Showing papers on "Conductance published in 1992"

Theoretical Considerations when Estimating the Mesophyll Conductance to CO2 Flux by Analysis of the Response of Photosynthesis to CO2

Abstract: The conductance for CO2 diffusion in the mesophyll of leaves can limit photosynthesis. We have studied two methods for determining the mesophyll conductance to CO2 diffusion in leaves. We generated an ideal set of photosynthesis rates over a range of partial pressures of CO2 in the stroma and studied the effect of altering the mesophyll diffusion conductance on the measured response of photosynthesis to intercellular CO2 partial pressure. We used the ideal data set to test the sensitivity of the two methods to small errors in the parameters used to determine mesophyll conductance. The two methods were also used to determine mesophyll conductance of several leaves using measured rather than ideal data sets. It is concluded that both methods can be used to determine mesophyll conductance and each method has particular strengths. We believe both methods will prove useful in the future.

Estimation of Mesophyll Conductance to CO2 Flux by Three Different Methods

Abstract: The resistance to diffusion of CO2 from the intercellular airspaces within the leaf through the mesophyll to the sites of carboxylation during photosynthesis was measured using three different techniques. The three techniques include a method based on discrimination against the heavy stable isotope of carbon, 13C, and two modeling methods. The methods rely upon different assumptions, but the estimates of mesophyll conductance were similar with all three methods. The mesophyll conductance of leaves from a number of species was about 1.4 times the stomatal conductance for CO2 diffusion determined in unstressed plants at high light. The relatively low CO2 partial pressure inside chloroplasts of plants with a low mesophyll conductance did not lead to enhanced O2 sensitivity of photosynthesis because the low conductance caused a significant drop in the chloroplast CO2 partial pressure upon switching to low O2. We found no correlation between mesophyll conductance and the ratio of internal leaf area to leaf surface area and only a weak correlation between mesophyll conductance and the proportion of leaf volume occupied by air. Mesophyll conductance was independent of CO2 and O2 partial pressure during the measurement, indicating that a true physical parameter, independent of biochemical effects, was being measured. No evidence for CO2-accumulating mechanisms was found. Some plants, notably Citrus aurantium and Simmondsia chinensis, had very low conductances that limit the rate of photosynthesis these plants can attain at atmospheric CO2 level.

Quantum transport in semiconductor-superconductor microjunctions

Abstract: A formula is derived that relates the conductance of a normal-metal--superconductor (NS) junction to the single-electron transmission eigenvalues. The formula is applied to a quantum point contact (yielding conductance quantization at multiples of 4${\mathit{e}}^{2}$/h), to a quantum dot (yielding a non-Lorentzian conductance resonance), and to quantum interference effects in a disordered NS junction (enhanced weak-localization and reflectionless tunneling through a potential barrier).

Relations between surface conductance and spectral vegetation indices at intermediate (100 m2 to 15 km2) length scales

Abstract: The relationship between surface conductance and spectral vegetation indices is investigated utilizing the FIFE data set, principally the surface flux station data and images from the TM instrument. It is found that the unstressed canopy conductance for a given site for a given day is near-linearly related to the incident PAR flux. Estimates of unstressed canopy conductance were acquired via a model inversion that separated the soil and vegetation contributions to evapotranspiration and made adjustments for the effects of vapor pressure deficit and soil moisture stress.

A simple method for the determination of the pore radius of ion channels in planar lipid bilayer membranes

Abstract: A new method of pore size determination is presented. The results of applying this simple method to ion channels formed by staphylococcal α-toxin and its N-terminal fragment as well as to cholera toxin channels are shown. The advantages and the difficulties of this method are discussed. It was found that (i) the mobility of ions in solutions depends only on the percentage of concentration of added non-electrolytes and practically not on their chemical nature (sugars or polyglycols) and molecular size; (ii) the proportional change of both ion channel conductance and bulk solution conductivity by low M. non-electrolytes may be used as an indication of a diffusion mechanism of ion transport through channels; (iii) the slope of the dependence of the ion channel conductance on the bulk conductivity of solutions containing different concentrations of non-electrolyte is a good measure of channel permeability for non-electrolytes.

Excess conductance of superconductor-semiconductor interfaces due to phase conjugation between electrons and holes.

