271 research outputs found
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A study of the effects of the use of perceptual methods in the supervision of intern teachers.
The role of reactive reaction intermediates in two-step heterogeneous electro-catalytic reactions: A model study
Experimental investigations of heterogeneous electrocatalytic reactions have been performed in flow cells which provide an environment with controlled parameters. Measurements of the oxygen reduction reaction in a flow cell with an electrode consisting of an array of Pt nanodisks on a glassy carbon substrate exhibited a decreasing fraction of the intermediate in the overall reaction products with increasing density of the nanodiscs. A similar result is true for the dependence on the catalyst loading in the case of a supported Pt/C catalyst thin-film electrode, where the fraction of the intermediate decreases with increasing catalyst loading. Similar effects have been detected for the methanol oxidation. We present a model of multistep heterogeneous electrocatalytic oxidation and reduction reactions based on an adsorption-reaction-desorption scheme using the Langmuir assumption and macroscopic transport equations. A continuum based model problem in a vertical cross section of a rectangular flow cell is proposed in order to explain basic principles of the experimental situation. It includes three model species A, B, C, which undergo adsorption and desorption at a catalyst surface, as well as adsorbate reactions from A to B to C. These surface reactions are coupled with diffusion and advection in the Hagen Poiseuille flow in the flow chamber of the cell. Both high velocity asymptotic theory and a finite volume numerical are used to obtain approximate solutions to the model. Both approaches show a behaviour similar to the experimentally observed. Working in more general situations, the finite volume scheme was applied to a catalyst layer consisting of a number of small catalytically active areas corresponding to nanodisks. Good qualitative agreement with the experimental findings was established for this case as well
Fluorescence staining of live cyanobacterial cells suggest non-stringent chromosome segregation and absence of a connection between cytoplasmic and thylakoid membranes
<p>Abstract</p> <p>Background</p> <p>In spite of their abundance and importance, little is known about cyanobacterial cell biology and their cell cycle. During each cell cycle, chromosomes must be separated into future daughter cells, i.e. into both cell halves, which in many bacteria is achieved by an active machinery that operates during DNA replication. Many cyanobacteria contain multiple identical copies of the chromosome, but it is unknown how chromosomes are segregated into future daughter cells, and if an active or passive mechanism is operative. In addition to an outer and an inner cell membrane, cyanobacteria contain internal thylakoid membranes that carry the active photosynthetic machinery. It is unclear whether thylakoid membranes are invaginations of the inner cell membrane, or an independent membrane system.</p> <p>Results</p> <p>We have used different fluorescent dyes to study the organization of chromosomes and of cell and thylakoid membranes in live cyanobacterial cells. FM1-43 stained the outer and inner cytoplasmic membranes but did not enter the interior of the cell. In contrast, thylakoid membranes in unicellular <it>Synechocystis </it>cells became visible through a membrane-permeable stain only. Furthermore, continuous supply of the fluorescent dye FM1-43 resulted in the formation of one to four intracellular fluorescent structures in <it>Synechocystis </it>cells, within occurred within 30 to 60 minutes, and may represent membrane vesicles. Using fluorescent DNA stains, we found that <it>Synechocystis </it>genomic DNA is compacted in the cell centre that is devoid of thylakoid membranes. Nucleoids segregated very late in the cell cycle, just before complete closing of the division septum. In striking contrast to <it>Bacillus subtilis</it>, which possesses an active chromosome segregation machinery, fluorescence intensity of stained nucleoids differed considerably between the two <it>Synechocystis </it>daughter cells soon after cell division.</p> <p>Conclusion</p> <p>Our experiments strongly support the idea that the cytoplasmic and thylakoid membranes are not directly connected, but separate entities, in unicellular cyanobacteria. Our findings suggest that a transport system may exist between the cytoplasmic membrane and thylakoids, which could mediate the extension of thylakoid membranes and possibly also protein transport from the cytoplasmic membrane to thylakoid membranes. The cell cycle studies in <it>Synechocystis </it>sp. PCC 6803 show that the multiple chromosome copies per cell segregate very late in the cell cycle and in a much less stringent manner than in <it>B. subtilis </it>cells, indicating that chromosomes may become segregated randomly and in a passive fashion, possibly through constriction of the division septum.</p
Herstellung eines Messgerätes zur Bestimmung der Leistung von Solarzellen
Zur Messung von Spannung, Strom und Leistung, die
von einem Solarmodul an einen veränderlichen
Verbraucher abgegeben werden können, wird eine
Schaltung beschrieben, die auf dem integrierten
Analogmultiplizierer RC4200 aufbaut.
