Photosynthetic Electron Transport System Promotes Synthesis of Au-Nanoparticles

In this communication, a novel, green, efficient and economically viable light mediated protocol for generation of Au-nanoparticles using most vital organelle, chloroplasts, of the plant system is portrayed. Thylakoids/chloroplasts isolated from Potamogeton nodosus (an aquatic plant) and Spinacia oleracea (a terrestrial plant) turned Au³⁺ solutions purple in presence of light of 600 µmol m⁻² s⁻¹ photon flux density (PFD) and the purple coloration intensified with time. UV-Vis spectra of these purple colored solutions showed absorption peak at ∼545 nm which is known to arise due to surface plasmon oscillations specific to Au-nanoparticles. However, thylakoids/chloroplasts did not alter color of Au³⁺ solutions in dark. These results clearly demonstrated that photosynthetic electron transport can reduce Au³⁺ to Au⁰ which nucleate to form Au-nanoparticles in presence of light. Transmission electron microscopic studies revealed that Au-nanoparticles generated by light driven photosynthetic electron transport system of thylakoids/chloroplasts were in range of 5–20 nm. Selected area electron diffraction and powder X-ray diffraction indicated crystalline nature of these nanoparticles. Energy dispersive X-ray confirmed that these nanoparticles were composed of Au. To confirm the potential of light driven photosynthetic electron transport in generation of Au-nanoparticles, thylakoids/chloroplasts were tested for their efficacy to generate Au-nanoparticles in presence of light of PFD ranging from 60 to 600 µmol m⁻² s⁻¹. The capacity of thylakoids/chloroplasts to generate Au-nanoparticles increased remarkably with increase in PFD, which further clearly demonstrated potential of light driven photosynthetic electron transport in reduction of Au³⁺ to Au⁰ to form nanoparticles. The light driven donation of electrons to metal ions by thylakoids/chloroplasts can be exploited for large scale production of nanoparticles.     

Mechanism of Linolenic Acid-induced Inhibition of Photosynthetic Electron Transport

The effect of linolenic acid on photosynthetic electron transport reactions in chloroplasts has been localized at a site on the donor side of photosystem I and at two functionally distinct sites in photosystem II.     

Role of Nitrate in Photosynthetic Electron Transport of Chlorella Vulgaris

Addition of nitrate to a suspension of NO₃ ^--depleted Chlorella vulgaris cells raised the O₂-evolving capacity of the organism by 60%. The rate of O₂-evolution under flash irradiation of the depleted cells was drastically reduced, which could be restored by addition of NO₃ ^-. The 2,5-dibromo-3-methyl-6-isopropyl-p-benzoquinone (DBMIB)-insensitive O₂-evolution, i.e., photosystem (PS) 2 activity of NO₃--depleted cells, showed a 75% stimulation by addition of NO₃ ^-. PS1-mediated electron transport was also stimulated (50%) by addition of NO₃ ^-. Fluorescence yields of the NO₃ ^--depleted cells were significantly reduced. A normal fluorescence response was restored by the addition of NO₃ ^-. The fluorescence yield of the NO₃ ^--depleted and DCMU-treated-cells increased significantly after addition of NO₃ ^- ions, indicating a further reduction of the primary acceptor of PS2 (Q). In addition, the low temperature fluorescence emission spectra showed that energy transfer to PS2 and PS1 was much higher when nitrate was present. Hence nitrate accelerates the light-induced charge transfer from the intact O₂-evolving system to the primary electron acceptor of PS2 and stimulates the PS1-mediated electron transport. This revised version was published online in June 2006 with corrections to the Cover Date.     

Mathematical Simulation of Electron Transport in the Primary Photosynthetic Processes

Biochemistry (Moscow) - Summarized results of investigation of regulation of electron transport and associated processes in the photosynthetic membrane using methods of mathematical and computer...     