Abstract: A semiclassical description is given of charge transport through a superconductor-semiconductor interface. As a result of the presence of a potential barrier both Andreev and normal reflection occur. Elastic scatterers in the semiconductor generate multiple reflections at the interface. The constructive quantum interference which results from the phase conjugation between electrons and holes enhances the (differential) conductance above its classical value. This excess conductance is suppressed by a magnetic field, or by a finite energy. The latter can be due to a finite voltage bias or a finite temperature.

Measurements of the Kapitza conductance between diamond and several metals.

Abstract: The Kapitza conductance between isotopically enriched synthetic diamond and Ti, Al, Au, and Pb has been studied using picosecond optical techniques. For Ti and Al, which have Debye temperatures roughly 1/5 of that of diamond, the measured heat flow is in reasonable agreement with calculations of the phonon heat transport based on a simple lattice dynamical model. However, for Au and Pb, which have Debye temperatures 0.07 and 0.04 times that of diamond, the measured Kapitza conductances are as much as 100 times larger than expected, suggesting the existence of a second conductance mechanism.

Anomalous Andreev conductance in InAs-AlSb quantum well structures with Nb electrodes

Abstract: The differential conductance of InAs-AlSb quantum wells with superconducting Nb electrodes exhibits a sharp single peak at zero bias, followed by a drop below the normal conductance at voltages an order of magnitude higher than the gap voltage, before returning to the normal value at yet higher bias. The behavior is interpreted in terms of Andreev reflections, modified by multiple normal reflections of both electrons and Andreev holes between the Nb electrodes and the bottom barrier of the quantum well, in the presence of a nonuniform potential along the well.

Anion permeation in an apical membrane chloride channel of a secretory epithelial cell

Abstract: Single channel currents though apical membrane Cl channels of the secretory epithelial cell line T84 were measured to determine the anionic selectivity and concentration dependence of permeation. The current-voltage relation was rectified with single channel conductance increasing at positive potentials. At 0 mV the single channel conductance was 41 +/- 2 pS. Permeability, determined from reversal potentials, was optimal for anions with diameters between 0.4 and 0.5 nm. Anions of larger diameter had low permeability, consistent with a minimum pore diameter of 0.55 nm. Permeability for anions of similar size was largest for those ions with a more symmetrical charge distribution. Both HCO3 and H2PO4 had lower permeability than the similar-sized symmetrical anions, NO3 and ClO4. The permeability sequence was SCN greater than I approximately NO3 approximately ClO4 greater than Br greater than Cl greater than PF6 greater than HCO3 approximately F much greater than H2PO4. Highly permeant anions had lower relative single channel conductance, consistent with longer times of residence in the channel for these ions. The conductance sequence for anion efflux was NO3 greater than SCN approximately ClO4 greater than Cl approximately I approximately Br greater than PF6 greater than F approximately HCO3 much greater than H2PO4. At high internal concentrations, anions with low permeability and conductance reduced Cl influx consistent with block of the pore. The dependence of current on Cl concentration indicated that Cl can also occupy the channel long enough to limit current flow. Interaction of Cl and SCN within the conduction pathway is supported by the presence of a minimum in the conductance vs. mole fraction relation. These results indicate that this 40-pS Cl channel behaves as a multi-ion pathway in which other permeant anions could alter Cl flow across the apical membrane.

Modulation of inner mitochondrial membrane channel activity.

Abstract: Three classes of inner mitochondrial membrane (IMM) channel activities have been defined by direct measurement of conductance levels in membranes with patch clamp techniques in 150 mM KCl. The "107 pS activity" is slightly anion selective and voltage dependent (open with matrix positive potentials). "Multiple conductance channel" (MCC) activity includes several levels from about 40 to over 1000 pS and can be activated by voltage or Ca2+. MCC may be responsible for the Ca(2+)-induced permeability transition observed with mitochondrial suspensions. A "low conductance channel" (LCC) is activated by alkaline pH and inhibited by Mg2+. LCC has a unit conductance of about 15 pS and may correspond to the inner membrane anion channel, IMAC, which was proposed from the results obtained from suspension studies. All of the IMM channels defined thus far appear to be highly regulated and have a low open probability under physiological conditions. A summary of what is known about IMM channel regulation and pharmacology is presented and possible physiological roles of these channels are discussed.

