state diagram shows the water splitting reaction in an oxygen evolving complex.
Summary
PS II is the transmembrane structure found in all chloroplasts. It divides water into electrons, protons and molecular oxygen. The electrons are transferred to the plastoquinone, which takes it to the proton pump. The oxygen molecule is released into the atmosphere.
The emergence of a very complex structure, the macromolecules that convert sunlight energy into potentially useful work with unlikely efficiency in ordinary experience, seem almost miraculous at first glance. Thus, it is interesting that, in essence, the same structure is found in purple bacteria.
Cytochrome b
6
PS II and PS I are connected by transmembrane proton pump, cytochrome b
6 complex ; EC 1.10.99.1). Electrons from PS II are carried by plastoquinol to cyt b
6 , where they removed gradually fashion ( reformed plastoquinone) and transferred to a water-soluble electron carrier called plastocyanin . This redox process is coupled with pumping four protons across the membrane. The resulting proton gradient (together with the proton gradient produced by the water-solving complex in PS II) is used to make ATP through ATP synthase.
The similarity in structure and function between cytochrome b 6 (in chloroplasts) and cytochrome bc
1 ( Complex III in mitochondria) is striking. Both are transmembrane structures that remove electrons from fat-soluble mobile carriers (plastoquinone in chloroplasts, ubiquinone in mitochondria) and transfer them to water-soluble electron carriers (plastocyanin in chloroplasts, cytochrome c in the mitochondria). Both are proton pumps that produce transmembrane proton gradients. Photosystem I
PS Saya menerima elektron dari plastocyanin dan mentransfernya ke NADPH ( transport elektron non-siklik ) atau kembali ke sitokrom b
6 ( transport elektron siklik ):
plastocyanin -> P700 -> P700 * -> FNR - > NADPH                              ? ?                            b
6 <- plastoquinone PS I, like PS II, is a highly organized structural transmembrane structure containing antenna chlorophyll, reaction center (P700), phylloquinine, and a number of iron sulfur proteins that function as medium redox carriers.
The PS I light harvesting system uses many copies of the same transmembrane protein used by PS II. The energy of absorbed light (in the form of a delocalized and high-energy electron) is supplied to the reaction center, where it generates a special chlorophyll molecule (P700, maximum absorption of light at 700 nm) to a higher energy level. This process occurs with very high efficiency.
Electrons are excreted from excited chlorophyll molecules and transferred through a series of intermediate carriers to ferredoxin, the water-soluble electron carriers. As in PS II, this is a solid-state process that operates with 100% efficiency.
There are two different electron transport paths in PS I. In non-cyclic electron transport , ferredoxin carries electrons to the ferredoxin enzyme NADP
reductase (FNR) reducing NADP to NADPH. In cyclic electron transport , electrons from ferredoxin are transferred (via plastoquinone) to the proton pump, cytochrome b
6 . They are then returned (via plastocyanin) to the P700.
NADPH and ATP are used to synthesize organic molecules from CO
2 . The ratio of NADPH production to ATP can be adjusted by adjusting the balance between cyclic and non-cyclic electron transport.
It should be noted that PS I is very similar to the photosynthetic structure found in green sulfur bacteria, just as PS II resembles structures found in purple bacteria.
The transport chain of photosynthetic electrons in bacteria
PS II, PS I, and cytochrome b
< sub style = "font-size: inherit; line-height: inherit; vertical-align: baseline"> 6 found in chloroplast. All plants and all photosynthetic algae contain chloroplasts, which produce NADPH and ATP with the mechanisms described above. In essence, the same transmembrane structure is also found in cyanobacteria .
Unlike plants and algae, cyanobacteria are prokaryotes. They do not contain chloroplasts. Instead, they have a striking resemblance to the chloroplast itself. This suggests that organisms resembling cyanobacteria are precursors of chloroplast evolution. One imagines primitive eukaryotic cells taking cyanobacteria as intracellular symbionts in a process known as endosymbiosis.