Das entwickelte Mess-System beinhaltet neben der
Stromversorgung und den Anzeigeelementen außerdem
ein Solarmodul, das aus kleinen Einzelzellen auf
einer Leiterplatte aufgebaut wurde
Exploring the Local Milky Way: M Dwarfs as Tracers of Galactic Populations
We have assembled a spectroscopic sample of low-mass dwarfs observed as part
of the Sloan Digital Sky Survey along one Galactic sightline, designed to
investigate the observable properties of the thin and thick disks. This sample
of ~7400 K and M stars also has measured ugriz photometry, proper motions, and
radial velocities. We have computed UVW space motion distributions, and
investigate their structure with respect to vertical distance from the Galactic
Plane. We place constraints on the velocity dispersions of the thin and thick
disks, using two-component Gaussian fits. We also compare these kinematic
distributions to a leading Galactic model. Finally, we investigate other
possible observable differences between the thin and thick disks, such as
color, active fraction and metallicity.Comment: 11 pages, 12 figures, Accepted by A
A new interferometric study of four exoplanet host stars : {\theta} Cygni, 14 Andromedae, {\upsilon} Andromedae and 42 Draconis
Studying exoplanet host stars is of the utmost importance to establish the
link between the presence of exoplanets around various types of stars and to
understand the respective evolution of stars and exoplanets.
Using the limb-darkened diameter (LDD) obtained from interferometric data, we
determine the fundamental parameters of four exoplanet host stars. We are
particularly interested in the F4 main-sequence star, {\theta} Cyg, for which
Kepler has recently revealed solar-like oscillations that are unexpected for
this type of star. Furthermore, recent photometric and spectroscopic
measurements with SOPHIE and ELODIE (OHP) show evidence of a quasi-periodic
radial velocity of \sim150 days. Models of this periodic change in radial
velocity predict either a complex planetary system orbiting the star, or a new
and unidentified stellar pulsation mode.
We performed interferometric observations of {\theta} Cyg, 14 Andromedae,
{\upsilon} Andromedae and 42 Draconis for two years with VEGA/CHARA (Mount
Wilson, California) in several three-telescope configurations. We measured
accurate limb darkened diameters and derived their radius, mass and temperature
using empirical laws.
We obtain new accurate fundamental parameters for stars 14 And, {\upsilon}
And and 42 Dra. We also obtained limb darkened diameters with a minimum
precision of \sim 1.3%, leading to minimum planet masses of Msini=5.33\pm 0.57,
0.62 \pm 0.09 and 3.79\pm0.29 MJup for 14 And b, {\upsilon} And b and 42 Dra b,
respectively. The interferometric measurements of {\theta} Cyg show a
significant diameter variability that remains unexplained up to now. We propose
that the presence of these discrepancies in the interferometric data is caused
by either an intrinsic variation of the star or an unknown close companion
orbiting around it.Comment: 10 pages + 2 pages appendix, 16 figures, accepted for publication in
A&
NICMOS Coronagraphic Observations of 55 Cancri
We present new near-infrared (1.1-micron) observations of the circumstellar
environment of the planet-bearing star 55 Cancri. With these Hubble Space
Telescope images we are unable to confirm the observation of bright scattered
radiation at longer NIR wavelengths previously reported by Trilling & Brown
(1998), and Trilling, Brown & Rivkin (2000). NICMOS coronagraphic images with
detection sensitivities to ~ 100 microJy arcsec at 1.1 microns in the
region 28 - 60 AU from the star fail to reveal any significant excess flux in
point- spread-function (PSF) subtracted images taken in two Hubble Space
Telescope orbits. These new observations place flux densities in the 19-28 AU
zone at a factor of ten or more below the reported ground-based observations.
