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Electron Flow (electron + flow)
Kinds of Electron Flow Selected AbstractsA PERSPECTIVE ON PHOTOSYNTHESIS IN THE OLIGOTROPHIC OCEANS: HYPOTHESES CONCERNING ALTERNATE ROUTES OF ELECTRON FLOW,JOURNAL OF PHYCOLOGY, Issue 4 2010Arthur R. Grossman Many regions of the open, oligotrophic oceans are depleted of nutrients, especially nitrogen and iron. The biogenesis and the functioning of the photosynthetic apparatus may be specialized and tailored to the various marine habitats. In this minireview, we discuss some new findings with respect to photosynthetic processes in the oceans. We focus on findings that suggest that some cyanobacteria may route electrons derived from the splitting of H2O to the reduction of O2 and H+ in a water-to-water cycle, and that other cyanobacteria that fix nitrogen during the day are likely missing PSII and enzymes involved in the fixation of inorganic carbon. Both of these proposed "variant" forms of photosynthetic electron flow provide new insights into ways in which marine phytoplankton satisfy their energetic and nutritive requirements. [source] Organization of Organic Molecules with Inorganic Nanoparticles: Hybrid Nanodiodes,ADVANCED FUNCTIONAL MATERIALS, Issue 5 2008Kallol Mohanta Abstract A monolayer of inorganic nanoparticles and a monolayer of organic molecules have been electrostatically assembled in sequence. Such assemblies or organizations exhibit electrical rectification. When the sequence of the organization is reversed, the direction of rectification becomes opposite. In both n-type ZnO/organic and organic/n-ZnO assemblies, electron flow is favorable from the n-ZnO nanoparticle to the (electron-accepting) organic molecule. While the individual components do not show any rectification, substitutes of the organic molecule tune electrical rectification. Junctions between a p-type ZnO nanoparticle and an electron-donating metal phthalocyanine favor current flow in the nanoparticle-to-phthalocyanine direction. The rectification in a junction between a nanoparticle and an organic molecule is due to the parity between free carriers in the former component and the type of carrier-accepting nature in the latter one. By observing electrical rectification with the tip of a scanning tunneling microscope, organic/inorganic hybrid nanodiodes or rectifiers on the molecular/nanoscale have been established. [source] A PERSPECTIVE ON PHOTOSYNTHESIS IN THE OLIGOTROPHIC OCEANS: HYPOTHESES CONCERNING ALTERNATE ROUTES OF ELECTRON FLOW,JOURNAL OF PHYCOLOGY, Issue 4 2010Arthur R. Grossman Many regions of the open, oligotrophic oceans are depleted of nutrients, especially nitrogen and iron. The biogenesis and the functioning of the photosynthetic apparatus may be specialized and tailored to the various marine habitats. In this minireview, we discuss some new findings with respect to photosynthetic processes in the oceans. We focus on findings that suggest that some cyanobacteria may route electrons derived from the splitting of H2O to the reduction of O2 and H+ in a water-to-water cycle, and that other cyanobacteria that fix nitrogen during the day are likely missing PSII and enzymes involved in the fixation of inorganic carbon. Both of these proposed "variant" forms of photosynthetic electron flow provide new insights into ways in which marine phytoplankton satisfy their energetic and nutritive requirements. [source] Accumulation of chlorophyll catabolites photosensitizes the hypersensitive response elicited by Pseudomonas syringae in ArabidopsisNEW PHYTOLOGIST, Issue 1 2010Luis A. J. Mur Summary ,The staygreen (SGR) gene encodes a chloroplast-targeted protein which promotes chlorophyll degradation via disruption of light-harvesting complexes (LHCs). ,Over-expression of SGR in Arabidopsis (SGR-OX) in a Columbia-0 (Col-0) background caused spontaneous necrotic flecking. To relate this to the hypersensitive response (HR), Col-0, SGR-OX and RNAi SGR (SGRi) lines were challenged with Pseudomonas syringae pv tomato (Pst) encoding the avirulence gene avrRpm1. Increased and decreased SGR expression, respectively, accelerated and suppressed the kinetics of HR-cell death. In Col-0, SGR transcript increased at 6 h after inoculation (hai) when tissue electrolyte leakage indicated the initiation of cell death. ,Excitation of the chlorophyll catabolite pheophorbide (Pheide) leads to the formation of toxic singlet oxygen (1O2). Pheide was first detected at 6 hai with Pst avrRpm1 and was linked to 1O2 