Hydrogen Peroxide Inhibits Photosynthetic Electron Transport in Cells of Cyanobacteria

The effect of H₂O₂ on photosynthetic O₂ evolution and photosynthetic electron transfer in cells of cyanobacteria Anabaena variabilis and Anacystis nidulans was studied. The following experiments were performed: 1) directly testing the effect of exogenous H₂O₂; 2) testing the effect of intracellular H₂O₂ generated with the use of methyl viologen (MV); 3) testing the effect of inhibiting intracellular H₂O₂ decomposition by salicylic acid (SA) and 3-amino-1,2,4-triazole (AT). H₂O₂ inhibited photosynthetic O₂ evolution and light-induced reduction of p-benzoquinone (BQ) + ferricyanide (FeCy) in the Hill reaction. The I₅₀ value for H₂O₂ was 0.75 mM. Photosynthetic electron transfer in the cells treated with H₂O₂ was not maintained by H₂O₂, NH₂OH, 1,5-diphenylcarbazide, tetraphenylboron, or butylated hydroxytoluene added as artificial electron donors for Photosystem (PS) II. The H₂O CO₂, H₂O MV (involving PSII and PSI) and H₂O BQ + FeCy (chiefly dependent on PSII) electron transfer reactions were inhibited upon incubation of the cells with MV, SA, or AT. The N,N,N",N"-tetramethyl-p-phenylenediamine MV (chiefly dependent on PSI) electron transfer was inhibited by SA and AT but was resistant to MV. The results show that H₂O₂ inhibits photosynthetic electron transfer. It is unlikely that H₂O₂ could be a physiological electron donor in oxygenic photosynthesis.     

Photosynthetic Electron Transport Inhibition by 3-Acyl-2,4,6-trihydroxybenzamide Derivatives

A series of 3-acyl-2,4,6-trihydroxybenzamides was synthesized, and the compounds’ PET inhibitory activities were examined in isolated chloroplasts. In general, the PET inhibitory activity was found to depend on the overall lipophilicity of the molecule. Low activities of the mono and dihydroxy derivatives indicated that the three hydroxyl groups on the nucleus were essential for high activity. The PET inhibition study, on chloroplasts isolated from an atrazine resistant biotype of Brassica napus and using thermoluminescence analysis, suggested that the trihydroxybenzamide derivatives would be classified as a urea type rather than a phenol type of PET inhibitor. However the trihydroxybenzamide derivatives, like the phenol type of PET inhibitor, showed a lag time before inhibition started, which was followed by constant activity. These results indicate that the binding domain for the trihy-droxybenzamide derivatives overlapped with those of both the urea type and phenol type of PET inhibitors.     

两种生态型芦苇叶绿体的光合电子传递和抗氧保护体系

分布于甘肃省河西走廊的两种不同生态型芦苇-水生芦苇(水芦,生长在深约1m的水滩里)和沙丘芦苇(沙芦,生长在高约5m的沙丘上)在其离体叶绿体的光化学活性上表现为前者高于后者。其中,沙芦全链电子传递速率及光系统Ⅱ(PSⅡ)电子传递速率明显低于水芦,而光系统Ⅰ(PSⅠ)电子传递速率却与水芦接近。沙芦叶片和叶绿体中抗氧化酶活性及叶片抗坏血酸含量均高于水芦。综合结果表明,喜水植物芦苇登陆后,在自然选择压力下,为适应长期的自然干旱胁迫环境,其抗氧保护系统在保护PSⅠ免受水分胁迫诱导的氧化伤害中可能发挥着重要作用。     

Photosynthetic Electron Transport Chain of Chlamydomonas reinhardi VI. Electron Transport in Mutant Strains Lacking Either Cytochrome 553 or Plastocyanin

A mutant strain of Chlamydomonas reinhardi, ac-206, lacks cytochrome 553, at least in an active and detectable form. Chloroplast fragments of this mutant strain are inactive in the photoreduction of NADP when the source of electrons is water, but they are active when the electron source is 2,6-dichlorophenolindophenol and ascorbate. The addition of either cytochrome 553 or plastocyanin, obtained from the wild-type strain, has no effect upon the photosynthetic activities of the mutant strain. Cells of the mutant strain lack both the soluble and insoluble forms of cytochrome 553, but they possess the mitochondrial type cytochrome c. Thus, the loss of cytochrome 553 appears to be specific.     

