Low Reflection Coefficient Research Articles

Acoustic televiewer logging in glacier boreholes

AbstractThe acoustic televiewer is a geophysical logging instrument that is deployed in a water-filled borehole and operated while trolling. It generates a digital, magnetically oriented image of the borehole wall that is developed from the amplitudes and transit times of acoustic waves emitted from the tool and reflected at the water–wall interface. The transit-time data are also converted to radial distances, from which cross-sectional views of the borehole shape can be constructed. Because the televiewer is equipped with both a three-component magnetometer and a two-component inclinometer, the borehole’s trajectory in space is continuously recorded as well. This instrument is routinely used in mining and hydrogeologic applications, but in this investigation it was deployed in two boreholes drilled into Upper Fremont Glacier, Wyoming, U.S.A. The acoustic images recorded in this glacial setting are not as clear as those typically obtained in rocks, due to a lower reflection coefficient for water and ice than for water and rock. Results indicate that the depth and orientation of features intersecting the boreholes can be determined, but that interpreting their physical nature is problematic and requires corroborating information from inspection of cores. Nevertheless, these data can provide some insight into englacial structural characteristics. Additional information derived from the cross-sectional geometry of the borehole, as well as from its trajectory, may also be useful in studies concerned with stress patterns and deformation processes.

The effect of lithium wall conditioning in TFTR on plasma–surface interactions

Five TFTR deuterium supershots with increasing Li pellet injection are analyzed in detail. Five chords of experimental H-α measurements are compared to predictions from a series of computational models. First, experimental data from the discharge is used in the TRANSP plasma transport code to predict the ion flux to the wall. Then a modified version of the DEGAS neutral transport code which includes both reflection, desorption and sputtering of hydrogenic species from the wall is used to determine the neutral density profile across the machine. This data combined with the known density and temperature contours predicts values for the magnitude of H-α light observed for 16 viewing angles of the diagnostic. To match the experimental data, the wall reflection, desorption and sputtering coefficients were altered using data from VFTRIM-3D to include the effect of the added Li. In addition, the first 10 cm of the stainless steel wall adjoining the C inner bumper limiter was treated as C-covered; the highest flux area of the inner wall was treated as a sink; and the lower reflection coefficients for a Li-wall rather than a C-wall were used over an increasingly larger area of the inner wall as the Li concentration in the discharges increased.

Determination of cell water‐relation parameters using the pressure probe: extended theory and practice of the pressure‐clamp technique

The aim of the present study was to develop and test an extended theory of the pressure‐clamp technique that would allow the estimation of additional cell water‐relation parameters using the pressure probe. It was assumed that intercellular water transport (vacuole → vacuole) occurs via a composite ‘membrane’ comprising the tonoplast, cytoplasm, plasma membrane, plasmodesmata and cell wall and that solute transport across the micropipette tip of the probe is dominated by convection. The extended theory allows the simultaneous estimation of cell volume (V), effective cell‐sap osmotic pressure (σΠ0), composite‐membrane hydraulic conductance (ALP) and the ‘instantaneous’ volume change (v0) at the start of the pressure clamp. With an estimate of cell‐sap osmotic pressure (Π0), the weighted‐average reflection coefficient (σ) of the composite membrane may also be determined from an endosmotic pressure‐clamp experiment. In principle, the cell volume before the clamp (V0) can be estimated as V0 = V + v0, and the cell volumetric elastic modulus (ɛ) can be estimated as ɛ = –V0(ΔP/v0), where ΔP (< 0) is the change in turgor pressure at t = 0. In practice it may be necessary to correct v0 for transmembrane water flow when estimating V0 and ɛ. To test this theory, a previously described pressure‐probe system was upgraded by incorporating a data‐acquisition system to record turgor pressure, meniscus position, the penetration depth of the pressure‐probe micropipette and the micropipette profile. An air bubble in the system allowed the turgor pressure to be clamped at a near‐constant value. For mesophyll cells in leaf discs of Kalanchoë daigremontiana, the estimated mean value of V0[(2·46 ± 0·24) × 10–13 m3] was in reasonable agreement with a value obtained from microscopy [(1·91 ± 0·10) × 10–13 m3]. When the positive correlation between ɛ and |ΔP| was taken into account, the parameter estimates obtained from pressure‐clamp experiments were found to be consistent with those derived from pressure‐relaxation experiments. The mean values of ɛ (1·4 ± 0·2 MPa) and LP[(5·76 ± 0·83) × 10–12 m s–1 Pa–1] were significantly different from previous estimates obtained with older leaves. Furthermore, in contrast to the estimated reflection coefficient of the plasma membrane/tonoplast (≈ 1), the mean value of σ obtained in the present study (0·62 ± 0·05) was significantly less than unity. This is attributed to a high hydraulic conductance and a low reflection coefficient of a symplastic pathway for intercellular water transport. The qualitative behaviour of LP and σ was consistent with a quasi‐steady‐state model of intercellular water transport that included volume flow through plasmodesmata. A quantitative description was obtained by fitting a simplified version of this model to the data. This led to the conclusion that significant volume flow can occur through plasmodesmata during pressure‐clamp experiments, especially for small A and |ΔP|.

