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140-690: PSII absorbs a photon to produce a so-called high energy electron which transfers via an electron transport chain to cytochrome b 6 f 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 , the first electron donor is water , creating oxygen (O 2 ) as a by-product. In anoxygenic photosynthesis , various electron donors are used. Cytochrome b 6 f and ATP synthase work together to produce ATP ( photophosphorylation ) in two distinct ways. In non-cyclic photophosphorylation, cytochrome b 6 f uses electrons from PSII and energy from PSI to pump protons from

280-596: A proton pump . The proton pump in all photosynthetic chains resembles mitochondrial Complex III . The commonly-held theory of symbiogenesis proposes that both organelles descended from bacteria. Ionization Ionization (or ionisation specifically in Britain, Ireland, Australia and New Zealand) is the process by which an atom or a molecule acquires a negative or positive charge by gaining or losing electrons , often in conjunction with other chemical changes. The resulting electrically charged atom or molecule

420-727: A classical electron in the laboratory frame. In other words, in the Kramers–Henneberger frame the classical electron is at rest. Starting in the lab frame (velocity gauge), we may describe the electron with the Hamiltonian: In the dipole approximation, the quiver motion of a classical electron in the laboratory frame for an arbitrary field can be obtained from the vector potential of the electromagnetic field: where α 0 ≡ E 0 ω − 2 {\displaystyle \alpha _{0}\equiv E_{0}\omega ^{-2}} for

560-421: A coherent superposition of the two states. Under subsequent action of the same pulse, due to interference in the transition amplitudes of the lambda system, the field cannot ionize the population completely and a fraction of the population will be trapped in a coherent superposition of the quasi degenerate levels. According to this explanation the states with higher angular momentum – with more sublevels – would have

700-514: A form of ionization in which an electron is removed from or added to an atom or molecule in its lowest energy state to form an ion in its lowest energy state. The Townsend discharge is a good example of the creation of positive ions and free electrons due to ion impact. It is a cascade reaction involving electrons in a region with a sufficiently high electric field in a gaseous medium that can be ionized, such as air . Following an original ionization event, due to such as ionizing radiation,

840-485: A higher probability of trapping the population. In general the strength of the trapping will be determined by the strength of the two photon coupling between the quasi-degenerate levels via the continuum. In 1996, using a very stable laser and by minimizing the masking effects of the focal region expansion with increasing intensity, Talebpour et al. observed structures on the curves of singly charged ions of Xe, Kr and Ar. These structures were attributed to electron trapping in

980-556: A linearly polarized laser with frequency ω {\displaystyle \omega } is given by where The coefficients f l m {\displaystyle f_{lm}} , g ( γ ) {\displaystyle g(\gamma )} and C n ∗ l ∗ {\displaystyle C_{n^{*}l^{*}}} are given by The coefficient A m ( ω , γ ) {\displaystyle A_{m}(\omega ,\gamma )}

1120-402: A lipid-soluble carrier, ubiquinone (Q). The reduced product, ubiquinol (QH 2 ), freely diffuses within the membrane, and Complex I translocates four protons (H ) across the membrane, thus producing a proton gradient. Complex I is one of the main sites at which premature electron leakage to oxygen occurs, thus being one of the main sites of production of superoxide . The pathway of electrons

1260-511: 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 H 2 S , which is oxidized to sulfur (hence the name "green sulfur bacteria"). Purple bacteria and green sulfur bacteria occupy relatively minor ecological niches in

1400-555: A lower to a higher redox potential , is used by the complexes in the electron transport chain to create an electrochemical gradient of ions. It is this electrochemical gradient that drives the synthesis of ATP via coupling with oxidative phosphorylation with ATP synthase . In eukaryotic organisms , the electron transport chain, and site of oxidative phosphorylation , is found on the inner mitochondrial membrane . The energy released by reactions of oxygen and reduced compounds such as cytochrome c and (indirectly) NADH and FADH 2

1540-516: A mobile electron carrier in the membrane called plastoquinone : Plastoquinol, in turn, transfers electrons to cyt b 6 f , which feeds them into PSI. The step H 2 O → 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

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1680-648: 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 b 6 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 c 1 are not homologous. PSI accepts electrons from plastocyanin and transfers them either to NADPH ( noncyclic electron transport ) or back to cytochrome b 6 f ( cyclic electron transport ): PSI, like PSII,

1820-413: A monochromatic plane wave. By applying a transformation to the laboratory frame equal to the quiver motion α ( t ) {\displaystyle \mathbf {\alpha } (t)} one moves to the ‘oscillating’ or ‘Kramers–Henneberger’ frame, in which the classical electron is at rest. By a phase factor transformation for convenience one obtains the ‘space-translated’ Hamiltonian, which

1960-462: A number of different electron acceptors, both organic and inorganic. As with other steps of the ETC, an enzyme is required to help with the process. If oxygen is available, it is most often used as the terminal electron acceptor in aerobic bacteria and facultative anaerobes. An oxidase reduces the O 2 to water while oxidizing something else. In mitochondria, the terminal membrane complex ( Complex IV )

2100-540: A process known as endosymbiosis . Cyanobacteria contain both PSI and PSII. 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 is, in essence, the same as the electron transport chain in chloroplasts. The mobile water-soluble electron carrier is cytochrome c 6 in cyanobacteria, having been replaced by plastocyanin in plants. Cyanobacteria can also synthesize ATP by oxidative phosphorylation, in