generation and correlated with reduced Pheide a oxygenase (PaO) protein concentrations. The maximum quantum efficiency of photosystem II (Fv/Fm), quantum yield of electron transfer at photosystem II (,PSII), and photochemical quenching (qP) decreased at 6 hai in Col-0 but not in SGRi. Disruption of photosynthetic electron flow will cause light-dependent H2O2 generation at 6 hai. ,We conclude that disruption of LHCs, possibly influenced by SGR, and absence of PaO produce phototoxic chlorophyll catabolites and oxidative stress leading to the HR. [source] Current imaging in quantum point contactsPHYSICA STATUS SOLIDI (A) APPLICATIONS AND MATERIALS SCIENCE, Issue 6 2006Alessandro Cresti Abstract The experimental imaging of microscopic currents in two-dimensional electron gas based systems has been recently realized exploiting an ingenious use of scanning microscope tips. By means of the Keldysh Green's function formalism in a tight-binding framework, I study the electron transport in a model quantum point contact device, obtaining detailed maps of the local current distribution. The results are then compared with those obtained by a direct simulation of the experimental process, i.e. introducing a suitable external potential to reproduce the effect of the coupled microscopic tip on the overall conductance and the electron flow. The analysis of the differences between the two calculations helps to interpret the experimental maps and sheds light on the interference effects of the tip. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] Knockout of major leaf ferredoxin reveals new redox-regulatory adaptations in Arabidopsis thalianaPHYSIOLOGIA PLANTARUM, Issue 3 2008Ingo Voss Ferredoxins are the major distributors for electrons to the various acceptor systems in plastids. In green tissues, ferredoxins are reduced by photosynthetic electron flow in the light, while in heterotrophic tissues, nicotinamide adenine dinucleotide (reduced) (NADPH) generated in the oxidative pentose-phosphate pathway (OPP) is the reductant. We have used a Ds -T-DNA insertion line of Arabidopsis thaliana for the gene encoding the major leaf ferredoxin (Fd2, At1g60950) to create a situation of high electron pressure in the thylakoids. Although these plants (Fd2-KO) possess only the minor fraction of leaf Fd1 (At1g10960), they grow photoautotrophically on soil, but with a lower growth rate and less chlorophyll. The more oxidized conditions in the stroma due to the formation of reactive oxygen species are causing a re-adjustment of the redox state in these plants that helps them to survive even under high light. Redox homeostasis is achieved by regulation at both, the post-translational and the transcriptional level. Over-reduction of the electron transport chain leads to increased transcription of the malate-valve enzyme NADP-malate dehydrogenase (MDH), and the oxidized stroma leads to an increased transcription of the OPP enzyme glucose-6-P dehydrogenase. In isolated spinach chloroplasts, oxidized conditions give rise to a decreased activation state of NADP-MDH and an activation of glucose-6-P dehydrogenase even in the light. In Fd2-KO plants, NADPH-requiring antioxidant systems are upregulated. These adjustments must be caused by plastid signals, and they prevent oxidative damage under rather severe conditions. [source] Some photosynthetic responses to salinity resistance are transferred into the somatic hybrid descendants from the wild soybean Glycine cyrtoloba ACC547PHYSIOLOGIA PLANTARUM, Issue 3 2007Yong Yang The somatic hybrid descendants between a cultivated soybean Glycine max Melrose and a wild species Glycine cyrtoloba ACC547 were found to possess some salinity-resistant traits of the wild soybean. Under salt stress, two of the descendants as well as their wild parent grew better than their cultivated parent. In addition, salinity-induced decline in the net photosynthetic rate and the maximum photochemical efficiency was much less in the wild species and the descendants than in Melrose when stressed for more than 5 days. Analysis of the postillumination transient increase in chlorophyll fluorescence and the dark rereduction of the oxidized primary electron donor in photosystem I (PSI) (P700+) indicated that salinity induced a significant upregulation of the cyclic electron flow around PSI (CEF1) in the wild species and the hybrid descendants. Similar to their wild parent, the descendants maintained higher non-photochemical dissipation of excess excitation energy than their