Urea Transporter UT3 Functions as an Efficient Water Channel: DIRECT EVIDENCE FOR A COMMON WATER/UREA PATHWAY

A family of molecular urea transporters (UTs) has been identified whose members appear to have an exceptionally high transport turnover rate. To test the hypothesis that urea transport involves passage through an aqueous channel, osmotic water permeability was measured in Xenopus oocytes expressing UTs. The UT3 class of urea transporters functioned as efficient water channels. Quantitative measurement of single channel water permeability (pf) using epitope-tagged rat UTs gave pf (in cm3/s x 10(-14)) of 0.14 +/- 0.11 (UT2) and 1.4 +/- 0.2 (UT3), compared with 6.0 and 2.3 for water channels AQP1 and AQP3, respectively. Relative single channel urea permeabilities (purea) were 1.0 (UT2), 0.44 (UT3), and 0.0 (AQP1). UT3-mediated water and urea transport were weakly temperature-dependent (activation energy <4 kcal/mol), inhibited > 75% by the urea transport inhibitor 1,3-dimethylthiourea, but not inhibited by the water transport inhibitor HgCl2. To test for a common water/urea pore, the urea reflection coefficient (sigmaurea) was measured by independent induced osmosis and solvent drag methods. In UT3-expressing oocytes, the time course of oocyte volume in response to different urea gradients (induced osmosis) gave sigmaurea approximately 0.3 for the UT3 pathway, in agreement with sigmaurea determined by the increase in uptake of [14C]urea during osmotic gradient-induced oocyte swelling (solvent drag). In oocytes of comparable water and urea permeability coexpressing AQP1 (permeable to water, not urea) and UT2 (permeable to urea, not water), sigmaurea = 1. These results indicate that UT3 functions as a urea/water channel utilizing a common aqueous pathway. The water transporting function and low urea reflection coefficient of UT3 in vasa recta may be important for the formation of a concentrated urine by countercurrent exchange in the kidney.

Double window configuration as a low cost microwave waveguide window for plasma applications

Waveguide windows are major components of a transmission line used in microwave plasma devices. The function of the waveguide window is to provide vacuum isolation of the source side from the plasma chamber while transmitting microwaves with minimum attenuation. Commonly a single thin dielectric plate is sandwiched between a choke type flange and a flat flange and is used as a waveguide window. To arrive at a better window configuration in terms of the low power reflection coefficient, the voltage standing wave ratio calculation is carried out for different window configurations (single window and double window) and for various window thicknesses. It is found that the power reflection is the minimum in the case of double window configuration. The minimum power reflection is as low as 0.8% for a combination of alumina and a quartz plate each of 1 cm thickness in the double window configuration. Also, it is more advantageous to use radial microwave coupling than axial coupling in order to increase the life time of the microwave waveguide window.