2240-410: A process that continues down the series until electrons are passed to oxygen, the terminal electron acceptor in the chain. Each reaction releases energy because a higher-energy donor and acceptor convert to lower-energy products. Via the transferred electrons, this energy is used to generate a proton gradient across the mitochondrial membrane by "pumping" protons into the intermembrane space, producing

2380-459: 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 . PSII and PSI are connected by

2520-544: A result, the electron re-scattering can be taken as the main mechanism for the occurrence of the NSI process. The ionization of inner valence electrons are responsible for the fragmentation of polyatomic molecules in strong laser fields. According to a qualitative model the dissociation of the molecules occurs through a three-step mechanism: The short pulse induced molecular fragmentation may be used as an ion source for high performance mass spectroscopy. The selectivity provided by

2660-406: A reverse electron transport chain. Green sulfur bacteria contain a photosystem that is analogous to PSI 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,

2800-416: A short pulse based source is superior to that expected when using the conventional electron ionization based sources, in particular when the identification of optical isomers is required. The Kramers–Henneberger frame is the non-inertial frame moving with the free electron under the influence of the harmonic laser pulse, obtained by applying a translation to the laboratory frame equal to the quiver motion of

2940-398: A single photosystem and do not produce oxygen. Purple bacteria contain a single photosystem that is structurally related to PSII in cyanobacteria and chloroplasts: 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 bc 1 complex; similar to

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3080-429: A special pigment molecule in a photosynthetic reaction center absorbs a photon, an electron in this pigment attains the excited state and then is transferred to another molecule in the reaction center. This reaction, called photoinduced charge separation , is the start of the electron flow and transforms light energy into chemical forms. In chemistry , many reactions depend on the absorption of photons to provide

3220-402: A state of higher free energy that has the potential to do work. This entire process is called oxidative phosphorylation since ADP is phosphorylated to ATP by using the electrochemical gradient that the redox reactions of the electron transport chain have established driven by energy-releasing reactions of oxygen. Energy associated with the transfer of electrons down the electron transport chain

3360-410: A state such as 6f of Xe which consists of 7 quasi-degnerate levels in the range of the laser bandwidth. These levels along with the continuum constitute a lambda system. The mechanism of the lambda type trapping is schematically presented in figure. At the rising part of the pulse (a) the excited state (with two degenerate levels 1 and 2) are not in multiphoton resonance with the ground state. The electron

3500-627: A total of four different electron transport chains operating simultaneously. A common feature of all electron transport chains is the presence of a proton pump to create an electrochemical gradient over a membrane. Bacterial electron transport chains may contain as many as three proton pumps, like mitochondria, or they may contain two or at least one. In the current biosphere, the most common electron donors are organic molecules. Organisms that use organic molecules as an electron source are called organotrophs . Chemoorganotrophs (animals, fungi, protists) and photolithotrophs (plants and algae) constitute

3640-399: A transmembrane proton pump, cytochrome b 6 f complex (plastoquinol—plastocyanin reductase; EC 1.10.99.1 ). Electrons from PSII are carried by plastoquinol to cyt b 6 f , 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

3780-428: A variety of equipment in fundamental science (e.g., mass spectrometry ) and in medical treatment (e.g., radiation therapy ). It is also widely used for air purification, though studies have shown harmful effects of this application. Negatively charged ions are produced when a free electron collides with an atom and is subsequently trapped inside the electric potential barrier, releasing any excess energy. The process

3920-401: 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)

4060-453: Is a parallel electron transport pathway to Complex I, but unlike Complex I, no protons are transported to the intermembrane space in this pathway. Therefore, the pathway through Complex II contributes less energy to the overall electron transport chain process. In Complex III ( cytochrome bc 1 complex or CoQH 2 -cytochrome c reductase; EC 1.10.2.2 ), the Q-cycle contributes to

4200-712: 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 NADP reductase (FNR) that reduces NADP to NADPH. In cyclic electron transport , electrons from ferredoxin are transferred (via plastoquinol) to a proton pump, cytochrome b 6 f . They are then returned (via plastocyanin) to P700. NADPH and ATP are used to synthesize organic molecules from CO 2 . The ratio of NADPH to ATP production can be adjusted by adjusting

4340-460: Is adopted from the field of ionization of atoms by X rays and electron projectiles where the SO process is one of the major mechanisms responsible for the multiple ionization of atoms. The SO model describes the NSI process as a mechanism where one electron is ionized by the laser field and the departure of this electron is so rapid that the remaining electrons do not have enough time to adjust themselves to

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4480-483: Is an extremely complex transmembrane structure that is embedded in the inner membrane. Three of them are proton pumps . The structures are electrically connected by lipid-soluble electron carriers and water-soluble electron carriers. The overall electron transport chain can be summarized as follows: In Complex I (NADH ubiquinone oxidoreductase, Type I NADH dehydrogenase, or mitochondrial complex I; EC 1.6.5.3 ), two electrons are removed from NADH and transferred to

4620-401: Is another channel A + L − > A + + {\displaystyle A+L->A^{++}} which is the main contribution to the production of doubly charged ions at lower intensities. The first observation of triple NSI in argon interacting with a 1  μm laser was reported by Augst et al. Later, systematically studying the NSI of all rare gas atoms,

4760-427: Is any process that creates a proton gradient across a membrane. Protons can be physically moved across a membrane, as seen in mitochondrial Complexes I and IV . The same effect can be produced by moving electrons in the opposite direction. The result is the disappearance of a proton from the cytoplasm and the appearance of a proton in the periplasm. Mitochondrial Complex III is this second type of proton pump, which