cultivated parent under salt stress. As a consequence, there were lower levels of superoxide radical and membrane lipid peroxidation in the plants of the descendants and the wild species. Based on these results, we proposed that the high salinity resistance of the descendants might be because of, at least partially, the trait inherited from the wild species of the enhanced CEF1 which contributed to the sufficient dissipation of excess excitation energy to protect photosynthetic apparatus from the damage of reactive oxygen species. [source] Chlororespiration and cyclic electron flow around PSI during photosynthesis and plant stress responsePLANT CELL & ENVIRONMENT, Issue 9 2007DOMINIQUE RUMEAU ABSTRACT Besides major photosynthetic complexes of oxygenic photosynthesis, new electron carriers have been identified in thylakoid membranes of higher plant chloroplasts. These minor components, located in the stroma lamellae, include a plastidial NAD(P)H dehydrogenase (NDH) complex and a plastid terminal plastoquinone oxidase (PTOX). The NDH complex, by reducing plastoquinones (PQs), participates in one of the two electron transfer pathways operating around photosystem I (PSI), the other likely involving a still uncharacterized ferredoxin-plastoquinone reductase (FQR) and the newly discovered PGR5. The existence of a complex network of mechanisms regulating expression and activity of the NDH complex, and the presence of higher amounts of NDH complex and PTOX in response to environmental stress conditions the phenotype of mutants, indicate that these components likely play a role in the acclimation of photosynthesis to changing environmental conditions. Based on recently published data, we propose that the NDH-dependent cyclic pathway around PSI participates to the ATP supply in conditions of high ATP demand (such as high temperature or water limitation) and together with PTOX regulates cyclic electron transfer activity by tuning the redox state of intersystem electron carriers. In response to severe stress conditions, PTOX associated to the NDH and/or the PGR5 pathway may also limit electron pressure on PSI acceptor and prevent PSI photoinhibition. [source] Stimulation of chlororespiration by heat and high light intensity in oat plantsPLANT CELL & ENVIRONMENT, Issue 8 2006MARÍA JOSÉ QUILES ABSTRACT High irradiance and moderate heat inhibit the activity of the photosynthetic apparatus of oat (Avena sativa L.) leaves. The incubation of oat leaves under high light intensity in conjunction with high temperatures strongly decreased the maximal quantum yield of photosystem (PS) II, indicating the close synergistic effect of both stress factors on PS II inhibition and the subsequent irreversible damage to the photosynthetic apparatus. The PS I A/B protein levels remained similar to control values in leaves incubated under high light intensity or moderate heat, and decreased only when both stress factors were simultaneously applied. Immunoblot analysis of thylakoid membranes using specific antibodies raised against the NDH-K subunit of the thylakoidal NADH dehydrogenase complex (NADH DH) and against plastid terminal oxidase (PTOX) revealed an increase in the amount of both proteins in response to high light intensity and/or heat treatments. In addition, these stress treatments were seen to stimulate the activity of electron donation by NADPH and ferredoxin to plastoquinone, the PTOX activity in plastoquinone oxidation and the NADH DH activity in thylakoid membranes. Incubation with n -propyl gallate (an inhibitor of PTOX) inhibited the increase of NDH-K and PTOX levels under high light intensity and heat, and slightly stimulated the activity of electron donation by NADPH and ferredoxin to plastoquinone. Antimycin A (an inhibitor of cyclic electron flow) increased the NADH DH activity and preserved the levels of NDH-K and PTOX in thylakoid membranes from leaves incubated under high light intensity and heat. The up-regulation of the PTOX and the thylakoidal NADH DH complex under these stress conditions supports a role for chlororespiration in the protection against high irradiance and moderate heat. [source] Effects of moderate heat stress on photosynthesis: importance of thylakoid reactions, rubisco deactivation, reactive oxygen species, and thermotolerance provided by isoprenePLANT CELL & ENVIRONMENT, Issue 3 2005THOMAS D. SHARKEY ABSTRACT Photosynthesis is particularly sensitive to heat stress and recent results provide important new insights into the mechanisms by which