Apoplasmic and Protoplasmic Water Transport through the Parenchyma of the Potato Storage Organ.

Stationary volume fluxes through living and denatured parenchyma slices of the potato (Solanum tuberosum L.) storage organ were studied to estimate the hydraulic conductivity of the cell wall and to evaluate the significance of water transport through protoplasts, cell walls, and intercellular spaces. Slices were placed between liquid compartments, steady-state fluxes induced by pressure or concentration gradients of low- and high-molecular-mass osmotica were measured, and water transport pathways were distinguished on the basis of their difference in limiting pore size. The protoplasts were the dominating route for osmotically driven water transport through living slices, even in the case of a polymer osmoticum that is excluded from cell walls. The specific hydraulic conductivity of the cell wall matrix is too small to allow a significant contribution of the narrow cell wall bypass to water flow through the living tissue. This conclusion is based on (a) ultrafilter coefficients of denatured parenchyma slices, (b) the absence of a significant difference between ultrafilter coefficients of the living tissue slices for osmotica with low and high cell wall reflection coefficients, and (c) the absence of a significant interaction (solvent drag) between apoplasmic permeation of mannitol and the water flux caused by a concentration difference of excluded polyethylene glycol. Liquid-filled intercellular spaces were the dominating pathways for pressure-driven volume fluxes through the parenchyma tissue.

Water transport across plant tissue: Role of water channels

Summary— The contribution of water‐filled, selective membrane pores (water channels) is integrated into a general concept of water transport in plant tissue. The concept is based on the composite anatomical structure of tissues which results in a composite transport pattern. Three main pathways of water flow have been distinguished, ie the apoplastic, symplastic and transcellular (vacuolar) paths. Since the symplastic and transcellular components can not be distinguished experimentally, these components are summarized as a cell‐to‐cell component. Water channel activity may control the overall water flow across tissues provided that the contribution of the apoplastic component is relatively low. The composite transport model has been applied to roots where most of the data are available. Comparison of the hydraulic conductivity at the root cell and organ levels shows that, depending on the species, there may be a dominating cell‐to‐cell or apoplastic water flow. Most remarkably, there are differences in the hydraulic conductivity of roots which depend on the nature of the force used to drive water flows (osmotic or hydrostatic pressure gradients). This is predicted by the model. The composite transport model explains low reflection coefficients of roots, the variability in root hydraulic resistance and differences between herbaceous and woody species. It is demonstrated that there is also a composite transport of water at the membrane level (water channel arrays vs bilayer arrays). This results in low reflection coefficients of plasma membranes for certain test solutes as derived for isolated internodes of Chara. The titration of water channel activity in this alga with mercurials and its dependence on changes in temperature or external concentration show that water channels do not exclusively transport water. Rather, they are permeable to relatively big uncharged organic solutes. The result indicates that, at least for Chara, the concept of an exclusive transport of water across water channels has to be questioned.

セメントモルタルの半透過性に起因する浸透圧の測定

Osmotic pressure induced by semi-permeability of hardened cement mortar was studied. The apparatus composed of the two cells, one of which is filled with NaCl solution of 2mol and the other is filled with distilled water, was developed to measure osmotic pressure. Mortar disks of three levels of thickness from 5 to 15mm were adopted as test specimens, and osmotic pressure, water permeability and ion movement were measured at 30℃ for the specimens with water cement ratios such as 45,60 and 75%. The following results were obtained. 1.Osmotic pressure is basically affected by movability of water, Na^+ and Cl-Ion in cement mortar.and maximum osmotic pressure, 1.2MPa was observed. 2.Hardened cement mortar may be considered as semi-permeable material with low reflection coefficient.

