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In oxygenic photosynthesis water is the electron donor and, ..

Bacteria are metabolically very diverse and some make use of
other electron acceptors (reducible substances), according to what is most readily available in their habitat.

they can use a variety of initial electron donors (oxidisable substances or fuels).

Photosynthesis is also a redox chain which harnesses light energy to build complex organic molecules (including fuels
for respiration) from simple carbon compounds, including carbon dioxide (as in plants) or simple water-soluble organic
carbon sources like acetate and other organic acids.

b) Other microorganisms use succinate as an electron donor fortheir catabolic processes.

Biocatalytic Photosynthesis with Water as an Electron Donor

In non-cyclic systems, the electron must arrive at a final electron-acceptor (NADP in plants).

positive and negative charges on opposite sides of its architecture, figuratively like the poles of a battery. Four more or less distinct steps constitute the process, after the chlorophyll pigment has absorbed the photon and donated its electron. The moving electron is accepted by the pigment pheophytin very quickly, "in roughly 4 picoseconds," explained Rees, which passes it to a primary quinone, QA, and then on to a secondary acceptor quinone, QB. Finally, a secondary electron donor gives up an electron to replace the one lost by the original donor, which is thereby reduced (that is, gains an electron). The light energy captured by the reaction center is ultimately utilized to drive the metabolic processes necessary for life.

Wrighton reported that "remarkable progress in establishing the molecular structure" of the reaction center has been made in recent years, citing Douglas Rees of the California Institute of Technology as a contributor in this area. There remain some important unanswered questions, however, said Wrighton: Why is the electron transfer from the photoexcited chlorophyll so fast? Why does the electron transfer take only one of two quite similar paths? Rees has focused on "one of the simplest systems for studying biological photosynthetic electron transfer," a system with "only a single photosystem," a type of bacteria that—while it does not produce oxygen or reduce carbon dioxide—nonetheless does photosynthesize, and employs in certain of its cells an electron transfer chain that is comparatively clear to study, and that will most likely yield insights about artificial systems that might be designed and built in the laboratory.

Biocatalytic photosynthesis with water as an electron donor.

Much of the detail has been observed directly, said Rees, who pointed out that "crystallographic studies provide a nice framework for rationalizing and understanding the kinetic sequence of events. But also, they raise a number of questions." The most significant of these involves the rates of these various steps. The atomic electrical attraction of positively and negatively charged actors in the process always threatens to draw the liberated electron back into its hole, a process called back electron transfer. If a step proceeds forward at too slow a rate, back transfer will occur and will short-circuit the entire process. In addition to increasing their speed, emphasized McLendon, experimenters also have to steer these freed-up electrons. "It doesn't do any good to move an electron around in a picosecond if it goes to the wrong place, because you will just short-circuit your electron transport chain. Then every cellular component gets to a common free energy, and you have a death by entropy."

In fermentation, build up of reduced electron carriers (NADH) isa problem because the cell needs to regenerate NAD+ to continue glycolysis.

Yet bacteria thatuse NH4+, H2S, or Fe+2 as anenergy source use O2 almost exclusively as a terminal electronacceptor.

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do not use water as an electron donor and therefore do ..

Hydrazine as an electron donor to the water-oxidation …

These "particles" of electromagnetic energy were observed to be proportional to the frequency of light in which they were traveling. Thus when a photon of a particular energy strikes a metal, for instance, that metal's outer electron(s) will be ejected by the photoelectric effect only when the incoming photon has sufficient energy to knock it loose. Light and the energy value of the photons it transmits vary according to its wavelength frequency; materials vary according to how easy it is to displace a valence electron. When this does occur, the photon is said to be absorbed by the substance, and actually ceases to exist as a particle. Aerobic plants absorb photons of light from the sun within a certain frequency range, and this drives the movement of electrons that yields the synthesis of carbohydrates and oxygen. This is the theoretical physics underlying photosynthesis. But it is the physical chemistry that interests Wrighton and his colleagues, who hope to develop analogous systems that would produce usable energy.

Nitrite, an Electron Donor for Anoxygenic Photosynthesis

The 2H+/Hcouple the most negative reduction potential, therefore providesthe most energy.b) Many H2-oxidizing bacteria (i.e., bacteria that can grow using H2as an energy source/electron donor) can use O2, NO3-1or SO42- as an electron acceptor.

Photosynthesis electron donor" Keyword Found …

Explain this observation. There is not enough of an electronpotential drop between NH4+, H2S, or Fe+2and the other electron acceptors to easily generate enough PMF.

15) Some bacteria, like spp , can use a numberof different electron acceptors, including oxygen, nitrate and iron oxides.

The driving forces for electron donor ..

(Refer to theelectron tower or Table A1.2, see above)e-donor + e-acceptor endproductsWhich terms apply for bacteria that are using each of the substratesabove?fermentation/ aerobic respiration/ anaerobic respiration / organotrophy/lithotrophya) anaerobic respiration/organotrophyb) aerobic respiration/lithotrophyc) aerobic respiration/ lithotrophyd) anaerobic respiration/ organotrophy14) Refer to the electron tower to answer the following questions aboutlithotrophs.

Photosynthetic electron donor H2O H2S, ..

First oxygen, because it has the most positive reduction potential,therefore the energy fall between the energy source (say acetate) and theterminal electron acceptor would be greatest.

Chemistry for Biologists: Photosynthesis

As Wrighton pointed out, "First, it is noteworthy that exposure of CO2 and H2O to sunlight alone does not lead to a reaction, since the photosynthetic apparatus involves light absorption by molecules other than CO2 and H2O, namely chlorophyll. Chlorophyll can therefore be regarded as a sensitizer, a light absorber which somehow channels the light to reaction pathways leading to the production of energy-rich products." It is the absorption by chlorophyll of light frequencies in the lower wavelengths that produces the green color of plants, he explained. But the crucial role played by chlorophyll and any sensitizer is to expand the bandwidth of the energy a system can accept. Wrighton added that since CO2 and H2O do not absorb in the visible frequency range, some sort of sensitization will be needed to exploit the full range of the sun's energy in the optical, or visible, range of the electromagnetic spectrum where photon energies tend to be sufficient to dislodge an electron, between 200 and 800 nanometers. This proviso is not limited to the carbon dioxide and water nature breaks down, however, but also applies, said Wrighton, to "all abundant, inexpensive fuel precursors" currently under consideration as candidates for a synthetic system. "Thus, efficient photoconversion systems will involve use of efficient visible light absorbers," he concluded.

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