Present PV technology has developed solar efficiencies that is, a certain percentage of the energy received from the sun deliverable as electricity of If the two natural photosynthetic inputs, CO2 and H2O, could be harnessed, said Wrighton, "fuel mixtures that would be useful within the existing technological framework—where combustion processes dominate the use of our existing fuels"—are foreseeable.
One ancillary benefit of such a process could be to "bring global CO2 concentrations to a steady-state value," manifestly a desirable goal, Page 28 Share Cite Suggested Citation:"2 Photosynthesis: Artificial Photosynthesis: Chemical and Biological Systems for Converting Light to Electricity and Fuels.
But as he pointed out, there are other candidates for an input source that are ubiquitous, including SiO2 silicon dioxide in rocks , N2 and O2 molecular nitrogen and oxygen from the air , and NaCl common table salt. If one of Earth's abundant natural resources could be energized by sunlight to produce probably by the breakdown and release of one of its elements a source that could be used for fuel, the entire fossil fuel cycle and the problems associated with it might be obviated.
If that resource were water, for example, and the resultant fuel source were hydrogen, burning liquid hydrogen in the air would produce only water as a combustion product. Liquid hydrogen is already in use as a fuel source and has always been the primary fuel powering space vehicles, since it produces more heat per gram of weight than any other known fuel. If a photosynthetic system delivering usable hydrogen could be developed, the process would regenerate the original water source, and an entirely new recycling of natural resources could be established.
This time, however, cultural rather than natural evolution would call the shots. With such a major new fuel production process, science would hopefully be able to provide a methodology to better anticipate and control the global impact of any byproducts or emissions.
The search for a new way to harness the sun's energy involves first the study of how photosynthesis works in nature, and then the attempt to devise a new system that to some extent will probably mimic or model the successful example. Wrighton and his colleagues provided a lucid description of both efforts. Clearly this energy is put to good use by plants and certain bacteria that have developed analogous photosynthesizing abilities.
Many studies of photosynthesis are conducted on these organisms, which are hardy and genetically manipulable under laboratory conditions. But of what does this energy shower of light consist?
How can certain structures convert it to chemical energy that is useful to them? The background to answering this question involves two of the giants of 20th-century physics—Planck and Einstein—whose work at the beginning of this century provided important fundamental insights into the energy of light.
Thus the energy E of a vibrating atom will vary according to its frequency of vibration v , but can assume only specific quantity values, namely whole integers times h approximately 6. These values, the various products of hv times whole integers, are known as quantum numbers. When one says the energies of atoms are quantized, one means that they can assume values from this set of numbers only.
Thus a quantum of energy—whether it be light or electromagnetic energy outside the optical spectrum—provides scientists a measure of the smallest piece of energy that seems to be involved in the transfer events they are trying to understand.
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. Harvesting Photons and Putting Them to Use Two fundamental constraints govern the system: the plant or photosynthesizing organism must possess a mechanism to register or receive the incoming photon; and since the energy content of a single photon is small, a way must also be found to collect and aggregate Page 30 Share Cite Suggested Citation:"2 Photosynthesis: Artificial Photosynthesis: Chemical and Biological Systems for Converting Light to Electricity and Fuels.
Plants have evolved mechanisms to overcome both of these problems. In plants, chlorophyll provides what chemists classify as a sensitizer, a species that absorbs light and effects subsequent chemical reactions. 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. 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 and 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. The sequence of molecular events occurs within a structure that biochemists classify as the Z-scheme Figure 2. This molecular arrangement accomplishes an oxidation-reduction, or redox, reaction that involves the actual or in some cases only the apparent transfer of electrons between species.
When these two phenomena occur together, the overall activity is described as a redox reaction, whereby in one half of the reaction a species loses electrons—is oxidized—and in the other half of the reaction a different species gains electrons—is reduced Ebbing, Nature uses photons to free electrons from chlorophyll and—through a series of steps—to oxidize H2O, and in the process O2 is created as a product of the reduction of CO2.
The Z-scheme provides an architecture of molecules, located in what biochemists call the reaction center of the plant, that facilitates the redox reaction. Crucial to this arrangement is a mechanism that will serve not only to separate an electron from its atomic species, but will also move it, once it has been separated, in a timed and coordinated way along a known path.