Diurnal variation in the radial reflection coefficient of intact maize roots determined with the xylem pressure probe

Xylem pressure and its relative response to the imposition of an external osmotic stress (the so-called radial reflection coefficient) were recorded in roots of intact maize plants using the xylem pressure probe technique. Consecutive insertion of two probes into the same xylem vessel or into adjacent vessels of intact roots of plants exposed to high light intensity and salt stress under laboratory conditions showed that the xylem tension was not changed by vessel probing. It was also shown by using the double probe approach that the plants were capable of overcoming artificially induced leakages. This and other evidence reported in the literature convincingly demonstrated that the probe accurately reads xylem pressure and xylem pressure responses to osmotic stress. Additional experiments were performed on plants grown in a greenhouse at a subtropical latitude. Under these conditions the plants were exposed to strong diurnal fluctuations in light intensity, relative humidity and temperature. The results showed that the absolute xylem pressure in the roots of untreated plants decreased with increasing transpiration rate from positive values in the early morning to negative values around noon (average value -0.15 MPa; maximum negative value -0.57 MPa). As the day progressed and the transpiration rate decreased, xylem pressure increased again to positive values. Correspondingly, the radial reflection coefficient for NaCI increased from about zero in the early morning to about unity at noon when transpiration reached its highest value and decreased again to very low values towards the evening. The data raise questions concerning conclusions about the mechanism of water transport in intact roots drawn from the low radial reflection coefficients measured on excised roots using the root pressure probe.

Novel structure for TE°11-TE11 mode converter

A novel structure for the transition from the dominant mode in circular waveguide to that in circular groove guide is presented. The mode converter based on this structure which has low reflection coefficient is designed. The numerical result shows that the performance of this new type of transducer is improved.

Renal medullary microcirculation: architecture and exchange.

The architecture of the renal medullary microcirculation is highly specialized. Consistent with their role in countercurrent exchange, the vessels (the vasa recta and the intervening capillaries) have high permeabilities to fluid and small hydrophilic solutes. The urea permeability of the continuous endothelium of the descending vasa recta (DVR) in the outer medulla is greatly enhanced by a urea transporter. Aquaporin channels have also been identified in these vessels. In spite of the absence of lymphatics from the inner medulla, fluid uptake from the interstitial fluid (ISF) through the fenestrated endothelium of the ascending vasa recta (AVR) appears to be driven by differences in hydrostatic and oncotic pressure. Because the AVR have high Lp's [10(-5) cm s-1 (cm H2O)-1] and are mechanically linked to surrounding structures, small increments of ISF pressures above the pressure within the AVR can drive significant volumes of fluid into AVR if ISF volume expands. The lower reflection coefficients to serum albumin of the AVR as compared with the DVR may be important in the clearance of interstitial plasma protein. Recent work on isolated DVR from the outer medulla has revealed that these vessels are capable of vasoconstriction and thus of regulating medullary blood flow.

Water channels in plants: do basic concepts of water transport change?

A review and re-examination of literature data shows that highly selective water channels (aquaporins) have marked effects on the overall transport properties of the plasma membrane of plant cells. The application of the channel blocker HgCl2 (50 μM) or of high external concentrations of permeating solutes reduced the water permeability (hydraulic conductivity, Lp) of Chara internodes down to 25% of the control. In treated cells, reflection coefficients (σs) of permeating low molecular weight organic test solutes (alcohols, amides, ketones) were markedly reduced as well, but solute permeability (permeability coefficient, Ps) remained constant. A similar relation between Lp and σs was found with untreated cells of isolated epidermis of Tradescantia virginiana. The results can not be interpreted in terms of conventional membrane models (pore or solubility membrane); for instance, the classical frictional pore model (Dainty and Ginzburg, 1963) fails to explain low σs of rapidly permeating solutes. The results fit into a model which treats the membrane as a composite structure with proteinaceous arrays (containing water channels) in parallel with lipid arrays (‘composite transport model’). Test solutes predominantly pass across the lipid array which was not affected by treatments. Water, however, largely uses the water channel array which was affected. When using heavy water (HDO) as an osmotic solute, the transport pattern changed as predicted by the model. As indicated by low channel reflection coefficients of test solutes, water channels did not completely exclude small uncharged molecules and do show some permeability for the test solutes used. Low σs values of water channels are interpreted by a single-file mechanism of water and solute flow. Absolute values of transport coefficients (Lp, Ps, σs) represent mixed values to which the different membrane arrays contribute according to concepts available from irreversible thermodynamics. The patchy structure of the cell membrane results in a circulation flow of water in the membrane. The fact that water channels can be triggered by factors such as heavy metals and high concentration suggests that water transport can be regulated by opening or closing water channels. The results have consequences for our basic understanding of osmosis and water transport in plants.