moderate heat stress reduces photosynthetic capacity. Perhaps most surprising is that there is little or no damage to photosystem II as a result of moderate heat stress even though moderate heat stress can reduce the photosynthetic rate to near zero. Moderate heat stress can stimulate dark reduction of plastoquinone and cyclic electron flow in the light. In addition, moderate heat stress may increase thylakoid leakiness. At the same time, rubisco deactivates at moderately high temperature. Relationships between effects of moderate heat on rubisco activation and thylakoid reactions are not yet clear. Reactive oxygen species such as H2O2 may also be important during moderate heat stress. Rubisco can make hydrogen peroxide as a result of oxygenase side reactions and H2O2 production by rubisco was recently shown to increase substantially with temperature. The ability to withstand moderately high temperature can be improved by altering thylakoid lipid composition or by supplying isoprene. In my opinion this indicates that thylakoid reactions are important during moderate heat stress. The deactivation of rubisco at moderately high temperature could be a parallel deleterious effect or a regulatory response to limit damage to thylakoid reactions. [source] Polyribosome loading of spinach mRNAs for photosystem I subunits is controlled by photosynthetic electron transportTHE PLANT JOURNAL, Issue 5 2002-untranslated region, A crucial cis element in the spinach PsaD gene is located in the Summary In light-, but not in dark-grown spinach seedlings, the mRNAs for the nuclear-encoded photosystem I subunits D, F and L are associated with polyribosomes and this association is prevented by the application of 3-(3,,4,-dichlorophenyl)-1,1,-dimethyl urea (DCMU), an inhibitor of the photosynthetic electron transport. To identify the cis -elements which are responsible for this regulation, we generated a series of chimeric PsaD constructs and tested them in transgenic tobacco. The spinach PsaD 5,-untranslated region is sufficient to confer light- and photosynthesis-dependent polyribosome association onto the uidA reporter gene, while the tobacco PsaD 5,-untranslated region directs constitutive polyribosome association. These results are discussed with regard to signals from photosynthetic electron flow which control processes in the cytoplasm. [source] Microbial community structure in a biofilm anode fed with a fermentable substrate: The significance of hydrogen scavengersBIOTECHNOLOGY & BIOENGINEERING, Issue 1 2010Prathap Parameswaran Abstract We compared the microbial community structures that developed in the biofilm anode of two microbial electrolysis cells fed with ethanol, a fermentable substrate,one where methanogenesis was allowed and another in which it was completely inhibited with 2-bromoethane sulfonate. We observed a three-way syntrophy among ethanol fermenters, acetate-oxidizing anode-respiring bacteria (ARB), and a H2 scavenger. When methanogenesis was allowed, H2 -oxidizing methanogens were the H2 scavengers, but when methanogenesis was inhibited, homo-acetogens became a channel for electron flow from H2 to current through acetate. We established the presence of homo-acetogens by two independent molecular techniques: 16S rRNA gene based pyrosequencing and a clone library from a highly conserved region in the functional gene encoding formyltetrahydrofolate synthetase in homo-acetogens. Both methods documented the presence of the homo-acetogenic genus, Acetobacterium, only with methanogenic inhibition. Pyrosequencing also showed a predominance of ethanol-fermenting bacteria, primarily represented by the genus Pelobacter. The next most abundant group was a diverse community of ARB, and they were followed by H2 -scavenging syntrophic partners that were either H2 -oxidizing methanogens or homo-acetogens when methanogenesis was suppressed. Thus, the community structure in the biofilm anode and suspension reflected the electron-flow distribution and H2 -scavenging mechanism. Biotechnol. Bioeng. 