4900-526: Is as follows: NADH is oxidized to NAD , by reducing flavin mononucleotide to FMNH 2 in one two-electron step. FMNH 2 is then oxidized in two one-electron steps, through a semiquinone intermediate. Each electron thus transfers from the FMNH 2 to an Fe–S cluster , from the Fe-S cluster to ubiquinone (Q). Transfer of the first electron results in the free-radical ( semiquinone ) form of Q, and transfer of

5040-489: Is blockage of ATP synthase, resulting in a build-up of protons and therefore a higher proton-motive force , inducing reverse electron flow . In eukaryotes, NADH is the most important electron donor. The associated electron transport chain is NADH → Complex I → Q → Complex III → cytochrome c → Complex IV → O 2 where Complexes I, III and IV are proton pumps, while Q and cytochrome c are mobile electron carriers. The electron acceptor for this process

5180-445: Is called an ion . Ionization can result from the loss of an electron after collisions with subatomic particles , collisions with other atoms, molecules, electrons, positrons , protons , antiprotons and ions, or through the interaction with electromagnetic radiation . Heterolytic bond cleavage and heterolytic substitution reactions can result in the formation of ion pairs. Ionization can occur through radioactive decay by

5320-414: Is cytochrome oxidase, which oxidizes the cytochrome. Aerobic bacteria use a number of differet terminal oxidases. For example, E. coli (a facultative anaerobe) does not have a cytochrome oxidase or a bc 1 complex. Under aerobic conditions, it uses two different terminal quinol oxidases (both proton pumps) to reduce oxygen to water. Bacterial terminal oxidases can be split into classes according to

5460-497: Is formed by one quinol ( 2 H 2 + e − {\displaystyle {\ce {2H+2e-}}} ) oxidations at the Q o site to form one quinone ( 2 H 2 + e − {\displaystyle {\ce {2H+2e-}}} ) at the Q i site. (In total, four protons are translocated: two protons reduce quinone to quinol and two protons are released from two ubiquinol molecules.) When electron transfer

5600-405: 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 , a water-soluble electron carrier. As in PSII, this

5740-568: Is given by where The quasi-static tunneling (QST) is the ionization whose rate can be satisfactorily predicted by the ADK model, i.e. the limit of the PPT model when γ {\displaystyle \gamma } approaches zero. The rate of QST is given by As compared to W P P T {\displaystyle W_{PPT}} the absence of summation over n, which represent different above threshold ionization (ATI) peaks,

Light-dependent reactions - Misplaced Pages Continue

5880-403: Is in the thylakoid membrane. It transfers absorbed light energy to a dimer of 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 centers. In PSII, it absorbs photons with a wavelength of 680 nm, and

6020-455: Is ionized through multiphoton coupling with the continuum. As the intensity of the pulse is increased the excited state and the continuum are shifted in energy due to the Stark shift. At the peak of the pulse (b) the excited states go into multiphoton resonance with the ground state. As the intensity starts to decrease (c), the two state are coupled through continuum and the population is trapped in

6160-411: Is known as electron capture ionization . Positively charged ions are produced by transferring an amount of energy to a bound electron in a collision with charged particles (e.g. ions, electrons or positrons) or with photons. The threshold amount of the required energy is known as ionization potential . The study of such collisions is of fundamental importance with regard to the few-body problem , which

6300-593: Is mediated by a quinone (the Q cycle ). Some dehydrogenases are proton pumps, while others are not. Most oxidases and reductases are proton pumps, but some are not. Cytochrome bc 1 is a proton pump found in many, but not all, bacteria (not in E. coli ). As the name implies, bacterial bc 1 is similar to mitochondrial bc 1 ( Complex III ). Cytochromes are proteins that contain iron. They are found in two very different environments. Some cytochromes are water-soluble carriers that shuttle electrons to and from large, immobile macromolecular structures imbedded in

6440-564: Is molecular oxygen. In prokaryotes ( bacteria and archaea ) the situation is more complicated, because there are several different electron donors and several different electron acceptors. The generalized electron transport chain in bacteria is: Electrons can enter the chain at three levels: at the level of a dehydrogenase , at the level of the quinone pool, or at the level of a mobile cytochrome electron carrier. These levels correspond to successively more positive redox potentials, or to successively decreased potential differences relative to

6580-400: Is one of the major unsolved problems in physics. Kinematically complete experiments , i.e. experiments in which the complete momentum vector of all collision fragments (the scattered projectile, the recoiling target-ion, and the ejected electron) are determined, have contributed to major advances in the theoretical understanding of the few-body problem in recent years. Adiabatic ionization is

6720-470: Is proportional to intensity) where ionization due to re-scattering can occur. The re-scattering model in Kuchiev's version (Kuchiev's model) is quantum mechanical. The basic idea of the model is illustrated by Feynman diagrams in figure a. First both electrons are in the ground state of an atom. The lines marked a and b describe the corresponding atomic states. Then the electron a is ionized. The beginning of

6860-661: Is reduced (by a high membrane potential or respiratory inhibitors such as antimycin A), Complex III may leak electrons to molecular oxygen, resulting in superoxide formation. This complex is inhibited by dimercaprol (British Anti-Lewisite, BAL), naphthoquinone and antimycin. In Complex IV ( cytochrome c oxidase ; EC 1.9.3.1 ), sometimes called cytochrome AA3, four electrons are removed from four molecules of cytochrome c and transferred to molecular oxygen (O 2 ) and four protons, producing two molecules of water. The complex contains coordinated copper ions and several heme groups. At

7000-433: Is remarkable. The calculations of PPT are done in the E -gauge, meaning that the laser field is taken as electromagnetic waves. The ionization rate can also be calculated in A -gauge, which emphasizes the particle nature of light (absorbing multiple photons during ionization). This approach was adopted by Krainov model based on the earlier works of Faisal and Reiss. The resulting rate is given by where: In calculating