Summarizing the three essential elements of the Z-scheme, Wrighton said that the two natural photosystems found in all aerobic plants work in series to 1 absorb four photons of light to energize chlorophyll, 2 release electrons by charge separation and move them by unidirectional charge transport Figure 2.
Courtesy of M. The fairly astounding concept at the heart of this series of events is that, in order for photosynthesis to occur, evolution had to develop a structure that would facilitate a chemical series of steps that are, even today, reproducible in the laboratory only with great difficulty. Fortunately, nature is not so skeptical, and biological energy is channelled in the photosynthetic and respiratory transport chains by just such long-distance reactions.
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.
The reaction center molecules are mostly proteins, specialized polypeptides, and strings of amino acid residues. The overall process begins, explained Rees, when "light is absorbed by a specialized pair of the bacterium's chlorophyll molecules. Such a structure is then said to be in an excited state and represents the positive component of the charge separation, referred to as a hole.
As the electron moves through the transport chain, its negative charge and the positively charged hole are separated by ever greater distance, and potential energy is created. 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. 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 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. Then every cellular component gets to a common free energy, and you have a death by entropy. Rees reported that, "rather surprisingly, in many of these mutant forms the reaction center still works," though with a reduced quantum efficiency. In sum, "this marriage of molecular biology and chemical physics has provided a good structural understanding of the reaction center," said Rees, who was also referring to major strides made in spectroscopy, theory, and x-ray crystallography.
The dynamics and the energetics of the process still remain imperfectly understood, but, he predicted, "the prognosis looks quite good for unravelling these details in the next 5 years or so.
The energetics Rees referred to have become an important area of inquiry called excited-state electron transfer, advances in which will aid chemists and molecular biologists who are already building actual molecular structures to achieve conversion of light to energy.
Thus far, the most promising synthetic systems have exploited the chemistry and physics of liquid-semiconductor junctions. Excited-state Electron Transfer in Synthetic Systems Quantum physics explains how the light energy of a photon is absorbed by the right sort of receptor, kicking an electron loose and commencing the process of photosynthesis by creating a source of potential energy between the separated electron and the hole it formerly occupied.
Gaining and losing electrons "is the name of the game in redox reactions," said Wrighton, who added, "It has long been known that the photoexcited molecules are both more potent oxidants and more potent reductants than the ground electronic state" Figure 2.
When a photon is absorbed to create an electron and a hole, something thermodynamically unstable is produced, and there's always the tendency for the electron and the hole to recombine. Back electron transfer is, metaphorically, a short circuit that bleeds the potential energy of the charge separation before it can aggregate at a distant site within the system and be put to use.
Figure 2. Protons released into the thylakoid lumen by water splitting and PQH2 oxidation by the cytb6f complex exit via the lumen the ATPase.
This process is driven by the transthylakoid proton motive force and results in the formation of ATP from ADP and inorganic phosphate Pi.
Other possible cyclic routes include the Fd-quinone reductase pathway in which electrons are transferred back into the cytb6f complex. Despite intensive efforts, FQR has not been found associated with any known chloroplast protein or protein complex. In contrast, it is now established that the NDH complex associates with PSI, a process that is considered to be important in facilitating NDH functions such as cyclic electron flow and chlororespiration Peng et al.
The NDH—PSI association may re-direct electron flow and so prevent over-reduction of the stroma in mutants such as pgr5, which are defective in PSI cyclic electron transport Peng et al.
Cyclic electron transport around PSI only predominates in some special cases, such as in the bundle sheath cells of some C4 leaves Shikanai, , though some mutants show clear up-regulation of cyclic electron transport e. Livingston et al. However, it is generally accepted that cyclic electron flow makes an important contribution to the overall regulation of electron transport in response to environmental variation and metabolic cues, even though the nature of these pathways and their precise functions remain controversial.
Cyclic electron transport serves only to support the production of the pmf and thus allows ATP generation without the net formation of reductant, such as reduced Fd or NADPH.
Flexibility in the formation of pmf and reductant is not only important in meeting the needs of the Benson—Calvin cycle and other metabolic pathways, but it is also vital to the activation and regulation of photosynthesis in the transition from darkness to light, in response to rapid i.