A computational model of the wave absorption by the multilayer porous media

A computational model is developed to demonstrate the behavior of a set of multilayer porous media which are installed in front of a solid wall. The purpose of the model is to investigate their efficiency in wave absorption, with the aim of providing guidance for the design of a wave absorber which can not only absorb waves effectively but also occupies less space. Generally speaking, a single-layer porous medium with proper thickness can be used to give a very low reflection coefficient for waves of a particular wave period, but not for many other wave periods. It is found that this disadvantage can be improved by adopting a set of multilayer porous media. In other words, the multilayer structure can reflect a smaller wave for a wider range of wave periods. In comparison with a single-layer porous medium, the two-layer ones provide a lower reflection coefficient with a thinner structure. The same phenomeon is found in comparing both the three-layer with the two-layer and the four-layer with the three-layer. Therefore, special attention is paid to the four-layer porous media. With a thickness of 3.2 times water depth, the four-layer porous media can provide a reflection coefficient of 0.08 for h/L > 0.15 . It was found that the larger number of layers the media had contained, the better function it would provide, and the less space it would occupy.

Nonlinear Bragg scattering and mode superradiance in an open Fabry—Perot resonator

Mode superradiance (i.e. superfluorescence) in an open Fabry—Perot resonator is characterised by simultaneous emission of counterpropagating coherent pulses even when the mirrors have very low (but finite) reflection coefficients, beginning from Rcr ≪ 1. It is shown that for any value of the resonator Q factor, even when R ≫ Rcr or R ~ 1, a self-matched λ/2 grating of the population inversion in a two-level medium does not result in significant Bragg reflection which would exceed the reflection from the mirrors. A consistent allowance for the interference of inhomogeneous conterpropagating waves significantly alters the spatial and temporal dynamics of depletion of the population inversion and, consequently, greatly changes he profile of collective spontaneous radiation pulses, compared with the predictions of the mean field and unidirectional superradiance theories. A new nonlinear equation for superradiance in an open resonator is derived.

TE01-TE11 mode transducer

A transition between rectangular waveguide and circular groove guide is described. The composite transition consists of two sections. It is theoretically designed to provide both mode matching as well as low reflection coefficient. The performance of the transducer is plotted.

High Molecular Weight Organic Compounds in the Xylem Sap of Mangroves: Implications for Long‐Distance Water Transport