7140-536: Is the electron source, the donor may be NADH or succinate, in which case electrons enter the electron transport chain via NADH dehydrogenase (similar to Complex I in mitochondria) or succinate dehydrogenase (similar to Complex II ). Other dehydrogenases may be used to process different energy sources: formate dehydrogenase, lactate dehydrogenase, glyceraldehyde-3-phosphate dehydrogenase, H 2 dehydrogenase ( hydrogenase ), electron transport chain. Some dehydrogenases are also proton pumps, while others funnel electrons into

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7280-457: Is the rate of quasi-static tunneling to i'th charge state and α n ( λ ) {\displaystyle \alpha _{n}(\lambda )} are some constants depending on the wavelength of the laser (but not on the pulse duration). Two models have been proposed to explain the non-sequential ionization; the shake-off model and electron re-scattering model. The shake-off (SO) model, first proposed by Fittinghoff et al.,

7420-499: Is the time-dependent energy difference between the two dressed states. In interaction with a short pulse, if the dynamic resonance is reached in the rising or the falling part of the pulse, the population practically remains in the ground state and the effect of multiphoton resonances may be neglected. However, if the states go onto resonance at the peak of the pulse, where d W / d t = 0 {\displaystyle \mathrm {d} W/\mathrm {d} t=0} , then

7560-441: Is therefore called P680 . 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

7700-477: Is unitarily equivalent to the lab-frame Hamiltonian, which contains the original potential centered on the oscillating point − α ( t ) {\displaystyle -\mathbf {\alpha } (t)} : The utility of the KH frame lies in the fact that in this frame the laser-atom interaction can be reduced to the form of an oscillating potential energy, where the natural parameters describing

7840-423: Is used by the electron transport chain to pump protons into the intermembrane space , generating the electrochemical gradient over the inner mitochondrial membrane . In photosynthetic eukaryotes, the electron transport chain is found on the thylakoid membrane. Here, light energy drives electron transport through a proton pump and the resulting proton gradient causes subsequent synthesis of ATP. In bacteria ,

7980-403: Is used to drive ATP synthesis, catalyzed by the F 1 component of the complex. Coupling with oxidative phosphorylation is a key step for ATP production. However, in specific cases, uncoupling the two processes may be biologically useful. The uncoupling protein, thermogenin —present in the inner mitochondrial membrane of brown adipose tissue —provides for an alternative flow of protons back to

8120-445: Is used to make ATP via ATP synthase . The overall process of the photosynthetic electron transport chain in chloroplasts is: PSII is extremely complex, a 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

8260-536: Is used to pump protons from the mitochondrial matrix into the intermembrane space, creating an electrochemical proton gradient ( ΔpH ) across the inner mitochondrial membrane. This proton gradient is largely but not exclusively responsible for the mitochondrial membrane potential (ΔΨ M ). It allows ATP synthase to use the flow of H through the enzyme back into the matrix to generate ATP from adenosine diphosphate (ADP) and inorganic phosphate . Complex I (NADH coenzyme Q reductase; labeled I) accepts electrons from

8400-476: Is used to reduce a chain of electron acceptors that have subsequently higher redox potentials. This chain of electron acceptors is known as an electron transport chain . 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 , because the redox diagram from P680 to P700 resembles

8540-557: The Krebs cycle electron carrier nicotinamide adenine dinucleotide (NADH) , and passes them to coenzyme Q ( ubiquinone ; labeled Q), which also receives electrons from Complex II ( succinate dehydrogenase ; labeled II). Q passes electrons to Complex III ( cytochrome bc 1 complex ; labeled III), which passes them to cytochrome c (cyt c ). Cyt c passes electrons to Complex IV ( cytochrome c oxidase ; labeled IV). Four membrane-bound complexes have been identified in mitochondria. Each

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8680-440: The inner mitochondrial membrane , electrons from NADH and FADH 2 pass through the electron transport chain to oxygen, which provides the energy driving the process as it is reduced to water. The electron transport chain comprises an enzymatic series of electron donors and acceptors. Each electron donor will pass electrons to an acceptor of higher redox potential, which in turn donates these electrons to another acceptor,

8820-465: The internal conversion process, in which an excited nucleus transfers its energy to one of the inner-shell electrons causing it to be ejected. Everyday examples of gas ionization occur within a fluorescent lamp or other electrical discharge lamps. It is also used in radiation detectors such as the Geiger-Müller counter or the ionization chamber . The ionization process is widely used in

8960-492: The quinone pool (Q) originating from succinate and transferred (via flavin adenine dinucleotide (FAD) ) to Q. Complex II consists of four protein subunits: succinate dehydrogenase (SDHA); succinate dehydrogenase [ubiquinone] iron–sulfur subunit mitochondrial (SDHB); succinate dehydrogenase complex subunit C (SDHC); and succinate dehydrogenase complex subunit D (SDHD). Other electron donors (e.g., fatty acids and glycerol 3-phosphate) also direct electrons into Q (via FAD). Complex II

9100-616: 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 b 6 f 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: PSI and PSII are light-harvesting complexes . If

9240-415: The F O turn one full revolution. For example, in humans, there are 8 c subunits, thus 8 protons are required. After c subunits, protons finally enter the matrix through an a subunit channel that opens into the mitochondrial matrix. This reflux releases free energy produced during the generation of the oxidized forms of the electron carriers (NAD and Q) with energy provided by O 2 . The free energy

