The possible influence of osmotic poration on cell membrane water permeability

It has been hypothesized that pores in the plasma membrane form under conditions of rapid water efflux, allowing extracellular ice to grow into the cytoplasm under conditions of rapid freezing. When cells with intracellular ice are thawed slowly, the transmembrane ice crystal expands through recrystallization causing the cell to lyse. One of the implications of this hypothesis is that osmotic pores will provide an alternative route for water movement under conditions of osmotically induced flow. We show that the plasma membrane water permeability of a fibroblast cell changes as a function of the osmotic pressure gradient that is used to drive water movement. It is further shown that cell volume is more important than the magnitude of water flux in causing this departure from a uniform water permeability. We suggest that these data provide evidence of a transient route for water movement across cell membranes.     

Analysis of the source of heterogeneity in the osmotic response of plant membrane vesicles

Plasma membrane vesicles have been widely employed to understand the biophysics of water movements, especially when active aquaporins are present. In general, water permeability coefficients in these preparations outcome from the analysis of the osmotic response of the vesicles by means of light scattering. As from now, this is possible by following a theoretical approach that assumes that scattered light follows a single exponential function and that this behavior is the consequence of vesicle volume changes due to an osmotic challenge. However, some experimental data do not necessarily fit to single exponentials but to double ones. It is argued that the observed double exponential behavior has two possible causes: different vesicle population in terms of permeability or in terms of size distribution. As classical models cannot identify this source of heterogeneity, a mathematical modeling approach was developed based on phenomenological equations of water transport. In the three comparative models presented here, it was assumed that water moves according to an osmotic mechanism across the vesicles, and there is no solute movement across them. Interestingly, when tested in a well described plasma membrane vesicle preparation, the application of these models indicates that the source of heterogeneity in the osmotic response is vesicles having different permeability, clearly discarding the variable size effect. In conclusion, the mathematical approach presented here allows to identify the source of heterogeneity; this information being of particular interest, especially when studying gating mechanisms triggered in water channel activity.     

The function of water channels in Chara: The temperature dependence of water and solute flows provides evidence for composite membrane transport and for a slippage of small organic solutes across water channels

Using the cell pressure probe, the effects of temperature on hydraulic conductivity (Lp; osmotic water permeability), solute permeability (permeability coefficient, P_s), and reflection coefficients (σ_s) were measured on internodes of Chara corallina, Klein ex Willd., em R.D.W.. For the first time, complete sets of transport coefficients were obtained in the range between 10 and 35 °C which provided evidence about pathways of water and solutes as they move across the plasma membrane (water channel and bilayer arrays). Test solutes used to check for the selectivity of water channels were monohydric alcohols of different molecular size and shape (ethanol, n-propanol, iso-propanol, and tert-butanol) and heavy water (HDO). Within the limits of accuracy, Q₁₀ values for Lp and for the diffusive water permeability (P_d) were identical (Q₁₀ for Lp = 1.29 ± 0.17 (± SD; n = 15 cells) and Q₁₀ for P_d = 1.25 ± 0.16 (n = 5 cells)). The Q₁₀ values were equivalent to activation energies of E_a = 16.8 ± 6.4 and 16.6 ± 10.0 kJ · mol⁻¹, respectively, which is similar to that of self-diffusion or of viscous flow of water. The Q₁₀ values and activation energies for P_s of the alcohols were significantly larger (ethanol: Q₁₀ = 1.68 ± 0.16, E_a = 37.1 ± 5.9 kJ · mol⁻¹; n-propanol: Q₁₀ =  1.75 ± 0.40, E_a = 43.1 ± 15.3 kJ · mol⁻¹; iso-propanol: Q₁₀ = 2.12 ± 0.42, E_a =  52.2 ± 14.6 kJ · mol⁻¹; tert-butanol: Q₁₀ = 2.13 ± 0.56, E_a = 51.6 ± 17.1 kJ · mol⁻¹; ±SD; n = 5 to 6 cells). Effects of temperature on reflection coefficients were most pronounced. With increasing temperature, σ_s values of the alcohols decreased and those of HDO increased. The data indicate that water and solutes use different pathways when crossing the membrane. Ordinary and isotopic water use water channels and the other test solutes use the bilayer array (composite transport model of membrane). Changes in σ_s values with temperature were found to be a sensitive measure for the open/closed state of water channels. The decrease of σ_s with temperature was theoretically predicted from the temperature dependence of P_s and Lp. Differences between predicted and measured values of σ_s allowed estimation of the bypass flow (slippage) of solutes through water channels which did not completely exclude test solutes. The permeability of channels depended on the structure and size of test solutes. It is concluded that water channels are much less selective than is usually thought. Since water channels represent single-file or no-pass pores, solutes drag along considerable amounts of water as they diffuse across channels. This results in low overall values of σ_s. The σ_s of HDO was extremely low. Its response to temperature was opposite to that for the σ_s of the alcohols. This suggested a stronger effect of temperature on the hydraulic (osmotic) than on the diffusive water flow across individual water channels, i.e. a differential sensitivity of different mechanisms to temperature. Received: 10 October 1996 / Accepted: 2 December 1996     