This process, which is called the Mehler reaction or pseudocyclic electron flow, is important in photosynthetic control because it contributes to the prevention of over-reduction of the PET carriers and it produces oxidative signals that regulate gene expression Foyer and Noctor, The addition of H2O2 to plant cells leads to substantial transcriptome re-programming Desikan et al.
Similarly, the expression of numerous genes was changed in mutants lacking a chloroplastic superoxide dismutase SOD; Rizhsky et al. Under these conditions, which would be experienced by a leaf with closed or partially closed stomata, the capacity of electron transport could exceed that of metabolism.
The second phase is observed at higher Ci values and corresponds to a limitation of photosynthesis by regeneration of ribulose 1, 5-bisphosphate RuBP , which is believed to be substantially due to a limitation of electron transport capacity. Under these conditions, down-regulation of electron transport would be minimal. View large Download slide The relationship between CO2 concentration at the site of carboxylation i. The rate of CO2 fixation is the minimum of the either the Rubisco-limited rate the Ac line or the ribulose-1,5-phosphate-limited rate the Aj line.
The supply of ribulose-1,5-bisphosphate is strongly influenced by the rate of electron transport and thus irradiance, so for any leaf there are an infinite number of possible Aj lines, corresponding to different irradiances only one is illustrated. The Ac line is determined by the amount of Rubisco, its activation state and temperature, so if the Rubisco is fully activated and temperature is constant there is only one Ac line.
The capacities for electron transport and metabolism are closely balanced by regulation of chloroplast structure and composition Evans and Terashima, and by physiological regulation that controls the rate constants for electron and proton transport Genty and Harbinson, ; Takizawa et al. For example, the Ci value of the transition point between the Rubisco- and electron transport-limited zones is offset relative to the growth Ci values Ainsworth and Rogers, Similarly, the light-saturated, maximum rate of CO2 fixation is proportional to the leaf content of the Rubisco and cytb6f proteins Onoda et al.
Considered in isolation from metabolism, electron transport is generally limited by the availability of light i. The key to the regulation of electron transport is the pH sensitivity of the reaction between PQH2 and the Rieske FeS of the cytb6f complex. PQH2 is able to form an H-bond with a specific histidine residue adjacent to the binding pocket of the Rieske FeS protein.
The formation of an H-bond between a hydroxyl group proton on the quinol head group of PQH2 and the lone pair of a nitrogen in the imidazole ring of the histidine residue is the first step in oxidation of the bound PQH2 Crofts et al. Protonation of the lone electron pair on the histidine residue blocks H-bond formation.
The pH optima for cytf reduction lie in the range 6. The pH sensitivity of the PQH2—cytb6f reaction combined with decreases in lumenal pH offers a model for the regulation of linear and cyclic electron transport that explains the observed changes in the oxidation and reduction of components of the electron transport chain during regulation.
This model, however, does not explain the mechanism that brings about the changes in lumen pH required for adjustment of the kinetics of the reaction between PQH2 and cytb6f. This mechanism will be discussed once the problem of regulating the ratio of ATP and reductant formation has been introduced because the mechanisms that regulate electron transport and the ratio of ATP to NADPH have much in common. These parameters, acting together, will determine the yield of ATP per unit linear electron transport.
Studies on the structure and operation of the thylakoid ATPase Seelert et al. The ATPases from E. Several such processes can operate. In particular, the occurrence of slippage reactions under physiological conditions has been disputed Junge et al. Uncertainty in the stoichiometry of proton transport into the thylakoid lumen coupled to electron transport hinges on the presence or absence of the Q-cycle in the cytb6f complex.
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. Why does the electron transfer take only one of two quite similar paths? When a photon is absorbed to create an electron and a hole, something thermodynamically unstable is produced, and there's always the tendency for the electron and the hole to recombine.
Photosynthesis Artificial Photosynthesis: Chemical and Biological Systems for Converting Light to Electricity and Fuels ''Natural photosynthesis is a process by which light from the sun is converted to chemical energy," began Mark Wrighton in his presentation to the Frontiers symposium. As designed by nature, it is the ultimate recycling process—since it uses the planet's two most abundant resources, CO2 and H2O, providing fuel and breaking down a pollutant. Figures 2. A diode failed in this manner is readily detected: it drops almost zero voltage when biased either way, like a piece of wire. If this can be provided by the energy of reaction, the reaction rate increases.