9380-529: The TDSE. In high frequency Floquet theory, to lowest order in ω − 1 {\displaystyle \omega ^{-1}} the system reduces to the so-called ‘structure equation’, which has the form of a typical energy-eigenvalue Schrödinger equation containing the ‘dressed potential’ V 0 ( α 0 , r ) {\displaystyle V_{0}(\alpha _{0},\mathbf {r} )} (the cycle-average of

9520-463: The above reaction possibly occur in the following way (Kok's diagram of S-states): (I) 2 H 2 O (monoxide) (II) OH. H 2 O (hydroxide) (III) H 2 O 2 (peroxide) (IV) HO 2 (super oxide)(V) O 2 (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

9660-441: The approach of Becker and Faisal (which is equivalent to Kuchiev's model in spirit), this drawback does not exist. In fact, their model is more exact and does not suffer from the large number of approximations made by Kuchiev. Their calculation results perfectly fit with the experimental results of Walker et al. Becker and Faisal have been able to fit the experimental results on the multiple NSI of rare gas atoms using their model. As

9800-534: The atom or molecule is interacting with near-infrared strong laser pulses. This process can be understood as a process by which a bounded electron, through the absorption of more than one photon from the laser field, is ionized. This picture is generally known as multiphoton ionization (MPI). Keldysh modeled the MPI process as a transition of the electron from the ground state of the atom to the Volkov states. In this model

9940-463: The atomic number, as summarized by ordering atoms in Mendeleev's table . This is a valuable tool for establishing and understanding the ordering of electrons in atomic orbitals without going into the details of wave functions or the ionization process. An example is presented in the figure to the right. The periodic abrupt decrease in ionization potential after rare gas atoms, for instance, indicates

10080-428: 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. PSII, PSI, and cytochrome b 6 f are found in chloroplasts. All plants and all photosynthetic algae contain chloroplasts, which produce NADPH and ATP by the mechanisms described above. In essence,

10220-527: 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

10360-408: The electron as light ( fluorescence ). The energy, but not the electron itself, may be passed onto another molecule; this is called resonance energy transfer . If an electron 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

10500-404: The electron dynamics are ω {\displaystyle \omega } and α 0 {\displaystyle \alpha _{0}} (sometimes called the “excursion amplitude’, obtained from α ( t ) {\displaystyle \mathbf {\alpha } (t)} ). From here one can apply Floquet theory to calculate quasi-stationary solutions of

10640-405: The electron transport chain and oxidative phosphorylation are coupled by a proton gradient across the inner mitochondrial membrane. The efflux of protons from the mitochondrial matrix creates an electrochemical gradient (proton gradient). This gradient is used by the F O F 1 ATP synthase complex to make ATP via oxidative phosphorylation. ATP synthase is sometimes described as Complex V of

10780-441: The electron transport chain can vary between species but it always constitutes a set of redox reactions that are coupled to the synthesis of ATP through the generation of an electrochemical gradient and oxidative phosphorylation through ATP synthase. Most eukaryotic cells have mitochondria , which produce ATP from reactions of oxygen with products of the citric acid cycle , fatty acid metabolism , and amino acid metabolism . At

10920-483: The electron transport chain. The F O component of ATP synthase acts as an ion channel that provides for a proton flux back into the mitochondrial matrix. It is composed of a, b and c subunits. Protons in the inter-membrane space of mitochondria first enter the ATP synthase complex through an a subunit channel. Then protons move to the c subunits. The number of c subunits determines how many protons are required to make

11060-441: 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 NADP to NADPH . Activities of the electron transport chain, especially from cytochrome b 6 f , lead to pumping of protons from the stroma to the lumen. The resulting transmembrane proton gradient

11200-458: The emergence of a new shell in alkali metals . In addition, the local maximums in the ionization energy plot, moving from left to right in a row, are indicative of s, p, d, and f sub-shells. Classical physics and the Bohr model of the atom can qualitatively explain photoionization and collision-mediated ionization. In these cases, during the ionization process, the energy of the electron exceeds

11340-439: The energy difference of the potential barrier it is trying to pass. The classical description, however, cannot describe tunnel ionization since the process involves the passage of electron through a classically forbidden potential barrier. The interaction of atoms and molecules with sufficiently strong laser pulses or with other charged particles leads to the ionization to singly or multiply charged ions. The ionization rate, i.e.

11480-418: The energy needed to overcome the activation energy barrier and hence can be labelled light-dependent. Such reactions range from the silver halide reactions used in photographic film to the creation and destruction of ozone in the upper atmosphere . This article discusses a specific subset of these, the series of light-dependent reactions related to photosynthesis in living organisms. The reaction center

11620-480: The energy of photons . 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,

11760-500: The energy of sunlight is used to create a high-energy electron donor which can subsequently reduce oxidized components and couple to ATP synthesis via proton translocation by the electron transport chain. Photosynthetic electron transport chains, like the mitochondrial chain, can be considered as a special case of the bacterial systems. They use mobile, lipid-soluble quinone carriers ( phylloquinone and plastoquinone ) and mobile, water-soluble carriers ( cytochromes ). They also contain

11900-423: 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

12040-441: The excited state is populated. After being populated, since the ionization potential of the excited state is small, it is expected that the electron will be instantly ionized. In 1992, de Boer and Muller showed that Xe atoms subjected to short laser pulses could survive in the highly excited states 4f, 5f, and 6f. These states were believed to have been excited by the dynamic Stark shift of the levels into multiphoton resonance with