H2-induced changes in electrical conductance of β-Ga2O3 thin-film systems

Abstract: H2-induced changes of electrical conductivity in polycrystalline, undoped β-Ga2O3 thin films in the temperature range of 400–650° C are described. The sheet conductance of these films depends reversibly, according to a power law σ□ ∼ p 1/3, on the partial pressure of hydrogen in the ambient atmosphere of the Ga2O3 film. A bulk vacancy mechanism is excluded by experiments and it is shown that the interaction is based on a surface effect. Changes in conductance are discussed to result from the formation of an accumulation layer due to chemisorption on the grain surfaces. Typical coverages are determined to be approximately 10−4 ML for pH2=0.05 bar and T=600° C. A possible explanation of the σ□ ∼ p 1/3 power law is provided.

Time-dependent changes in kinetics of Na+ current in single canine cardiac Purkinje cells.

Abstract: The spontaneous hyperpolarizing shift in Na+ channel kinetics that occurs during a series of voltage-clamp recordings was characterized in single canine cardiac Purkinje cells at 10-13.5 degrees C. The change in the half-point of voltage-dependent availability, in the half-point of peak conductance, in the voltage dependence of deactivation and time to peak Na+ channel current (INa), and in the time constants of INa decay in response to step depolarizations were examined. The half points of availability and conductance shifted similarly, -0.41 +/- 0.13 and -0.47 +/- 0.19 mV/min, respectively (n = 14). These were directly correlated (slope 1.14 +/- 0.06, R2 = 0.81) with conductance shifting on average only -0.05 mV/min faster than availability. The deactivation time constant-voltage relationship shifted similarly to availability and conductance. Tail current decay time constants predicted the voltage dependence of the open to closed transition to be 0.9e-. Time to peak INa in response to step depolarizations changed e-fold for 25 mV but plateaued at positive potentials (531 microseconds, n = 22). INa decay was multiexponential between -40 and 80 mV. Decay time constants changed little as a function of voltage at positive potentials. The contribution of the second time constant to decay amplitude was 15-20% over the entire voltage range. Time to peak INa shifted in a curvilinear fashion, changing less late in an experiment. We conclude that the channel-voltage sensor responds to a changing fraction of the applied voltage during an experiment, producing similar rates of shift of voltage-dependent availability, conductance, and deactivation time constants.

Transport spectroscopy of a confined electron system under a gate tip.

Abstract: Conductance resonances measured for split-gate structures around threshold are interpreted in terms of single-electron tunneling through a quantum dot beneath the tip of the gate finger. Measurements at different values of source-drain voltage allow the spectroscopy of excited states for a fixed number of electrons in the quantum dot. The measured magnetic-field dependence of the conductance resonances is consistent with a minimal number of seven electrons in the quantum dot.

Electrical and magneto-optical of MBE InAs on GaAs

Abstract: The electrical quality of InAs films grown on GaAs substrates by MBE is found to be optimum for growth temperatures close to 490 degrees C. The Hall mobility for such samples is 80000 cm2 V-1 s-1 at 77 K for film thicknesses of 5 mu m but falls to about 10000 cm2V-1s-1 at a thickness of 0.05 mu m. The carrier concentration in the bulk of the films is believed to be less than 1015 cm-3. The carrier concentration rises and the mobility falls as the growth temperature is varied on either side of this optimum value, reaching 2.5 *1016 cm-3 and 15000 cm2 V-1 s-1 at 77 K respectively for a growth temperature of 350 degrees C. Extremely sharp free-carrier cyclotron resonance and shallow donor lines are observed from the bulk of the film in far-infrared magneto-optical measurements, together with a very broad but strong cyclotron resonance line from an electron accumulation layer believed to be at the surface. The width of the cyclotron resonance line is consistent with a bulk mobility of the order of 200000 cm2 V-1 s-1 and the decrease in Hall mobility, together with the apparent increase in carrier concentration with decreasing film thickness, can be explained by the parallel conductance from the two-dimensional electron gas at the surface. There is no evidence for a significant reduction in mobility from the high density of threading dislocations caused by the mismatch with the GaAs substrate. The sharpness of the cyclotron resonance allows an accurate value for the band edge effective mass to be determined of 0.0236+or-0.0003 me with a pressure coefficient of +2.0% kbar-1. The donor lines are sufficiently sharp that central cell structure due to two different donor contaminants can be detected, and these donors are thought to be sulphur and selenium originating from the As source material. Certain of the transitions detected are too energetic to be from the shallow donors and these are thought to arise from singly ionized double donors which may be arsenic antisites. Silicon is found to act as a donor dopant up to high concentrations (6*1019 cm-3 where the mobility is 2000 cm2 V-1 s-1).

