Membrane channels water

Parker, M., Buckley, J., Postma, J., et al., 1994. Structure of the Aeromonas toxin proaerolysin in its water-soluble and membrane-channel states. Nature 367 292-295. 

Based on GebeTs calculations for Nafion (where lEC = 0.91 meq/g),i isolated spheres of ionic clusters in the dry state have diameters of 15 A and an intercluster spacing of 27 A. Because the spheres are isolated, proton transport through the membrane is severely impeded and thus low levels of conductivity are observed for a dry membrane. As water content increases, the isolated ionic clusters begin to swell until, at X, > 0.2, the percolation threshold is reached. This significant point represents the point at which connections or channels are now formed between the previously isolated ionic clusters and leads to a concomitant sharp increase in the observed level of proton conductivity. 

Figure 4.20.A shows a more recent cell reported by Cobben et al. It consists of three Perspex blocks, of which two (A) are identical and the third (B) different. Part A is a Perspex block (1) furnished with two pairs of resilient hooks (3) for electrical contact. With the aid of a spring, the hooks press at the surface of the sensor contact pads (4), the back side of which rests on the Perspex siuface, so the sensor gate is positioned in the centre of the block, which is marked by an engraved cross as in the above-described wall-jet cell. Part B is a prismatic Perspex block (2) (85 x 24 x 10 mm ) into which a Z-shaped flow channel of 0.5 mm diameter is drilled. Each of the wedges of the Z reaches the outside of the block. The Z-shaped flow-cell thus built has a zero dead volume. As a result, the solution volume held between the two CHEMFETs is very small (3 pL). The cell is sealed by gently pushing block A to B with a lever. The inherent plasticity of the PVC membrane ensures water-tight closure of the cell. The closeness between the two electrodes enables differential measurements with no interference from the liquid junction potential. The differential signal provided by a potassium-selective and a sodium-selective CHEMFET exhibits a Nemstian behaviour and is selective towards potassium in the presence of a (fixed) excess concentration of sodium. The combined use of a highly lead-selective CHEMFET and a potassium-selective CHEMFET in this type of cell also provides excellent results.

During cell/stack operation, water content in the membrane is affected by the local intensive variables, such as local temperature, water vapor concentration in the gas phase, gas temperature and velocity in the channel, and the properties of the electrode and gas diffusion media. The power fluctuation can result in temperature variation inside the cell/stack, which will subsequently change the local membrane water content. As the water content in the membrane tends to be non-uniform and unsteady, this results in operation stresses. When the membrane uptakes water from a dry state, it tends to expand as there is no space for it to extend in plane and it can wrinkle up as schematically shown in Fig. 4 when the membrane dries out, the wrinkled part may not flatten out, and this ratcheting effect can cause the pile up of wrinkles at regions where membrane can find space to fold. The operation stress is typically cyclic in nature due to startup-shutdown cycles, freeze-thaw cycles, and power output cycles. 

Transport through membrane channels or pores requires the substance to be water soluble and to have a cross-sectional diameter less than the diameter of the pore or channel. This... 

Hydration forces, the long-range interactions between surfaces in water, may participate in membrane function, specifically, in the response to diacylglycerol, in membrane fusion, and in the gating of membrane channels (Rand etal., 1985 Zimmerberg and Parsegian, 1987). 

Apart from the co-transporters, astrocytic perivascular system in the brain involves membrane-bound water channels, called aquaporins (Nagelhus et al., 2004). These water channels... 

A series of experiments was conducted to study the AS transfer rate through the dialysis membrane by pumping a concentrated AS solution into the upper channel at 1 mL/min and water into the lower channel at varied flow rates ranging from 1 to 0.1 mL/min without sample injection. In these experiments, the AS input concentration into the upper channel and the AS output concentration from the lower channel were compared. The rate of the AS transfer rose, as expected, with the decreased flow rate through the water channel, and at a flow rate of 0.1 mL/min, the AS concentration collected through the lower channel reached nearly 100% that of the AS input in the upper channel. Whereas AS diffuses from the upper channel toward the lower channel, water in the lower channel is... 

Specific Channels Increase the Permeability of Some Membranes to Water... 

Some tissues contain proteins that increase the permeability of membranes to water. Each water-channel-forming protein, termed an aqua-porin, consists of six membrane-spanning a helices and a central channel lined with hydrophilic residues that allow water molecules to pass in single file. Aquaporins do not transport protons. 

Sodium, potassium, and chloride ions do not move freely across the plasma membrane, but water channels (aquaporins) in the membrane permit the flow of water in the direction dictated by the ion concentration of the surrounding medium, (a) When the medium is isotonic, there is no net flux of water into or out of the cell, (b) When the medium is hypotonic, water flows into the cell (red arrow) until the ion concentration inside and outside the cell is the same. Because of the influx of water, the cell volume increases, (c) When the medium is hypertonic, water flows out of the cell until the ion concentration inside and outside the cell is the same. Because water is lost, the cell volume decreases. 

Pure phospholipid bilayers are essentially impermeable to water, but most cellular membranes contain water-channel proteins that facilitate the rapid movement of water in and out of cells. Such movement of water across the epithelial layer lining the kidney tubules of vertebrates is responsible for concentrating the urine. If this did not happen, one would excrete several liters of urine a day In higher plants, water and minerals are absorbed from the soil by the roots and move up the plant through conducting tubes (the xylem) water loss from the plant, mainly by evaporation... 

In a cell-free system inhibition can be shown to occur on both 70S and 80S ribosomes. However, in a more realistic in vitro setting intact prokaryotic (i.e., bacterial) cells are much more sensitive. The reason for this selectivity is that tetracyclines are actively transported into bacterial but not mammalian cells. In Gm- bacteria, at least, the more water-soluble compounds seem to cross through membrane channels (pores). The more lipid-soluble drugs (particularly MNC, Table 6-9) diffuse more readily through the lipoidal phases of the membranes. This energy-coupled process then leads to intracellular antibiotic accumulations. 

Almost all cell membranes exhibit some water permeability. Our understanding of the movement of water through cell membranes has been greatly advanced by the discovery of a family of water-specific, membrane-channel proteins—the aquaporins. At least 12 different aquaporins (0 to 11) have been identified. These proteins are present in organisms at all levels of life, and their unique permeability characteristics and distribution in numerous tissues indicate diverse roles in the regulation of water homeostasis. Agre (Figure 16.1) of Johns Hopkins University School of Medicine received the 2003 Nobel Prize in Chemistry for the discovery of the water channels. 

The Preferential Sorption/Capillary Flow Model (Sourirajan and Matsuufa (1985)) is based on the assumption that a layer of water sorbs at the membrane surface, creating a deficit of solute at the surface. The membrane is viewed as a microporous medium, and transport is controlled by the surface chemistry of the membrane and water transport through the membrane. Ions with large hydrated radii are retained better, since they also have to overcome more energy to strip off the water. Ions diffuse through the laj et of strucmred water at the membrane surface and through water cluster channels in the membrane (Staude (1992)), where B is the pure water permeability of the membrane. 

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