Electrogenic membrane transport in plants a review

This review treats some examples of electrogenic transport across the outer plasmamembrane (plasmalemma) of plant cells. The selection includes primary active uniport by membrane ATPases (e.g., the proton pump), secondary active transport of hexoses by proton-dependent cotransport, and passive uniport of amines. Primacy is given to the presentation of electrophysiological data and to the discussion of voltage-dependence of the transport mechanisms.Lecture from the Annual Meeting of the Deutsche Gesellschaft für Biophysik at Konstanz     

Intracellular solute gradients during osmotic water flow: An electron-microprobe analysis

Summary In an attempt to quantify possible intracellular water activity gradients during ADH-induced osmotic water flow, we employed energy dispersive X-ray microanalysis to thin, freezedried cryosections obtained from fresh, shock-frozen tissue of the toad urinary bladder. The sum of all detectable small ions (Na + K + Cl) in the cellular water space was taken as an index of the intracellular osmolarity. Presuming that all ions are osmotically active, they comprise about 90% of the cellular solutes. When the cells were exposed to dilute serosal medium, the reduction in the sum of the ions agreed well with the expected reduction in osmolarity. After inducing water flow by addition of ADH and dilution of the mucosal medium, all epithelial cells showed a fall in osmolarity. The change was more pronounced in granular cells than in basal or mitochondria-rich cells, consistent with the notion that granular cells represent the main transport pathway. Most significantly, intracellular osmolarity gradients, largely caused by an uneven distribution of K and Na, were detectable in granular cells. The gradients were not observed after ADH or mucosal dilution alone, or when the direction of transepithelial water flow was reversed. We conclude from these results that there is a significant cytoplasmic resistance to water flow which may lead to intracellular gradients of water activity. Concentration gradients of diffusible cations can be explained by a flow-induced Donnan-type distribution of fixed negative charges. With regard to transepithelial Na transport, the data suggest that ADH stimulates transport by increasing the Na permeability of the apical membranes of granular cells specifically.     

A ditopic receptor for cation binding and facilitated transport through a supported liquid membrane

The binding properties of an artificial receptor towards a series of cations including Li⁺, Na⁺, K⁺, Mg²⁺, Ca²⁺, Ba²⁺, Fe²⁺ and Al³⁺ in acetonitrile are described. The receptor comprises a photo-responsive pyrene unit connected via a short spacer to a 2,2′:6′,2″-terpyridine metal ion binding site. Interaction of cations with the receptor was monitored by changes in absorption profile and the association constants calculated for 1:1 and 1:2 cation:ligand binding fall within the range log β = 3-12. The receptor is highly fluorescent and quenching of the emission is observed upon cation binding. The potassium picrate transport properties of the membrane-bound receptor are also described. This receptor when immobilised in a polymer support, which separates two aqueous solutions, has been shown to transport potassium ions in the dark with a flux rate of 1.5 × 10⁸ mol/s m². In contrast, when the membrane-bound receptor is selectively illuminated with light (λ > 400 nm), the flux increases to 2.0 × 10⁸ mol/s m². The transport efficiency depends on the nature of the trap used in the receiver phase.     