Surface structures and conductance at epitaxial growths of Ag and Au on the Si(111) surface

Abstract: In situ measurements of surface conductance, combined with simultaneous observations of reflection high-energy electron diffraction, clearly demonstrated strong dependence of the conductance on substrate-surface structures and epitaxial growth styles at early stages of Ag and Au depositions on Si(111) surface at room temperature. The conductance showed a large change with a small amount of deposition (<0.1 monolayer) on the substrate of a metal-induced superstructure (√3×√3-Ag or 5×2-Au), while it scarcely changed for a clean 7×7 substrate

The properties of ion channels formed by zervamicins

Abstract: The zervamicins (Zrv) are a family of 16 residue peptaibol channel formers, related to the 20 residue peptaibol alamethicin (Alm), but containing a higher proportion of polar sidechains. Zrv-IIB forms multi-level channels in planar lipid (diphytanoyl phosphatidyl- choline) bilayers in response to cis positive voltages. Analysis of the voltage and concentration dependence of macroscopic conductances induced by Zrv-IIB suggests that, on average, channels contain ca. 13 peptide monomers. Analysis of single channel conductance levels suggests a similar value. The pattern of successive conductance levels is consistent with a modified helix bundle model in which the higher order bundle are distorted within the plane of the bilayer towards a "torpedo" shaped cross-section, The kinetics of intra-burst switching between adjacent conductance levels are shown to be approximately an order of magnitude faster for Zrv-IIB than for Ahn. The channel forming properties of the related naturally occurring peptaibols, Zrv-Leu and Zrv-IC, have also been demonstrated, as have those of the synthetic apolar analogue Zrv-Al-16. The experimental studies on channel formation are combined with the known crystallographic structures of Zrv-Al-16 and Zrv- Leu to develop a molecular model of Zrv-IIB channels.

Chloride-dependent cation conductance activated during cellular shrinkage.

Abstract: A chloride (Cl-)-dependent, nonselective cation conductance was activated during cellular shrinkage and inhibited during cellular swelling or by extracellular gadolinium. The shrinking-induced, nonselective cation conductance and the swelling-induced anion conductance appear to function in the regulation of cell volume in airway epithelia. The shrinking-induced cation conductance had an unusual dependence on Cl-: partial replacement of extracellular Cl- with aspartate reduced the magnitude of the shrinking-enhanced current without accompanying changes in the reversal potential. The Cl- dependence of the nonselective cation conductance could provide a mechanism that tightly regulates Cl- secretion and sodium reabsorption in cells under osmotic stress.

Characterization of large-conductance chloride channels in rabbit colonic smooth muscle.