Thermal Acclimation of Photosynthetic Electron Transport Activity by Thylakoids of Saxifraga cernua

Thermal acclimation by Saxifraga cernua to low temperatures results in a change in the optimum temperature for gross photosynthetic activity and may directly involve the photosynthetic apparatus. In order to test this hypothesis photosynthetic electron transport activity of S. cernua thylakoids acclimated to growth temperatures of 20°C and 10°C was measured in vitro. Both populations exhibited optimum temperatures for whole chain and PSII electron transport activity at temperatures close to those at which the plants were grown. Chlorophyll a fluorescence transients from 10°C-acclimated leaves showed higher rates in the rise and subsequent quenching of variable fluorescence at low measuring temperatures; 20°C-acclimated leaves showed higher rates of fluorescence rise at higher measuring temperatures. At these higher temperatures, fluorescence quenching rates were similar in both populations. The kinetics of State 1-State 2 transitions in 20°C- and 10°C-acclimated leaf discs were measured as changes in the magnitude of the fluorescence emission maxima measured at 77K. Leaves acclimated at 10°C showed a larger F730/F695 ratio at low temperatures, while at higher temperatures, 20°C-acclimated leaves showed a higher F730/F695 ratio after the establishment of State 2. High incubation temperatures also resulted in a decrease in the F695/F685 ratio for 10°C-acclimated leaves, suggesting a reduction in the excitation transfer from the light-harvesting complex of photosystem II to photosystem II reaction centers. The relative amounts of chlorophyll-protein complexes and thylakoid polypeptides separated electro-phoretically were similar for both 20°C- and 10°C-acclimated leaves. Thus, photosynthetic acclimation to low temperatures by S. cernua is correlated with an increase in photosynthetic electron transport activity but does not appear to be accompanied by major structural changes or different relative amounts in thylakoid protein composition.     

Effects of Lipoxygenase on the Activities of Photosynthetic Electron Transport in Cucumber Leaves

Photosynthesis and electron transport activity decreased with leaf aging, and however, lipoxygenase (Lox) activity increased correspondingly. Soybean Lox-1 inhibited significantly PSⅡ electron transport activity of chloroplasts isolated from cucumber (Cucumis sativus L. ) cotyledon. But the inhibition could be eliminated by the addition of propyl gallate (PG) or 3, 3, 4, 5, 7-pentahydroxyflavon (PF). The inhibition of PSⅠ activity by soybean Lox-1 was enhanced in the presence of 3, 4, dichlorophenyl-1, 1-dimethylurea (DCMU) or 2, 5-dibromothymoquinone (DBMIB), bfft could be restored to its original level when PG was added. Addition of 2, 2-diphenylcarbonic dihydrazide (DPC) to the mixture of isolated chloroplasts and Lox-1, PSⅡ activity resumed obviously. Chlorophyll a fluorescence study showed that Fm was decreased by Lox-1 and resumed slightly by DPC. Based on the above results, it was suggested that Lox might act at least on three sensitive sites located on Q, PQ and the oxidative side of PSⅠ . The bleaching of chlorophyll and carotenoid stimulated by Lox-l, and the inhibition of PSⅠ electron transport activity by active oxygen might be. one of the important reasons to explaine the effect of Lox on the function of photosynthetic membrane.     

Responses of Photosynthetic Electron Transport to Drought and Re-watering in Two Maize Genotypes

Russian Journal of Plant Physiology - Effects of drought stress on photosynthesis have been well-documented. However, photosynthetic electron transport process in response to combined drought...     

Sodium Dependency of the Photosynthetic Electron Transport in the Alkaliphilic Cyanobacterium Arthrospira platensis

Arthrospira (Spirulina) platensis (A. platensis) is a model organism for investigation of adaptation of photosynthetic organisms to extreme environmental conditions: the cell functions in this cyanobacterium are optimized to high pH and high concentration (150–250 mM) of Na⁺. However, the mechanism of the possible fine-tuning of the photosynthetic functions to these extreme conditions and/or the regulation of the cellular environment to optimize the photosynthetic functions is poorly understood. In this work we investigated the effect of Na-ions on different photosynthetic activities: linear electron transport reactions (measured by means of polarography and spectrophotometry), the activity of photosystem II (PS II) (thermoluminescence and chlorophyll a fluorescence induction), and redox turnover of the cytochrome b ₆ f complex (flash photolysis); and measured the changes of the intracellular pH (9-aminoacridine fluorescence). It was found that sodium deprivation of cells in the dark at pH 10 inhibited, within 40 min, all measured photosynthetic reactions, and led to an alkalinization of the intracellular pH, which rose from the physiological value of about 8.3–9.6. These were partially and totally restored by readdition of Na-ions at 2.5–25 mM and about 200 mM, respectively. The intracellular pH and the photosynthetic functions were also sensitive to monensin, an exogenous Na⁺/H⁺ exchanger, which collapses both proton and sodium gradients across the cytoplasmic membrane. These observations explain the strict Na⁺-dependency of the photosynthetic electron transport at high extracellular pH, provide experimental evidence on the alkalization of the intracellular environment, and support the hypothesized role of an Na⁺/H⁺ antiport through the plasma membrane in pH homeostasis (Schlesinger et al. (1996). J. Phycol. 32, 608–613). Further, we show that (i) the specific site of inactivation of the photosynthetic electron transport at alkaline pH is to be found at the water splitting enzyme; (ii) in contrast to earlier reports, the inactivation occurs in the dark and, for short periods, without detectable damage in the photosynthetic apparatus; and (iii) in contrast to high pH, Na⁺ dependency in the neutral pH range is shown not to originate from PSII, but from the acceptor side of PSI. These data permit us to conclude that the intracellular environment rather than the machinery of the photosynthetic electron transport is adjusted to the extreme conditions of high pH and high Na⁺ concentration.     