Formulation of a coupled mechanism between solute diffusion,phosphatase-kinase reactions and membrane potentials for the primary active transport of phosphorylated substrates through biological membranes

Coupled interrelations occurring between a phosphatase/kinase reaction sequence acting in unstirred layers and on both sides of a charged biomembrane pore structure are presented as a plausible kinetic model for the primary active transport of phosphorylated molecules. Simulations conducted at the cell level and with credible numerical values demonstrate that the enzymes positions strongly regulate the membrane permeability for the transported substrate. Depending on both the enzymes positions (more or less far from the membrane) and the membrane charges, the membrane may appear either impervious, either permeable or able to actively transport a phosphorylated substrate. Globally all happens as if, in function of the enzymes positions, a permanent pore may be regulated, changing from a more closed to a more open conformation.     

Trophic structure and energy flow in a Texas pond

Annual energy flow and mean annual biocontent of eighteen compartments were determined for a 1.0 ha north central Texas pond ecosystem. Annual primary production was 7,789 kcal m^–2 yr^–1, and community production-to-respiration ratio was 1.49. One-third of annual primary production accumulated on the substrate as silt and sedimentation. Biocontents of four trophic levels in the pond were all of the same order of magnitude, averaging 50 kcal m^–2. Wind and water-borne organic material for primary consumers and terrestrial insects for tertiary consumers were substantial allochthonous energy imports into the pond system.     

Barrier height of free energy on confined polymer translocation through a short nano-channel

The translocation of a confined polymer chain through a nano-channel has been simulated by using two-dimensional bond fluctuation model (BFM) with Monte Carlo dynamics. It is found that the trapping time for the polymer chain to overcome the free energy barrier during the translocation, tautrap, depends exponentially on the chain length N and the channel length M, respectively. The results suggest that the barrier height of free energy depends linearly on N and M, which is different from that predicted for the Gaussian chain.     

New QTLs identified for plant water status, water-soluble carbohydrate and osmotic adjustment in a barley population grown in a growth-chamber under two water regimes

Quantitative trait locus (QTL) analysis was carried out with 167 recombinant inbred lines (RILs) of barley derived from a cross between Tadmor and Er/Apm to identify the genomic regions controlling traits related to plant water status and osmotic adjustment (OA). The experiment was conducted in a growth chamber using a random incomplete block design (nine blocks). Relative water content (RWC) and leaf osmotic potential (ψ_π) were measured at 100% and 14% of the field capacity on 105 RILs in each block. In addition, the water-soluble carbohydrate concentration (WSC) was measured in the four first-blocks. The leaf osmotic potential at full turgor (ψ_π100), the water-soluble carbohydrate concentration at full turgor (WSC100), and also OA, the accumulation of water-soluble carbohydrates (dWSC100), the contribution of a change in water content to OA (CWC) and of the net solute accumulation to OA (SA) have also been calculated. In a previous paper (Teulat et al. 1998), 12 QTLs were identified for RWC, ψ_π, ψ_π100 and OA with adjusted means (block effects and pot-within-block effects fixed) with an incomplete genetic map. In the present paper, a more-saturated and improved map is described. A new QTL analysis as been performed with adjusted means. The new QTLs identified for previous evaluated traits, as well as the QTLs for the new traits, are presented. Eight additional regions (22 QTLs) were identified which increased to 13 the total number of chromosomal regions (32 QTLs) controlling traits related to plant water status and/or osmotic adjustment in this barley genetic background. The results emphasise the value of the experimental design employed for the evaluation of traits difficult to assess in genetic studies. The putative target regions for drought-tolerance improvement are discussed combining arguments on the consistency of QTLs and, when possible, the physiological value of QTLs (trait relevance, syntenic relationships and clustering of QTLs). Received: 8 March 2000 / Accepted: 18 October 2000     

Considerations on the flow configuration of membrane chromatography devices for the purification of human basic fibroblast growth factor from crude lysates

Membrane chromatography possesses numerous advantages such as operation at high flow rates, low back pressure, ease of handling and scale up, which make the membrane adsorber process a viable alternative to conventional packed column chromatography. A purification process for the isolation of human recombinant basic fibroblast growth factor (FGF‐2) based on membrane chromatography was investigated using devices with different flow configurations. In the first process, the FGF‐2 capture step was performed with an axial flow device, while the alternative method achieved direct capture of FGF‐2 from unclarified cell lysate with a tangential flow device. In both processes, FGF‐2 purities exceeded 82% and the purified cytokine displayed high biological activity. Binding capacity (BC) from fermentation broth of the axial flow device was 28 mg/mL. This was 50% higher than the BC obtained with the tangential flow device under particle‐free supernatant conditions (18 mg/mL) and 150% higher compared to the BC achieved with unclarified cell lysate (11 mg/mL). While membrane chromatography in tangential flow mode omits clarification and thus reduces the number of stages in the downstream process, it displays lower peak resolution and leads to a lower overall process yield.     