12180-646: The experimental point of view, the NS double ionization refers to processes which somehow enhance the rate of production of doubly charged ions by a huge factor at intensities below the saturation intensity of the singly charged ion. Many, on the other hand, prefer to define the NSI as a process by which two electrons are ionized nearly simultaneously. This definition implies that apart from the sequential channel A + L − > A + + L − > A + + {\displaystyle A+L->A^{+}+L->A^{++}} there

12320-411: The field during the rising part of the laser pulse. Subsequent evolution of the laser pulse did not completely ionize these states, leaving behind some highly excited atoms. We shall refer to this phenomenon as "population trapping". We mention the theoretical calculation that incomplete ionization occurs whenever there is parallel resonant excitation into a common level with ionization loss. We consider

12460-596: The first order correction in the quasi-classical action. Larochelle et al. have compared the theoretically predicted ion versus intensity curves of rare gas atoms interacting with a Ti:Sapphire laser with experimental measurement. They have shown that the total ionization rate predicted by the PPT model fit very well the experimental ion yields for all rare gases in the intermediate regime of the Keldysh parameter. The rate of MPI on atom with an ionization potential E i {\displaystyle E_{i}} in

12600-503: The flow of electrons terminates with molecular oxygen as the final electron acceptor. In anaerobic respiration , other electron acceptors are used, such as sulfate . In an electron transport chain, the redox reactions are driven by the difference in the Gibbs free energy of reactants and products. The free energy released when a higher-energy electron donor and acceptor convert to lower-energy products, while electrons are transferred from

12740-434: The generalized Rabi frequency, Γ ( t ) = Γ m I ( t ) m / 2 {\displaystyle \Gamma (t)=\Gamma _{m}I(t)^{m/2}} coupling the two states. According to Story et al., the probability of remaining in the ground state, P g {\displaystyle P_{g}} , is given by where W {\displaystyle W}

12880-410: 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. 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

13020-403: The ground state. Within the dressed atom picture, the ground state dressed by m {\displaystyle m} photons and the resonant state undergo an avoided crossing at the resonance intensity I r {\displaystyle I_{r}} . The minimum distance, V m {\displaystyle V_{m}} , at the avoided crossing is proportional to

13160-594: The inner mitochondrial matrix. Thyroxine is also a natural uncoupler. This alternative flow results in thermogenesis rather than ATP production. Reverse electron flow is the transfer of electrons through the electron transport chain through the reverse redox reactions. Usually requiring a significant amount of energy to be used, this can reduce the oxidized forms of electron donors. For example, NAD can be reduced to NADH by Complex I. There are several factors that have been shown to induce reverse electron flow. However, more work needs to be done to confirm this. One example

13300-457: The ion excitation to a discrete or continuum state. Figure b describes the exchange process. Kuchiev's model, contrary to Corkum's model, does not predict any threshold intensity for the occurrence of NS ionization. Kuchiev did not include the Coulomb effects on the dynamics of the ionized electron. This resulted in the underestimation of the double ionization rate by a huge factor. Obviously, in

13440-537: The ionization probability in unit time, can be calculated using quantum mechanics . (There are classical methods available also, like the Classical Trajectory Monte Carlo Method (CTMC) ,but it is not overall accepted and often criticized by the community.) There are two quantum mechanical methods exist, perturbative and non-perturbative methods like time-dependent coupled-channel or time independent close coupling methods where

13580-425: The ionization process is shown by the intersection with a sloped dashed line. where the MPI occurs. The propagation of the ionized electron in the laser field, during which it absorbs other photons (ATI), is shown by the full thick line. The collision of this electron with the parent atomic ion is shown by a vertical dotted line representing the Coulomb interaction between the electrons. The state marked with c describes

13720-466: 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 picoseconds (10 seconds). In their high-energy states,

13860-399: The laser at larger distances from the nucleus. This is in contrast to the approximation made by neglecting the potential of the laser at regions near the nucleus. Perelomov et al. included the Coulomb interaction at larger internuclear distances. Their model (which we call the PPT model) was derived for short range potential and includes the effect of the long range Coulomb interaction through

14000-432: The laser intensity is sufficiently high, the detailed structure of the atom or molecule can be ignored and analytic solution for the ionization rate is possible. Tunnel ionization is ionization due to quantum tunneling . In classical ionization, an electron must have enough energy to make it over the potential barrier, but quantum tunneling allows the electron simply to go through the potential barrier instead of going all

14140-400: 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 b 6 f transfers the electron chain to PSI through plastocyanin molecules. PSI can continue the electron transfer in two different ways. It can transfer

14280-507: The level of a mobile cytochrome or quinone carrier. For example, electrons from inorganic electron donors (nitrite, ferrous iron, electron transport chain) enter the electron transport chain at the cytochrome level. When electrons enter at a redox level greater than NADH, the electron transport chain must operate in reverse to produce this necessary, higher-energy molecule. As there are a number of different electron donors (organic matter in organotrophs, inorganic matter in lithotrophs), there are

14420-455: 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. When the excited chlorophyll P 680 passes the electron to pheophytin, it converts to high-energy P 680 , which can oxidize the tyrosine Z (or Y Z ) molecule by ripping off one of its hydrogen atoms. The high-energy oxidized tyrosine gives off its energy and returns to

14560-628: The manner of other bacteria. The electron transport chain is where the mobile electron carriers are plastoquinol and cytochrome c 6 , while the proton pumps are NADH dehydrogenase, cyt b 6 f and cytochrome aa 3 (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

14700-413: 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 process, not a typical chemical reaction. It occurs within an essentially crystalline environment created by

