Permeability, of bilayer membranes

The macrocyclic polyether 14 increases the permeability of bilayer membranes to cations in the sequence... 

Ion Permeability of Bilayer Membranes in the Iodine/Iodide System Controlled by Redox Reactions at the Interface... 

Iodide ion-selective electrode The iodide electrode has broad application both in the direct determination of iodide ions present in various media as well as for the determination of iodide in various compounds. It is, for example, important in the determination of iodide in milk [44,64,218, 382, 442], This electrode responds to Hg ions [150, 306, 439] and can be used for the indirect determination of oxidizing agents that react with iodide, such as 10 [305], lOi [158], Pd(II) [117, 347,405] and for the determination of the overall oxidant content, for example in the atmosphere [393], It can also be used to monitor the iodide concentration formed during the reactions of iodide with hydrogen peroxide or perborate, catalyzed by molybdenum, tungsten or vanadium ions, permitting determination of traces of these metals [12,192,193, 194, 195]. The permeability of bilayer lipid membranes for iodide can be measured using an I"... 

Supporting the bilayer surfaces by synthetic polymer scaffolds (Figure 4.6) enables one to enhance the vesicle stability and to control the permeability of their membranes. The polymers can be embedded within the vesicles among... 

The rate and character of the molecular motions of both the molecules embedded in the lipid bilayer and lipid molecules themselves are strongly dependent on the temperature [19, 203], At a certain temperature tm, the gel-liquid crystal phase transition is known to occur for the membrane made of a synthetic lipid. For example, tm = 41.5 °C for the membranes from DPL. In the vesicles formed by a mixture of lipids, e.g. egg lecithin, the phase transition occurs smoothly rather than jumpwise and starts below 0 °C. Note that the permeability of lipid membranes increases notably upon transition from the liquid crystal state to the gel state [204]. 

Bozic and Svetina [36] analysed a different situation, where addition of membrane constituents happens from the external milieu, and there is no metabolism inside, but there is limited permeability. They supposed that the membrane assumes spontaneous membrane curvature. This is non-zero if the properties of the inside and outside solutions differ, or if the two layers of a bilayer membrane differ in composition, or if some membrane-embedded constituents are asymmetrically shaped. They were able to show that under these assumptions membrane division is possible provided TLkC4 > 1.85, where T is the time taken to double the membrane area, L is the hydraulic permeability of the membrane, k is the bending modulus, and C is the spontaneous membrane curvature. In this model growing vesicles first retain spherical shape, then are distorted to a dumbbell, then to a pair of asymmetric vesicles coupled by a narrow neck, and finally to a pair of spherical vesicles linked by a narrow neck. Separation of the two daughter vesicles occurs as a result of mechanical agitation in the solution. 

The passive permeability of lipid membranes is another fluidity related parameter. In general, two mechanisms of membrane permeability can operate in the membrane (8). For many nonpolar molecules, the predominant permeation pathway is solubility-diffusion, which is a combination of partitioning and diffusion across the bilayer, both of which depend on lipid fluidity. In a few cases, such as permeation of positively charged ions through thin bilayers, an alternative pathway prevails (9, 10). It is permeation through transient pores produced in the bilayer by thermal fluctuations. This mechanism, in general, correlates with membrane fluidity. However, for model membranes undergoing the main phase transition, permeation caused by this mechanism exhibits a clear maximum near the phase transition point (11). 

The rate of transmembrane diffusion of ions and molecules across a membrane is usually described in terms of a permeability constant (P), defined so that the unitary flux of molecules per unit time [J) across the membrane is 7 = P(co - f,), where co and Ci are the concentrations of the permeant species on opposite sides of membrane correspondingly, P has units of cm s. Two theoretical models have been proposed to account for solute permeation of bilayer membranes. The most generally accepted description for polar nonelectrolytes is the solubility-diffusion model [24]. This model treats the membrane as a thin slab of hydrophobic matter embedded in an aqueous environment. To cross the membrane, the permeating particle dissolves in the hydrophobic region of the membrane, diffuses to the opposite interface, and leaves the membrane by redissolving in the second aqueous phase. If the membrane thickness and the diffusion and partition coefficients of the permeating species are known, the permeability coefficient can be calculated. In some cases, the permeabilities of small molecules (water, urea) and ions (proton, potassium ion) calculated from the solubility-diffusion model are much smaller than experimentally observed values. This has led to an alternative model wherein permeation occurs through transient hydrophilic defects, or pores , formed by thermal fluctuations of surfactant monomers in the membrane [25]. 