Quantum Requirements of Photosynthetic Electron Transport In Sitka Spruce from Different Light Environments

Quantum requirements of photosynthetic electron transport have been measured in shoots of Picea sitchensis (Bong.) Carr. (Sitka spruce) from different levels in a forest canopy and in shoots from plants grown in contrasting light environments in controlled environment chambers. Neutral density filters were used to obtain very low photon flux densities. The light absorbed by the chloroplast suspensions was calculated from measurements of the transmittance of the suspensions. The shoots from the top of the forest canopy (“sun” shoots) had lower quantum requirements for photosystems I and II than the shoots from the bottom of the forest canopy (“shade” shoots). High light grown plants and “sun” shoots had higher rates of electron transport at light saturation than low light grown plants and “shade” shoots. Thus a higher potential for electron transport was found to exist in “sun” shoots than in “shade” shoots at both high and low photon flux densities.     

Studies of Electron Transport Dynamics in Photosynthetic Reaction Centers Using Fast Temperature Changes

Rates of thermoinduced conformational transitions of reaction center (RC) complexes providing effective electron transport were studied in chromatophores and isolated RC preparations of various photosynthesizing purple bacteria using methods of fast freezing and laser-induced temperature jump. Reactions of electron transfer from the primary to secondary quinone acceptors and from the multiheme cytochrome c subunit to photoactive bacteriochlorophyll dimer were used as probes of electron transport efficiency. The thermoinduced transition of the acceptor complex to the conformational state facilitating electron transfer to the secondary quinone acceptor was studied. It was shown that neither the characteristic time of the thermoinduced transition within the temperature range 233-253 K nor the characteristic time of spontaneous decay of this state at 253 K exceeded several tens of milliseconds. In contrast to the quinone complex, the thermoinduced transition of the macromolecular RC complex to the state providing effective electron transport from the multiheme cytochrome c to the photoactive bacteriochlorophyll dimer within the temperature range 220-280 K accounts for tens of seconds. This transition is thought to be mediated by large-scale conformational dynamics of the macromolecular RC complex.     

Photosynthetic Electron Transport in Single Guard Cells as Measured by Scanning Electrochemical Microscopy

Scanning electrochemical microscopy (SECM) is a powerful new tool for studying chemical and biological processes. It records changes in faradaic current as a microelectrode ([less than equal]7 [mu]m in diameter) is moved across the surface of a sample. The current varies as a function of both distance from the surface and the surface's chemical and electrical properties. We used SECM to examine in vivo topography and photosynthetic electron transport of individual guard cells in Tradescantia fluminensis, to our knowledge the first such analysis for an intact plant. We measured surface topography at the micrometer level and concentration profiles of O2 evolved in photosynthetic electron transport. Comparison of topography and oxygen profiles above single stomatal complexes clearly showed photosynthetic electron transport in guard cells, as indicated by induction of O2 evolution by photosynthetically active radiation. SECM is unique in its ability to measure topography and chemical fluxes, combining some of the attributes of patch clamping with scanning tunneling microscopy. In this paper we suggest several questions in plant physiology that it might address.     