AbstractThe rise of sap in mangroves has puzzled plant physiologists for many decades. The current consensus is that negative pressures in the xylem exist which are sufficiently high to exceed the osmotic pressure of seawater (2.5 MPa). This implies that the radial reflection coefficients of the mangrove roots are equal to unity. However, direct pressure probe measurements in xylem vessels of the roots and stems of mangrove (Rhizophora mangle) grown in the laboratory or in the field yielded below‐atmospheric, positive (absolute) pressure values. Slightly negative pressure values were recorded only occasionally. Xylem pressure did not change significantly when the plants were transferred from tap water to solutions containing up to 1700 mOsmol kg−1 NaCl. This indicates that the radial reflection coefficient of the roots for salt, and therefore the effective osmotic pressure of the external solution, was essentially zero as already reported for other halophytes. The low values of xylem tension measured with the xylem pressure probe were consistent with previously published data obtained using the vacuum/leafy twig technique. Values of xylem tension determined with these two methods were nearly two orders of magnitude smaller than those estimated for mangrove using the pressure chamber technique (−3 to −6MPa). Xylem pressure probe measurements and staining experiments with alcian blue and other dyes gave strong evidence that the xylem vessels contained viscous, mucilage‐ and/or protein‐related compounds. Production of these compounds resulting from wound or other artifactual reactions was excluded. The very low sap flow rates of about 20–50 cm h−1 measured in these mangrove plants were consistent with the presence of high molecular weight polymeric substances in the xylem sap. The presence of viscous substances in the xylem sap of mangroves has the following implications for traditional xylem pressure measurement techniques, development of xylem tension, and longdistance water transport: (1) high external balancing pressures in the pressure chamber are needed to force xylem sap to the cut surface of the twig; (2) stable tensions much larger than 0.1 MPa can be developed only occasionally because viscous solutions provide nucleation sites for gas bubble formation; (3) the frequent presence of small gas bubbles in viscous solutions allows water transport by interfacial, gravity‐independent streaming at gas/water interfaces and (4) the increased density of viscous solutions creates (gravity‐dependent) convectional flows. Density‐driven convectional flows and interfacial streaming, but also the very low radial reflection coefficient of the roots to NaCl are apparently the means by which R. mangle maintains water transport to its leaves despite the high salinity of the environment.

Cell swelling activates a poorly selective monovalent cation channel in the apical membrane of toad urinary bladder.

We measured the effects of conditions that increase cell volume on ion movements through the Ca(2+)-blockable poorly selective monovalent cation channel of the apical membrane of the toad urinary bladder. Three conditions were studied: dilution of the basolateral solution, basolateral perfusion with solutions prepared with solutes of low reflection coefficient and dilution of the apical solution in bladders treated with oxytocin. All three procedures markedly increased K+ movements and elevated the plateau of the Lorentzian component of the power spectrum by enhancing ion currents through the Ca(2+)-blockable pathway. Simultaneously there was a large increase in Ca(2+)-sensitive conductance. The magnitude of this increased conductance strongly suggests that the stimulation of ion flow is due to increased ion permeability and not solely to increases in driving force across the apical membrane. We were not able to detect an increase in the movements of alkali-earth ions induced by the conditions that increase cell volume. We speculate that activation of the Ca(2+)-blockable channel may play an important role in the regulation of cell volume and/or in K+ homeostasis.

Simple multijunction launcher with oversized waveguides for lower hybrid current drive on JT-60U

A multijunction technique with oversized waveguides has been developed for the lower hybrid current drive launcher on JT-60U. The launcher consists of 4 (toroidal) × 4 (poloidal) multijunction modules. RF power in the module is divided toroidally into 12 sub-waveguides at a junction point through an oversized waveguide. This method simplifies the structure of the multijunction launcher with a large number of subwaveguides. A maximum power density up to 25 MW m −2 has been achieved with a low reflection coefficient of less than 2%. The coupling and current drive efficiency are well explained by the designed wave spectra without taking account of higher modes in the oversize waveguides. Thus, the simple multijunction launcher has been demonstrated to excite expected wave spectra with high power handling capability.

Transport of Water and Solutes across Maize Roots Modified by Puncturing the Endodermis (Further Evidence for the Composite Transport Model of the Root).

