Water filled pores

The nAChR is comprised of five subunits, each of which spans the lipid bilayer to create a water-filled pore or channel (Fig. la). Each subunit consists of four transmembrane segments, the second transmembrane segment (M2) lines the ion channel (Fig. lb). The extracellular N-terminal domain of every subunit... 

It was extended to describe spontaneous rupture of lipid bilayers [72,73] and electroporation [74] (see also Ref. 70 and references therein). The energy of the membrane with a virtual water-filled pore of a radius r subject to an applied voltage V is [70,74]... 

MR has been used to measure the pore size distribution by exploiting the dependence of T) and T2 on the surface to volume ratio of water-filled pores [20,... 

There are two pathways by which a drug molecule can cross the epithelial cell the transcellular pathway, which requires the drug to permeate the cell membranes, and the paracellular pathway, in which diffusion occurs through water-filled pores of the tight junctions between the cells. Both the passive and the active transport processes may contribute to the permeability of drugs via the transcellular pathway. These transport pathways are distinctly different, and the molecular properties that influence drug transport by these routes are also different (Fig. 

Dobbie KE, Smith KA (2003) Nitrous oxide emission factors for agricultural soils in Great Britain the impact of soil water-filled pore space and other controlling variables. Glob Change Biol 9 204-218... 

The complications for fhe fheoretical description of proton fransporf in the interfacial region befween polymer and water are caused by the flexibility of fhe side chains, fheir random distributions at polymeric aggregates, and their partial penetration into the bulk of water-filled pores. The importance of an appropriate flexibilify of hydrated side chains has been explored recently in extensive molecular modeling studies. Continuum dielectric approaches and molecular dynamics simulations have been utilized to explore the effects of sfafic inferfacial charge distributions on proton mobility in single-pore environments of Molecular level simulations were employed... 

Oxygen Availability in Degrading Films. A major difference between natural materials and starch-plastic or cellulose-plastic blends is that the hydrophilic and relatively permeable matrix of materials like wood and hydrated polysaccharide films allows diffusion of O2 and release of nutrients from sites at a distance from the invasion site. As colonization proceeds, pore enlargement occurs when the pore walls are degraded (8) or as the polymer matrix of amylose or PVA films is hydrolyzed (10.12). In contrast, the LDPE matrix supplies no nutrients, hinders diffusion of water and O2, and the pore diameter cannot be increased. The consequence of impermeability is that the sole means of obtaining O2 and nutrients is by diffusion through water-filled pores. 

The anomalous behaviour for potassium chloride matrices with a value <0.5 emphasizes the complex release of this drug. Peppas [8] did not interpret values of <0.5 but stated that such occurrences were an indication of statistical analysis problems or were due to diffusion through a polymeric network where diffusion occurred partially through a swollen matrix and partly through water-filled pores. In this case, in order to investigate the mechanism of release, the percentage release v. time profile was evaluated for goodness of fit. The details of the use of this statistical... 

Theoretically, release of drug through the water-filled pores of matrix systems is expected to show a square-root-of-time dependence, with a time exponent of 0.5 and hence continual declining release rates. However, there have been many research groups finding remarkable deviations. 

We should also note that microorganisms have water-filled pores (porins) through their exterior membranes (Fig. 17.13) which permit the passive entrance of small hydrophilic substances (Madigan et al., 2000). In studies of enteric bacteria, passive glucose uptake exhibited transmembrane uptake time scales of less than a millisecond (Nikaido, 1979). Thus, the rate of passive uptake of small, hydrophilic molecules (< 500 molecular mass units) via membrane pores of bacteria is not likely to cause them to avoid biodegradation for prolonged times. 

First note that benzene is primarily moving through the gas-filled pores. Diffusion through the water-filled pores is too slow to account for much of the total flux. To calculate the steady-state diffusive flux through the 3-meter-thick gas-filled pores, use Eq. 18-56 and replace Dipm by diffusivity in the unsaturated zone, Djuz, and ( > by 0g. The latter is the gas-filled void which amounts to 75% of the 40% total porosity. That is, 0 = 0.30. 

Physicochemical Structure. Water-filled pores and capillaries of differing diameters permeate the organic gel material that makes up asmined lignite. 

Because of the complexity of hydrated PEMs, a full atomistic modeling of proton transport is impractical. The generic problem is a disparity of time and space scales. While elementary molecular dynamics events occur on a femtosecond time scale, the time interval between consecutive transfer events is usually 3 orders of magnitude greater. The smallest pore may be a few tenth of nanometer while the largest may be a few tens of nanometers. The molecular dynamics events that protons transfer between the water filled pores may have a timescale of 100-1000 ns. This combination of time and spatial scales are far out of the domain for AIMD but in the domain of MD and KMC as shown in Fig. 2. Because of this difficulty, in the models the complexity of the systems is restricted. In fact in many models the dynamics of excess protons in liquid water is considered as an approximation for proton conduction in a hydrated Nation membrane. The conformations and energetics of proton dissociation in acid/water clusters were also evaluated as approximations for those in a Nation membrane.16,19 20 22 24 25... 

In bioerodible drug delivery systems various physicochemical processes take place upon contact of the device with the release medium. Apart from the classical physical mass transport phenomena (water imbibition into the system, drug dissolution, diffusion of the drug, creation of water-filled pores) chemical reactions (polymer degradation, breakdown of the polymeric structure once the system becomes unstable upon erosion) occur during drug release. 

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