Secondary productivity and energy flow in a tropical pond

Monthly changes in density and biomass of a Pila globosa population were estimated in the littoral area of the pond Idumban. Mean density of active snail was 10.4, equivalent to 76 g dry weight/m² during 1973 and 6.5, equivalent to 45 g/m² during 1974. Total population size of the snail decreased from 9.2 × 10⁶ individuals, equivalent to 6.5 ton during 1973 to 6.3 × 10⁶ snails, equivalent to 4.4 ton during 1974. The period from December to May represented the time of abundance and active growth. Mortality assessed from marking and recapture as well as from monthly changes in population density, averaged to 2.7 snails/m²/month or 20% of the density. Growth estimated by marking and recapture suggested that the snail required a period of over 4 years to attain a body (wet) weight of 35 g. Laboratory experiments revealed that young (<4 g), intermediate (4 g><24 g) and old (>24 g) P. globosa grew at the rate of 4.0, 1.5 and 0.3 mg dry weight/g live weight/day. Using these values and the size-wise population density data, net productivity of the snail was estimated as 74 and 40 g/m²/year in 1973 and 74, respectively. The snail exhibited an efficiency of 70% for absorption and 10% for conversion. Using these values, it was further possible to estimate rates of feeding and absorption for the population. Consumption amounted to 1039 g/m² in 1973 and 560 g/m² in 1974. The efficiencies of exploitation, gross and net productions were 21, 7 and 10%, respectively; ecological efficiency amounted to 1.4% only.     

Leaf litter processing and energy flow through macroinvertebrates in a woodland pond (Switzerland)

Energy generated by leaf litter processing and its flow through the associated macroinvertebrate community was quantified in a pond near Geneva (Switzerland). Annual density, biomass, and production on oak (Quercus robur) leaf litter were assessed for all macroinvertebrate taxa with emphasis on predators. Empirical energetic relations provided an energy budget for the macroinvertebrate community. On 1 m² of pond bottom, the processing of 5641 kJ of oak leaf litter resulted in 8.5% of leachate (6 days), and after 1 year 32% of material remained; the other 59.5% was biologically (animal or microbial) converted, including 11.2% processed by shredders. The mean annual density of associated macroinvertebrates was 51374 individuals, mean biomass was 3.53 g (dry mass) and production was about 1451 kJ (or 65 g). Predator production was 170 kJ/m², non-chironomid primary consumer production was 101 kJ/m² (including 57 kJ from shredders) and chironomid primary consumer production was estimated at 1180 kJ/m². Predators contributed to a high proportion of total biomass (39%) but to a smaller amount of production (12%) or density (6%). In this two-stepped food-chain mainly based on detritus, the transfer coefficient between first level (detritus + primary producers) and third level (secondary consumers) was high (2–2.5%) and indicated efficient conversion of energy. This high efficiency was partly related to the reutilization of fine particulate organic matter by the collectors. The production estimate measured on leaf litter was compared with two other predominant substrates (Typha latifolia stems and Chara sp.), and exhibited the highest value. This study shows how leaf litter can constitute a direct source for high secondary production and be an efficient energy source in freshwater ecosystems. It is also demonstrated that a woodland pond can support a high macroinvertebrate production as compared with other freshwater ecosystems.     

Unconstrained enhanced sampling for free energy calculations of biomolecules: a review

Abstract

Free energy calculations are central to understanding the structure, dynamics and function of biomolecules. Yet insufficient sampling of biomolecular configurations is often regarded as one of the main sources of error. Many enhanced sampling techniques have been developed to address this issue. Notably, enhanced sampling methods based on biasing collective variables (CVs), including the widely used umbrella sampling, adaptive biasing force and metadynamics, have been discussed in a recent excellent review (Abrams and Bussi, Entropy, 2014). Here, we aim to review enhanced sampling methods that do not require predefined system-dependent CVs for biomolecular simulations and as such do not suffer from the hidden energy barrier problem as encountered in the CV-biasing methods. These methods include, but are not limited to, replica exchange/parallel tempering, self-guided molecular/Langevin dynamics, essential energy space random walk and accelerated molecular dynamics. While it is overwhelming to describe all details of each method, we provide a summary of the methods along with the applications and offer our perspectives. We conclude with challenges and prospects of the unconstrained enhanced sampling methods for accurate biomolecular free energy calculations.     