Effect of High Temperature on Photosynthetic Electron Transport Activities of the Cyanobacterium Spirulina Platensis

The activities of photosystem 2 (PS2) and whole chain electron transport declined in high temperature treated cells at the room temperature beyond 35 °C, while photosystem 1 (PS1) showed increased activity. Thylakoid membrane studies did not exhibit increase in PS1 activity indicating that the enhancement of PS1 activity is due to permeability change of cell membranes. However, the electron transport activity measured from reduced duroquinone to methylviologen which involves intersystem electron transport was extremely sensitive to high temperature. The activity of PS2 at different irradiance, which was accompanied by alterations in absorption and fluorescence emission properties, indicated changes in the energy transfer processes within phycobilisomes. Thus high temperature has multiple target sites in photosynthetic electron transport system of Spirulina platensis.     

Fluorescence Transients as Indicator for the Existence of Regulatory Mechanisms in Photosynthetic Electron Transport

In green algae several characteristic differences in the slope of the fast 685 nm fluorescence transient indicate the existence of different mechanisms for the regulation of the photosynthetic electron transport in vivo with respect to the requirements for ATP and NADPH. Autotrophically cultivated Chlamydobotrys stellata exhibits a normal time curve of the fluorescence yield. Anaerobiosis and C0₂-deficiency raise the O-, I- and S-level, whereas the P- level is lowered and the I-D-decay disappears. The readdition of oxygen increases the fluorescence significantly. Supplementation of aerobic cells with CO₂ restores the normal fluorescence transients. The replacement of carbon dioxide by acetate as a carbon source in the light lowers the overall fluorescence emission and abolishes the D-P-increase and the P-S-decline. The presence of DCMU increases fluorescence only at high intensities of incedent light. Anaerobiosis in these photoheterotrophic algae lowers the fluorescence emission. In this case DCMU increases fluorescence even at low light intensities. In Gonium multicoccum, which shows a normal fluorescence transient when cultivated autotrophically, CO₂-deficiency abolishes the O-level and increases the I- and S-niveau. Additional anaerobiosis in CO₂-deficient cells raises the steady state emission. Readdition of oxygen to these cells raises the I- and S-level even more and prevents the build up of the P-level. In Gonium     

Photosynthetic Electron Transport under Phosphorylating Conditions as Influenced by Different Concentrations of Various Salts

Osman, M. E-A. H. and El-Shentenawy, F. 1988. Photosyntheticelectron transport under phosphorylating conditions as influencedby different concentrations of various salts.—J. exp.Bot. 39: 859–863. The rate of light-induced electron transport by isolated spinachthylakoids under phosphorylating conditions, as affected bydifferent concentrations of Br^–, Cl^–, NO₃^–,HCO₃^–, SO₄^2– and CO₃^2– has been investigated.The results show that both low and high concentrations of HCO₃^2–stimulated the oxygen evolution capacity under phosphorylatingconditions, whereas only low concentrations of CO₃^2–,SO₄^2– and Cl^– stimulated the oxygen evolution capacity.However, irrespective of concentration, both Br^– and NO₃^–reduced this capacity. The rate of photosynthetic electron transportwas generally stimulated by addition of ADP, even in cases whereelectron transport was inhibited by addition of bromide andnitrate. The different concentrations of these anions also causedreduction of the power generated by proton pumping and usedfor phosphorylation. The greatest level of reduction was observedin the presence of high concentrations of Cl^– and HCO₃^–. Key words: Spinach thylakoids, photosynthetic electron transport, phosphorylation     

Changes of Photochemical Activities and Photosynthetic Electron Transport of Chloroplasts in the Course of Ageing

Chloroplasts were isolated from Spinacia olerecea L. and Doblichos lablab L. Chloroplasts suspension was stored in refrigerator at 5–8 ℃. Photochemical activities and chlorophyll content of chloroplasts at different times of storage were deter- mined. The results can be summarized as follows: 1. In the course of chloroplasts ageing, the lost of K3Fe(CN)6 photo reduction activity was more than that of DCPIP photoreduction activity. 2. The activity of K3Fe(CN)6 photoreduction during storage began to decrease markedly after 12 hours, but activity of DCPIP photoreduction began to decrease markedly after 24 hours. 3. The DCPIP photoreduction activity of aged chloroplasts was stimulated by the addition of 1.5-diphenylcarbazide. 4. Destruction of oxidized side of PSⅡ was earlier and higher than that of the other side (from the active center of PSⅡ to the reduced side of PSⅠ). 5. During chloroplasts ageing, the decrease of chlorophyll content was less than the rate of decrease of photochemical activities.