14840-498: The membrane. Bacteria use ubiquinone (Coenzyme Q, the same quinone that mitochondria use) and related quinones such as menaquinone (Vitamin K 2 ). Archaea in the genus Sulfolobus use caldariellaquinone. The use of different quinones is due to slight changes in redox potentials caused by changes in structure. The change in redox potentials of these quinones may be suited to changes in the electron acceptors or variations of redox potentials in bacterial complexes. A proton pump

14980-411: The membrane. The mobile cytochrome electron carrier in mitochondria is cytochrome c . Bacteria use a number of different mobile cytochrome electron carriers. Other cytochromes are found within macromolecules such as Complex III and Complex IV . They also function as electron carriers, but in a very different, intramolecular, solid-state environment. Electrons may enter an electron transport chain at

15120-540: 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 b 6 f (in chloroplasts) is very similar to cytochrome bc 1 ( 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

15260-468: The model can be understood easily from Corkum's version. Corkum's model describes the NS ionization as a process whereby an electron is tunnel ionized. The electron then interacts with the laser field where it is accelerated away from the nuclear core. If the electron has been ionized at an appropriate phase of the field, it will pass by the position of the remaining ion half a cycle later, where it can free an additional electron by electron impact. Only half of

15400-671: The molecules act as terminal electron acceptors. Class I oxidases are cytochrome oxidases and use oxygen as the terminal electron acceptor. Class II oxidases are quinol oxidases and can use a variety of terminal electron acceptors. Both of these classes can be subdivided into categories based on what redox-active components they contain. E.g. Heme aa3 Class 1 terminal oxidases are much more efficient than Class 2 terminal oxidases. Mostly in anaerobic environments different electron acceptors are used, including nitrate, nitrite, ferric iron, sulfate, carbon dioxide, and small organic molecules such as fumarate. When bacteria grow in anaerobic environments,

15540-445: The molecules of table sugar dissociate in water (sugar is dissolved) but exist as intact neutral entities. Another subtle event is the dissociation of sodium chloride (table salt) into sodium and chlorine ions. Although it may seem as a case of ionization, in reality the ions already exist within the crystal lattice. When salt is dissociated, its constituent ions are simply surrounded by water molecules and their effects are visible (e.g.

15680-520: The new energy states. Therefore, there is a certain probability that, after the ionization of the first electron, a second electron is excited to states with higher energy (shake-up) or even ionized (shake-off). We should mention that, until now, there has been no quantitative calculation based on the SO model, and the model is still qualitative. The electron rescattering model was independently developed by Kuchiev, Schafer et al , Corkum, Becker and Faisal and Faisal and Becker. The principal features of

15820-445: The next collisions occur; and so on. This is effectively a chain reaction of electron generation, and is dependent on the free electrons gaining sufficient energy between collisions to sustain the avalanche. Ionization efficiency is the ratio of the number of ions formed to the number of electrons or photons used. The trend in the ionization energy of atoms is often used to demonstrate the periodic behavior of atoms with respect to

15960-410: The oscillating potential). The interpretation of the presence of V 0 {\displaystyle V_{0}} is as follows: in the oscillating frame, the nucleus has an oscillatory motion of trajectory − α ( t ) {\displaystyle -\mathbf {\alpha } (t)} and V 0 {\displaystyle V_{0}} can be seen as

16100-415: The perturbation of the ground state by the laser field is neglected and the details of atomic structure in determining the ionization probability are not taken into account. The major difficulty with Keldysh's model was its neglect of the effects of Coulomb interaction on the final state of the electron. As it is observed from figure, the Coulomb field is not very small in magnitude compared to the potential of

16240-408: The positive ion drifts towards the cathode , while the free electron drifts towards the anode of the device. If the electric field is strong enough, the free electron gains sufficient energy to liberate a further electron when it next collides with another molecule. The two free electrons then travel towards the anode and gain sufficient energy from the electric field to cause impact ionization when

16380-488: The potential of the smeared out nuclear charge along its trajectory. The KH frame is thus employed in theoretical studies of strong-field ionization and atomic stabilization (a predicted phenomenon in which the ionization probability of an atom in a high-intensity, high-frequency field actually decreases for intensities above a certain threshold) in conjunction with high-frequency Floquet theory. A substance may dissociate without necessarily producing ions. As an example,

16520-451: The presence of light. This led later to the discovery of photosystems I and II. Electron transport chain An electron transport chain ( ETC ) is a series of protein complexes and other molecules which transfer electrons from electron donors to electron acceptors via redox reactions (both reduction and oxidation occurring simultaneously) and couples this electron transfer with

16660-421: The present day biosphere. They are of interest because of their importance in precambrian ecologies, and because their methods of photosynthesis were the likely evolutionary precursors of those in modern plants. The first ideas about light being used in photosynthesis were proposed by Jan IngenHousz in 1779 who recognized it was sunlight falling on plants that was required, although Joseph Priestley had noted

16800-599: 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 CO 2 . Then in 1939, Robin Hill demonstrated that isolated chloroplasts would make oxygen, but not fix CO 2 , 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

16940-422: The proton gradient by an asymmetric absorption/release of protons. Two electrons are removed from QH 2 at the Q O site and sequentially transferred to two molecules of cytochrome c , a water-soluble electron carrier located within the intermembrane space. The two other electrons sequentially pass across the protein to the Q i site where the quinone part of ubiquinone is reduced to quinol. A proton gradient

17080-400: The quadruple NSI of Xe was observed. The most important conclusion of this study was the observation of the following relation between the rate of NSI to any charge state and the rate of tunnel ionization (predicted by the ADK formula) to the previous charge states; where W A D K ( A i + ) {\displaystyle W_{ADK}\left(A^{i+}\right)}