Measuring molecular order for lipid membrane phase studies: Linear relationship between Laurdan generalized polarization and deuterium NMR order parameter

Two dimensional phase separation in lipid membranes and cell membranes is of interest to biology because of the idea of membrane rafts — compositionally heterogeneous liquid crystal domains with cellular functions. Few quantitative tools exist for characterizing and differentiating coexisting phases on a molecular scale. Lipid acyl chain order can be measured directly using deuterium nuclear magnetic resonance spectroscopy (²H NMR), or inferred using fluorescence microscopy along with the environment-sensitive probe Laurdan. We found a linear relationship between the ²H NMR order parameter and Laurdan generalized polarization. This observed correlation supports the idea that lipid chain order is tightly associated with the amount and dynamics of water molecules at the glycerol backbone level of the membrane.     

Characterization of membrane foulants in a pilot-scale powdered activated carbon–membrane bioreactor for drinking water treatment

The external and internal foulants of a pilot-scale powdered activated carbon–membrane bioreactor (PAC–MBR) used for drinking water treatment were systematically examined by scanning electron microscopy (SEM), three-dimensional excitation emission matrix (EEM) fluorescence spectroscopy, Fourier transform infrared spectroscopy (FTIR) and energy diffusive X-ray (EDX) analysis. The results showed that external fouling, which comprised 31.68% of the total fouling, was caused by the deposition of a large amount of biological PAC on the membrane surface. Bacteria and organic matter comprised only a small fraction of the external foulants. Biologically derived proteins and polysaccharides were the major constituents of the internal foulants. EDX analysis indicated that the external and internal foulants also included inorganic elements such as Mg, Al, Si, Ca, Mn and Fe. During the operation of PAC–MBR, low flux and effective physical cleaning protocols should be adopted; proteins, polysaccharides and inorganic elements in the bioreactor should also be controlled.     

Production and energy flow in the benthic community of a Texas pond

Energy flow (A = R + P; assimilation = respiration + production) and life-history/density-biomass cycles were followed for 1 year in a mayfly (Brachycercus sp.) population and two chironomid (Procladius sp. and Tendipes decorus) populations in a 0.94 ha pond located in north-central Texas. Temperature (5, 10, 15, 20, 25 and 30°C) effects on R and patterns of metabolic compensation/acclimatization were determined for the mayfly and chironomids. Chironomid and mayfly populations dominated the nine benthic macroinvertebrates identified in the pond, with greatest abundances in winter and winter-spring respectively. Annual energy flow was 51 kcal m^–2 (R = 40 kcal, P = 11 kcal or 1.9 g) in Brachycercus sp., 23 kcal m^–2 (R = 5 kcal, P = 18 kcal or 2.4 g) in Procladius sp. and 74 kcal m^–2 (R = 17 kcal, P = 57 kcal or 6.0 g) in T. decorus. Each species metabolically compensated partially to temperatures en countered during their life histories.     

Linear free energy relationship for the anomeric effect: MP2, DFT and ab initio study of 2-substituted-1,4-dioxanes

The anomeric effect of 2-substituted 1,4-dioxane derivatives was calculated and compared with the values for substituted cyclohexane. The bond lengths, bond angles, torsion angles, and relative energies of axial and equatorial conformers of 2-substituted 1,4-dioxanes were calculated by the second-order Møller–Plesset (MP2), density functional theory (DFT/B3LYP), and Hartree–Fock (HF) methods using 6-31G^∗ basis set. The energy differences between the axial and equatorial conformers, endo and exo-anomeric effects, repulsive non-bond and H-bonding interactions were investigated. A linear free energy relationship (LFER) between calculated (MP2/6-31G^∗) anomeric effect and inductive substituent constants (σ_I) was obtained for 2-substituted-1,4-dioxanes (slope = 6.19 and r² = 0.967). The calculated energy differences indicate lower equatorial orientation for 2-substituted-1,4-dioxanes compared to the 2-substituted-tetrahydropyrans. The contribution of resonance, hyperconjugation, inductive, steric, hydrogen bonding, electrostatic interaction, and level of theory influences the anomeric effect.