Abstract: 1. A large-conductance Cl- channel was characterized in cell-free membrane patches from the rabbit longitudinal colonic smooth muscle using the patch clamp technique. In addition, the regulation of these channels by neurokinin-1 (NK-1) receptor agonists and G proteins was studied. 2. No spontaneous channel activity was observed in cell-attached patches at the cell resting potential, or in excised patches at pipette potentials (Vp) between -20 and 20 mV. In excised patches, channel activity could be induced in thirty-six out of ninety-six patches by holding the patch at Vp values more negative than -60 mV or more positive than 60 mV. Once induced, the channel showed a bell-shaped voltage activation curve in high symmetric [Cl-], with maximal open probability between 20 and -5 mV. Varying cytosolic calcium concentration ([Ca2+]) between 5 x 10(-8) M and 1.0 mM had no effect on the voltage activation of the channel. 3. In inside-out and outside-out patches, when pipette and bath solutions contained equal [Cl-] (130 mM), the anion channel showed a linear current-voltage (I-V) relationship between -60 and 60 mV with a slope conductance of 309 +/- 20 pS (n = 13). Reversal potential measurements indicated that the channel was selective for Cl- over Na+ and K+ (PCl/PNa = 6:1). 4. Channel openings from the closed state to the full open state as well as transitions through smaller conductance states were observed. The smallest detectable substate had a conductance of 15.6 pS. Based on the similarities in selectivity and linearity of the I-V curve of the smaller conductances with the full open state, and kinetic analysis of channel activity, it is concluded that the large conductance channel is composed of multiple substates which can either open and close independently, or simultaneously via a main gate. 5. The stilbene derivative diiso-thiocyanato-stilbene-disulphonic acid (DIDS) and the diphenylamine-2-carboxylate analogue 5-nitro-2-(3-phenylpropylamino)-benzoate (NPPB) caused a dose-dependent, reversible flicker block of the small conductance and significantly reduced the macroscopic current flow through the channel. 6. In quiescent outside-out patches, when the pipette contained a 140 mM-CsCl solution with 10(-6) M-CaCl2, 1.2 mM-MgCl2 and 1 mM-GTP, and the bath contained Ringer solution, addition of the NK-1 receptor antagonists substance P methylester resulted in activation of the full conductance state and of smaller substates.(ABSTRACT TRUNCATED AT 400 WORDS)

Polymyxin B induces transient permeability fluctuations in asymmetric planar lipopolysaccharide/phospholipid bilayers.

Abstract: The interaction of the polycationic decapeptide polymyxin B with asymmetric planar bilayers from lipopolysaccharide and phospholipid monolayers, which resemble the lipid matrix of the outer membrane of Gram-negative bacteria, was investigated. The addition of polymyxin B in micromolar amounts to the lipopolysaccharide side of the asymmetric bilayers resulted, under voltage-clamp conditions, in a fast macroscopic increase of their ionic conductance, whereas the polymyxin B nonapeptide induced no significant conductance changes. The polymyxin B induced macroscopic conductance exhibited large fluctuations and was strongly dependent on the amplitude and polarity of the transmembrane potential. The temporal pattern and amplitudes of the fluctuations were characterized by power spectra of the membrane currents and their variances, respectively. In the initial phase following peptide addition, the conductance changes appeared to be channellike discrete fluctuations. The lifetimes of the fluctuations were exponentially distributed, and the mean lifetimes were strongly voltage-dependent, ranging from approximately 30 ms at +80 mV (positive at the side opposite to peptide addition) to less than 5 ms at reverse polarity. The conductance amplitudes of the single fluctuations exhibited a broad distribution with a mean of 2 nS. A comparison of the features of the macroscopic conductance and of the discrete fluctuations showed that the former can basically be understood as a superposition of a large number of the latter. From the amplitudes of the fluctuations, the diameter of the polymyxin-induced lesions was estimated to about 3 nm. The experimental findings can be understood by assuming a detergent-like action of polymyxin B.

The electrical properties of metal‐sandwiched Langmuir–Blodgett multilayers and monolayers of a redox‐active organic molecular compound

Abstract: Both ac and dc conduction processes through thin Langmuir–Blodgett (LB) films of the dodecyloxyphenylurethane of 2‐bromo‐5 (2’‐hydroxyethoxy) tetracyanoquinodimethan (DDOP‐C‐TCNQ, a redox‐active molecule) sandwiched between dissimilar metal (Pt and Mg) electrodes have been studied. The dc conduction changed from a linear I/V characteristic for the lowest applied voltages (±20 mV) to a symmetric nonlinear characteristic obeying a ln I∝V1/4 dependence for voltages up to ±1.5 V, in as‐prepared samples. For larger positive voltages, a large increase in current was observed with the dependence changing to a ln I∝V3 law, a dependence not reported previously for metal/LB film/metal systems. For increased negative voltages, the ln I∝V1/4 was again observed. For low applied ac fields, the conductance was found to follow a ωn law with a value of n close to 0.8. The effect of heat annealing the samples was also studied with significant differences in the observed changes in conductance between bilayer and monolayer ...

Conductance and statistical properties of metallic spectra.

Abstract: We present a new expression of the dissipative conductance and study under which conditions it is equal to the Thouless conductance defined as the curvature of the energy levels for a change of the boundary conditions. This equality is related to the transition between different classes of universality of random matrices.