17220-549: The quinone pool. Most dehydrogenases show induced expression in the bacterial cell in response to metabolic needs triggered by the environment in which the cells grow. In the case of lactate dehydrogenase in E. coli , the enzyme is used aerobically and in combination with other dehydrogenases. It is inducible and is expressed when the concentration of DL-lactate in the cell is high. Quinones are mobile, lipid-soluble carriers that shuttle electrons (and protons) between large, relatively immobile macromolecular complexes embedded in

17360-459: The rate of MPI of atoms only transitions to the continuum states are considered. Such an approximation is acceptable as long as there is no multiphoton resonance between the ground state and some excited states. However, in real situation of interaction with pulsed lasers, during the evolution of laser intensity, due to different Stark shift of the ground and excited states there is a possibility that some excited state go into multiphoton resonance with

17500-456: 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

17640-432: 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

17780-519: The same time, eight protons are removed from the mitochondrial matrix (although only four are translocated across the membrane), contributing to the proton gradient. The exact details of proton pumping in Complex IV are still under study. Cyanide is an inhibitor of Complex IV. According to the chemiosmotic coupling hypothesis , proposed by Nobel Prize in Chemistry winner Peter D. Mitchell ,

17920-427: 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

18060-576: The second electron reduces the semiquinone form to the ubiquinol form, QH 2 . During this process, four protons are translocated from the mitochondrial matrix to the intermembrane space. As the electrons move through the complex an electron current is produced along the 180 Angstrom width of the complex within the membrane. This current powers the active transport of four protons to the intermembrane space per two electrons from NADH. In Complex II ( succinate dehydrogenase or succinate-CoQ reductase; EC 1.3.5.1 ) additional electrons are delivered into

18200-445: The special pigment and the acceptor could undergo charge recombination; that is, the 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. In the case of PSII, this backflow of electrons can produce reactive oxygen species leading to photoinhibition . Three factors in

18340-489: The strong laser field. A more unambiguous demonstration of population trapping has been reported by T. Morishita and C. D. Lin . The phenomenon of non-sequential ionization (NSI) of atoms exposed to intense laser fields has been a subject of many theoretical and experimental studies since 1983. The pioneering work began with the observation of a "knee" structure on the Xe ion signal versus intensity curve by L’Huillier et al. From

18480-411: The structure of the reaction center work together to suppress charge recombination nearly completely: 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 . The photosynthesis process in chloroplasts begins when an electron of P680 of PSII attains a higher-energy level. This energy

18620-471: The surface of Earth. Because of their volume of distribution, lithotrophs may actually outnumber organotrophs and phototrophs in our biosphere . The use of inorganic electron donors such as hydrogen as an energy source is of particular interest in the study of evolution . This type of metabolism must logically have preceded the use of organic molecules and oxygen as an energy source. Bacteria can use several different electron donors. When organic matter

18760-635: The terminal electron acceptor is reduced by an enzyme called a reductase. E. coli can use fumarate reductase, nitrate reductase, nitrite reductase, DMSO reductase, or trimethylamine-N-oxide reductase, depending on the availability of these acceptors in the environment. Most terminal oxidases and reductases are inducible . They are synthesized by the organism as needed, in response to specific environmental conditions. In oxidative phosphorylation , electrons are transferred from an electron donor such as NADH to an acceptor such as O 2 through an electron transport chain, releasing energy. In photophosphorylation ,

18900-589: The terminal electron acceptor. In other words, they correspond to successively smaller Gibbs free energy changes for the overall redox reaction. Individual bacteria use multiple electron transport chains, often simultaneously. Bacteria can use a number of different electron donors, a number of different dehydrogenases, a number of different oxidases and reductases, and a number of different electron acceptors. For example, E. coli (when growing aerobically using glucose and oxygen as an energy source) uses two different NADH dehydrogenases and two different quinol oxidases, for

19040-420: The time the electron is released with the appropriate phase and the other half it never return to the nuclear core. The maximum kinetic energy that the returning electron can have is 3.17 times the ponderomotive potential ( U p {\displaystyle U_{p}} ) of the laser. Corkum's model places a cut-off limit on the minimum intensity ( U p {\displaystyle U_{p}}

19180-402: The transfer of protons (H ions) across a membrane . Many of the enzymes in the electron transport chain are embedded within the membrane . The flow of electrons through the electron transport chain is an exergonic process . The energy from the redox reactions creates an electrochemical proton gradient that drives the synthesis of adenosine triphosphate (ATP). In aerobic respiration ,

19320-407: The vast majority of all familiar life forms. Some prokaryotes can use inorganic matter as an electron source. Such an organism is called a (chemo)lithotroph ("rock-eater"). Inorganic electron donors include hydrogen , carbon monoxide , ammonia , nitrite , sulfur , sulfide , manganese oxide , and ferrous iron . Lithotrophs have been found growing in rock formations thousands of meters below

19460-431: The wave function is expanded in a finite basis set. There are numerous options available e.g. B-splines or Coulomb wave packets. Another non-perturbative method is to solve the corresponding Schrödinger equation fully numerically on a lattice. In general, the analytic solutions are not available, and the approximations required for manageable numerical calculations do not provide accurate enough results. However, when

19600-412: The way over it because of the wave nature of the electron. The probability of an electron's tunneling through the barrier drops off exponentially with the width of the potential barrier. Therefore, an electron with a higher energy can make it further up the potential barrier, leaving a much thinner barrier to tunnel through and thus a greater chance to do so. In practice, tunnel ionization is observable when

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