The phospholipid bilayer, the basic structural unit of biomembranes, is essentially impermeable to most water-soluble molecules, ions, and water itself. After describing the factors that influence the permeability of lipid membranes, we briefly compare the three major classes of membrane proteins that Increase the permeability of biomembranes. We then examine operation of the simplest type of transport protein to Illustrate basic features of protein-mediated transport. Finally, two common experimental systems used in studying the functional properties of transport proteins are described. 

A low level of proton leak across the inner mitochondrial membrane occurs in our mitochondria all of the time, and our mitochondria thus are normally partially uncoupled. It has been estimated that more than 20% of our resting metabolic rate is the energy expended to maintain the electrochemical gradient dissipated by our basal proton leak (also referred to as global proton leak). Some of the proton leak results from permeability of the membrane associated with proteins embedded in the lipid bilayer. An unknown amount may result from uncoupling proteins. 

Figure 5.11 Facilitated transport proteins in cell membranes. Unlike simple diffusion through the membrane bilayer, facilitated transport systems become saturated as the solute concentration difference increases. In this hypothetical example, the permeability of the membrane in simple diffusion is 0.4 (units of flux/concentration).

Ion permeability of this type was discovered when investigating the permeability of bilayer phospholipid membranes in the iodine/ iodide system,3,4 as well as the photopotentials of membranes containing chlorophyll and plastoquinone.122... 

Membrane lipids, and particularly cholesterol, are instrumental not only in the control of diffusion across biological membranes but also in the determination of the activity of membrane-bound enzymes, their modulation by hormones and other agents, and the determination of membrane fluidity (for original references, see [4,6]). It is generally accepted that incorporation of cholesterol in a lipid bilayer membrane tends to decrease significantly the permeability of these membranes to water. Movement of water across these membranes occurs primarily by dissolution in the membrane matrix. The decrease in the rate of water transport as a result of cholesterol incorporation is due mainly to a decrease in membrane fluidity. As a general rule, it is found that the presence of cholesterol in membranes or the incorporation of cholesterol into dispersions composed of phosphatidylserine or ganglioside lead to a decrease in the fluidity of the hydrocarbon chains of lipid membranes which are in the liquid-crystalline state [4,20]. 

Red cell deformation takes place under two important constraints fixed surface area and fixed volume. The constraint of fixed volume arises from the impermeability of the membrane to cations. Even though the membrane is highly permeable to water, the inability of salts to cross the membrane prevents significant water loss because of the requirement for colloidal osmotic equilibrium [Lew and Bookchin, 1986). The constraint of fixed surface area arises from the large resistance of bilayer membranes to changes in area per molecule [Needham and Nunn, 1990]. These two constraints place strict limits on the kinds of deformations that the cell can undergo and the size of the aperture that the cell can negotiate. Thus, a major determinant of red cell deformability is its ratio of surface area to volume. One measure of this parameter is the sphericity, defined as the dimensionless ratio of the two-thirds power of the cell volume to the cell area times a constant that makes its maximum value 1.0 ... 

The physical properties of bilayers are different from cell membranes in that they have extremely high resistance (10 J2/cm ) and usually a low capacitance (typically 0.5 tF/cm ). However, when the ionic permeability of bilayers is increased with various agents the resistance falls and the capacitance may increase producing models whose physical characteristics are, superficially at least, similar to biological membranes. 

This circuit includes multiple branches but only two nodes. Channels for different ions are equivalent to voltage sources, whose electromotive forces are equal to their respective equilibrium potentials determined by the Nemst formula and whose internal resistances depend on the permeability of the membrane for, and the diffusion coefficients as well as the concentrations of, respective ions. The ion pumps can be represented by corresponding current sources, all of which can be summed up forming one current source as shown in Fig. 1. If there exist transporters in the membrane, they can be electrically modeled (omitted in Fig. 1) in the same way as pumps. The capacitor represents the effect of the lipid bilayer of the membrane together with the extracellular solution (or the bath solution under artificial conditions) and intracellular solutiOTi. All these branches are arranged in parallel. 

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