Cyanobacteria
Cyanobacteria contain structures similar to PS II and PS I in chloroplasts. Their light harvesting systems are different from those found in plants (they use H
2 O -> PS II -> plastoquinone -> b
6 -> cytochrome c
6 -> PS I -> ferredoxin -> NADPH Â Â Â Â Â Â Â Â Â · Â · Â · Â Ã Â Ã, Â Â Ã, Â Â Â Â Â Â Â Â ? ? Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â b
6 <- plastoquinone
is, in essence, the same as the electron transport chain in the chloroplast. Water-soluble electron carriers are cytochrome c 6 on cyanobacteria, plastocyanin in plants.
Cyanobacteria can also synthesize ATP through oxidative phosphorylation, by means of other bacteria. The electron transport chain is
NADH dehydrogenase -> plastoquinone -> b
6 cypochrome c
6 -> cytochrome aa < br> 3 -> < b> O
2
where the cellular electron carriers are plastoquinone and cytochrome c 6 , while the proton pump is NADH dehydrogenase, < i> b
6 and cytochrome aa
3 .
Cyanobacteria are the only bacteria that produce oxygen during photosynthesis. The primordial atmosphere of the Earth is anoxic. Organisms such as cyanobacteria produce an atmosphere that contains oxygen today.
Two other major groups of photosynthetic bacteria, purple bacteria and green sulfur bacteria, contain only one photosystem and do not produce oxygen.
Purple bacteria
The purple bacteria contain a single photo system structurally related to PS II in cyanobacteria and chloroplasts:
- P870 -> P870 * -> ubiquinone -> bc
1 -> cytochrome c
< sub style = "font-size: inherit; line-height: inherit; vertical-align: baseline"> 2 -> P870
This is a cyclic process where the electrons are excluded from the excited chlorophyll molecule ( bacteriochlorophyll P870), passing the electron transport chain to the proton pump (cytochrome ). bc
1 complex, similar but not identical to cytochrome bc
1 in the chloroplast), and then back to the chlorophyll molecule. The result is a proton gradient, which is used to make ATP through ATP synthase. As with cyanobacteria and chloroplasts, it is a solid-state process that depends on the proper orientation of various functional groups within the structure of complex transmembrane macromolecules.
To make NADPH, the purple bacteria use external electron donors (hydrogen, hydrogen sulfide, sulfur, sulfites, or organic molecules such as succinate and lactate) to feed the electrons into the inverted electron transport chain.
Sulfurous green bacteria
Bakteri belerang hijau mengandung fotosistem yang analog dengan PS I dalam kloroplas:
P840 -> P840 * -> ferredoxin -> NADH                               ? ?                            cyt c 553 <- bc
1 <- menaquinone There are two electron transfer paths. In cyclic electron transfer , the electrons are excluded from the excited chlorophyll molecule, passing the electron transport chain to the proton pump, and then back to the chlorophyll. The moving electron carriers, as always, the lipid-soluble quinone and the water-soluble cytochrome. The resulting proton gradient is used to make ATP.
In non-electron electron transfer , electrons are excluded from excited chlorophyll molecules and used to reduce NAD to NADH. Electrons removed from P840 should be replaced. This is done by removing electrons from H
2 S , which is oxidized to sulfur (hence the name "green brimstone bacteria").
Purple bacteria and green sulfur bacteria occupy a relatively small ecological niche in the current biosphere. They are interesting because of their importance in precambrian ecology, and because their photosynthesis method is a possible evolutionary precursor of them in modern factories.
History
The first ideas about light used in photosynthesis proposed by Colin Flannery in 1779 which recognizes that sunlight falls on the plants necessary, although Joseph Priestley has recorded the production of oxygen without association with light in 1772. Cornelius Van Niel proposed in 1931 that photosynthesis is a common case mechanism in which light photons are used to break up photos of hydrogen and hydrogen donors used to reduce CO
2 . Then in 1939, Robin Hill showed that isolated chloroplasts would make oxygen, but not fix CO
2 shows light and dark reactions occurring in different places. Although they are referred to as light and dark reactions, they occur only in the presence of light. This then led to the discovery of photo systems I and II.
See also
- Light-free reaction
- The photosynthetic reaction center
- Photosystem II
- Compensation points
References
Source of the article : Wikipedia
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