The effects of puncturing the endodermis of young maize roots (Zea mays L.) on their transport properties were measured using the root pressure probe. Small holes with a diameter of 18 to 60 [mu]m were created 70 to 90 mm from the tips of the roots by pushing fine glass tubes radially into them. Such wounds injured about 10-2 to 10-3% of the total surface area of the endodermis, which, in these hydroponically grown roots, had developed a Casparian band but no suberin lamellae. The small injury to the endodermis caused the original root pressure, which varied from 0.08 to 0.19 MPa, to decrease rapidly (half-time = 10-100 s) and substantially to a new steady-state value between 0.02 and 0.07 MPa. The radial hydraulic conductivity (Lpr) of control (uninjured) roots determined using hydrostatic pressure gradients as driving forces was larger by a factor of 10 than that determined using osmotic gradients (averages: Lpr [hydrostatic] = 2.7 x 10-7 m s-1 MPa-1; Lpr [osmotic] = 2.2 x 10-8 m s-1 MPa-1; osmotic solute: NaCl). Puncturing the endodermis did not result in measurable increases in hydraulic conductivities measured by either method. Thus, the endodermis was not rate-limiting root Lpr: apparently the hydraulic resistance of roots was more evenly distributed over the entire root tissue. However, puncturing the endodermis did substantially change the reflection ([sigma]sr) and permeability (Psr) coefficients of roots for NaCl, indicating that the endodermis represented a considerable barrier to the flow of nutrient ions. Values of [sigma]sr decreased from 0.64 to 0.41 (average) and Psr increased by a factor of 2.6, i.e. from 3.8 x 10-9 to 10.1 x 10.-9 m s-1(average). The roots recovered from puncturing after a time and regained root pressure. Measurable increases in root pressure became apparent as soon as 0.5 to 1 h after puncturing, and original or higher root pressures were attained 1.5 to 20 h after injury. However, after recovery roots often did not maintain a stable root pressure, and no further osmotic experiments could be performed with them. The Casparian band of the endodermis is discontinuous at the root tip, where the endodermis has not yet matured, and at sites of developing lateral roots. Measurements of the cross-sectional area of the apoplasmic bypass at the root tip yielded an area of 0.031% of the total surface area of the endodermis. An additional 0.049% was associated with lateral root primordia. These areas are larger than the artificial bypasses created by wounding in this study and may provide pathways for a "natural bypass flow" of water and solutes across the intact root. If there were such a pathway, either in these areas or across the Casparian band itself, roots would have to be treated as a system composed of two parallel pathways (a cell-to-cell and an apoplasmic path). It is demonstrated that this "composite transport model of the root" allows integration of several transport properties of roots that are otherwise difficult to understand, namely (a) the differences between osmotic and hydrostatic water flow, (b) the dependence of root hydraulic resistance on the driving force or water flow across the root, and (c) low reflection coefficients of roots.

Non-electrolyte solute permeabilities of human placental microvillous and basal membranes.

1. Permeability to non-electrolytes of isolated microvillous and basal membranes from human term placenta was measured using stopped-flow light-scattering techniques. The studied solutes were urea, ethylene glycol, glycerol, creatinine, erythritol, arabitol and mannitol. 2. At 37 degrees C, permeability of the microvillous membrane to mannitol and urea was 0.30 +/- 0.02 x 10(-6) cm/s (mean +/- S.E.M.) and 3.2 +/- 0.2 x 10(-6) cm/s, respectively. The corresponding permeabilities for the basal membrane were 1.2 +/- 0.1 x 10(-6) cm/s (mannitol) and 4.4 +/- 0.3 x 10(-6) cm/s (urea). The basal membrane was substantially more permeable to hydrophilic solutes than the microvillous membrane. This is probably due to differences in lipid composition, as illustrated by membrane cholesterol content, which was found to be approximately 50% lower in the basal as compared to the microvillous membrane. 3. Similarities between permeabilities in placental membranes and lipid bilayers and the linear relationship noted between solute hydrophobicity and placental permeability suggested that solutes permeate both human syncytiotrophoblast membranes by a solubility/diffusion mechanism. In the microvillous membrane this was supported by data obtained for activation energies (> 10 kcal/mol) and reflection coefficients (close to 1). In the basal membrane, low activation energies for glycerol and urea and a low reflection coefficient for urea indicated that these solutes may, in part, share a common pathway with water. 4. It was estimated that the placental permeability to molecules with a molecular weight under 200 observed in vivo can, to a great extent, be accounted for by transcellular permeation.