Applying a suite of a dozen well-matched coronagraphic reference PSFs,
including one obtained in the same orbits as the observations of 55 Cnc,
yielded consistently null results in detecting a disk. We also searched for,
and failed to find, a suggested flux-excess anisotropy in the ratio of ~1.7:1
in the circumstellar background along and orthogonal to the plane of the
putative disk. We suggest that, if such a disk does exist, then the total
1.1-micron spectral flux density in an annular zone 28 - 42 AU from the star
must be no more than ~0.4mJy, at least ten times smaller than suggested by
Trilling and Brown, upon which their very large estimate for the total dust
mass (0.4~M_{\earth}) was based. Based on the far infrared and submillimeter
flux of this system and observations of scattered light and thermal emission
from other debris disks, we also expect the intensity of the scattered light to
be at least an order of magnitude below our upper limits.Comment: 40 pages, accepted to AJ 03OCT2000; 7 figures available at:
http://nicmosis.as.arizona.edu:8000/PREPRINTS/55CNC/55_CNC_PREPRINT.htm
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An Outer Membrane Vesicle-Based Permeation Assay (OMPA) for Assessing Bacterial Bioavailability
When searching for new antibiotics against Gram-negative bacterial infections, a better understanding of the permeability across the cell envelope and tools to discriminate high from low bacterial bioavailability compounds are urgently needed. Inspired by the phospholipid vesicle-based permeation assay (PVPA), which is designed to predict non-facilitated permeation across phospholipid membranes, outer membrane vesicles (OMVs) of Escherichia coli either enriched or deficient of porins are employed to coat filter supports for predicting drug uptake across the complex cell envelope. OMVs and the obtained in vitro model are structurally and functionally characterized using cryo-TEM, SEM, CLSM, SAXS, and light scattering techniques. In vitro permeability, obtained from the membrane model for a set of nine antibiotics, correlates with reported in bacterio accumulation data and allows to discriminate high from low accumulating antibiotics. In contrast, the correlation of the same data set generated by liposome-based comparator membranes is poor. This better correlation of the OMV-derived membranes points to the importance of hydrophilic membrane components, such as lipopolysaccharides and porins, since those features are lacking in liposomal comparator membranes. This approach can offer in the future a high throughput screening tool with high predictive capacity or can help to identify compound- and bacteria-specific passive uptake pathways
The Metallicity Distribution Functions of SEGUE G and K dwarfs: Constraints for Disk Chemical Evolution and Formation
We present the metallicity distribution function (MDF) for 24,270 G and
16,847 K dwarfs at distances from 0.2 to 2.3 kpc from the Galactic plane, based
on spectroscopy from the Sloan Extension for Galactic Understanding and
Exploration (SEGUE) survey. This stellar sample is significantly larger in both
number and volume than previous spectroscopic analyses, which were limited to
the solar vicinity, making it ideal for comparison with local volume-limited
samples and Galactic models. For the first time, we have corrected the MDF for
the various observational biases introduced by the SEGUE target selection
strategy. The SEGUE sample is particularly notable for K dwarfs, which are too
faint to examine spectroscopically far from the solar neighborhood. The MDF of
both spectral types becomes more metal-poor with increasing |Z|, which reflects
the transition from a sample with small [alpha/Fe] values at small heights to
one with enhanced [alpha/Fe] above 1 kpc. Comparison of our SEGUE distributions
to those of two different Milky Way models reveals that both are more
metal-rich than our observed distributions at all heights above the plane. Our
unbiased observations of G and K dwarfs provide valuable constraints over the
|Z|-height range of the Milky Way disk for chemical and dynamical Galaxy
evolution models, previously only calibrated to the solar neighborhood, with
particular utility for thin- and thick-disk formation models.Comment: 70 pages, 25 figures, 7 tables. Accepted by The Astrophysical Journa
Genetic Diversity and Phylogeny of Antagonistic Bacteria against Phytophthora nicotianae Isolated from Tobacco Rhizosphere
The genetic diversity of antagonistic bacteria from the tobacco rhizosphere was examined by BOXAIR-PCR, 16S-RFLP, 16S rRNA sequence homology and phylogenetic analysis methods. These studies revealed that 4.01% of the 6652 tested had some inhibitory activity against Phytophthora nicotianae. BOXAIR-PCR analysis revealed 35 distinct amplimers aligning at a 91% similarity level, reflecting a high degree of genotypic diversity among the antagonistic bacteria. A total of 25 16S-RFLP patterns were identified representing over 33 species from 17 different genera. Our results also found a significant amount of bacterial diversity among the antagonistic bacteria compared to other published reports. For the first time; Delftia tsuruhatensis, Stenotrophomonas maltophilia, Advenella incenata, Bacillus altitudinis, Kocuria palustris, Bacillus licheniformis, Agrobacterium tumefaciens and Myroides odoratimimus are reported to display antagonistic activity towards Phytophthora nicotianae. Furthermore, the majority (75%) of the isolates assayed for antagonistic activity were Gram-positives compared to only 25% that were Gram-negative bacteria
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