2010;105: 69,78. © 2009 Wiley Periodicals, Inc. [source] Oriented Electric Fields Accelerate Diels,Alder Reactions and Control the endo/exo SelectivityCHEMPHYSCHEM, Issue 1 2010Rinat Meir Abstract Herein we demonstrate that an external electric field (EEF) acts as an accessory catalyst/inhibitor for Diels,Alder (DA) reactions. When the EEF is oriented along the "reaction axis" (the coordinate of approach of the reactants in the reaction path), the barrier of the DA reactions is lowered by a significant amount, equivalent to rate enhancements by 4,6 orders of magnitude. Simply flipping the EEF direction has the opposite effect, and the EEF acts as an inhibitor. Additionally, an EEF oriented perpendicular to the "reaction axis" in the direction of the individual molecule dipoles can change the endo/exo selectivity, favouring one or the other depending on the positive/negative directions of the EEF vis-à-vis the individual molecular dipole. At some critical value of the EEF along the "reaction axis", there is a crossover to a stepwise mechanism that involves a zwitterionic intermediate. The valence bond diagram model is used to comprehend these trends and to derive a selection rule for EEF effects on chemical reactions: an EEF aligned in the direction of the electron flow between the reactants will lower the reaction barrier. It is shown that the exo/endo control by the EEF is not associated with changes in secondary orbital interactions. [source] In Situ Spectroscopic Characterization of Rectifying Molecular Monolayers Self-Assembled on GoldCHEMPHYSCHEM, Issue 15 2007Alberto Girlando Prof. Abstract We report visible, Raman, and infrared spectra of self-assembled monolayers (SAMs) formed by the donor-(,-bridge)-acceptor chromophore, Z -,-[N -(,-acetylthioalkyl)-4-quinolinium]-,-cyano-4-styryldicyanomethanide (CH3CO-S-CnH2n -Q3CNQ where n=8, 10), on gold-coated substrates. The data are compared with the spectra collected for the same compound in solution and in the solid state, and with those obtained for a Langmuir,Blodgett (LB) monolayer of C16H33 -Q3CNQ deposited on gold. Spectral analysis confirms that in solution, in the solid state and in the LB film the chromophore has a zwitterionic (D+ -,-A,) ground state. At variance with this well-known result, our data show that in SAMs deposited on gold the chromophore has a more neutral, quinoid ground state. We relate this difference to the different packing of the molecules in the two different films: in SAMs in fact the chromophores stand almost vertical with respect to the substrate, whereas in LB films they make an angle of about 45 degrees. The Q3CNQ molecule is a well-known molecular rectifier, and for SAMs we were able to check the direction of electron flow at forward bias on the same samples that have been characterized spectroscopically, shedding light on the rectification mechanism. [source] An electron-flow model can predict complex redox reactions in mixed-culture fermentative BioH2: Microbial ecology evidenceBIOTECHNOLOGY & BIOENGINEERING, Issue 4 2009Hyung-Sool Lee Abstract We developed the first model for predicting community structure in mixed-culture fermentative biohydrogen production using electron flows and NADH2 balances. A key assumption of the model is that H2 is produced only via the pyruvate decarboxylation-ferredoxin-hydrogenase pathway, which is commonly the case for fermentation by Clostridium and Ethanoligenens species. We experimentally tested the model using clone libraries to gauge community structures with mixed cultures in which we did not pre-select for specific bacterial groups, such as spore-formers. For experiments having final pHs 3.5 and 4.0, where H2 yield and soluble end-product distribution were distinctly different, we established stoichiometric reactions for each condition by using experimentally determined electron equivalent balances. The error in electron balancing was only 3% at final pH 3.5, in which butyrate and acetate were dominant organic products and the H2 yield was 2.1,mol,H2/mol,glucose. Clone-library analysis showed that clones affiliated with Clostridium sp. BL-22 and Clostridium sp. HPB-16 were dominant at final pH 3.5. For final pH 4.0, the H2 yield was 0.9,mol,H2/mol,glucose, ethanol, and acetate were the dominant organic products, and the electron balance error was 13%. The significant error indicates that a second pathway for H2 generation was active. The most abundant clones were affiliated with Klebsiella pneumoniae, which uses the formate-cleavage pathway for H2 production. Thus, the clone-library analyses confirmed that the model predictions for when the pyruvate decarboxylation-ferredoxin-hydrogenase pathway was (final pH 3.5) or was not (final pH 4.0) dominant. With the electron-flow model, we can easily assess the main mechanisms for H2 formation and the dominant H2 -producing bacteria in mixed-culture fermentative bioH2. Biotechnol. Bioeng. 2009; 104: 687,697 © 2009 Wiley Periodicals, Inc. [source] | ||||||||||