Water transport across membranes

The pressure difference between the high and low pressure sides of the membrane is denoted as AP the osmotic pressure difference across the membrane is defined as Att the net driving force for water transport across the membrane is AP — (tAtt, where O is the Staverman reflection coefficient and a = 1 means 100% solute rejection. The standardized terminology recommended for use to describe pressure-driven membrane processes, including that for reverse osmosis, has been reviewed (24). 

Phagocytosis is an important mechanism for the organism to rid itself of bacteria and pathogenic material, as well as cell debris and remnants of apoptosis. However, it can also provide a route for the uptake of pollutant particulate material. It is seen to be especially important in the incorporation of airborne particulate material, which often has serious health consequences (see Section 6.4). In terrestrial invertebrates, food is obtained either from particulate matter in the soil or from molecules dissolved in interstitial water. Most of these organisms have extracellular digestion, with nutrients and foreign material being absorbed by one or more of the routes available for transport across membranes, such as diffusion, channels or pinocytosis. There have been few studies to establish which route is taken. 

Water transport across the luminal and basolateral membranes of collecting duct cells. Above, low water permeability exists in the absence of antidiuretic hormone (ADH). Below, in the presence of ADH, aquaporins are inserted into the apical membrane, greatly increasing water permeability. (AQP2, apical aquaporin water channels AQP3,4, basolateral aquaporin water channels V2, vasopressin V2 receptor.)... 

Fig 29. A simple equivalent circuit for the artificial permeable membrane. Physical meaning of the elements C, membrane capacitance (dielectric charge displaceme-ment) R, membrane resistance (ion transport across membrane) f pt, Phase transfer resistance (ion transport across interface) Zw, Warburg impedance (diffusion through aqueous phase) Ctt, adsorption capacitance (ion adsorption at membrane side of interface) Cwa, aqueous adsorption capacitance (ion adsorption at water side of interface). From ref. 109. 

Zeuthen, T., and Stein, W.D. 2002. Molecular mechanisms of water transport across biological membranes. Int. Rev. Cytol. 215 1—442. 

Many solute properties are intertwined with those of the ubiquitous solvent, water. For example, the osmotic pressure term in the chemical potential of water is due mainly to the decrease of the water activity caused by solutes (RT In aw = —V ri Eq. 2.7). The movement of water through the soil to a root and then to its xylem can influence the entry of dissolved nutrients, and the subsequent distribution of these nutrients throughout the plant depends on water movement in the xylem (and the phloem in some cases). In contrast to water, however, solute molecules can carry a net positive or negative electrical charge. For such charged particles, the electrical term must be included in their chemical potential. This leads to a consideration of electrical phenomena in general and an interpretation of the electrical potential differences across membranes in particular. Whether an observed ionic flux of some species into or out of a cell can be accounted for by the passive process of diffusion depends on the differences in both the concentration of that species and the electrical potential between the inside and the outside of the cell. Ions can also be actively transported across membranes, in which case metabolic energy is involved. 

Haines, T. H. (1994). Water transport across biological membranes. FEBS Lett. 546 115. 

Transient water pores in cellular membranes are involved in several relevant processes, such as maintenance of osmotic balance, drug and antibody delivery into cells, and ion transport across the membrane. Understanding ion transport across membranes is especially important, because membranes strive to maintain a cationic electrochemical gradient used for ATP synthesis. Yet, ions leak through lipid membranes, and understanding the mechanisms associated with ion leakage would allow one to control membrane properties better in related applications. 

Reverse osmosis membranes are characterized by an MWCO of -100 Da, and the process involves transmembrane pressures (TMP) of 10-50 bar (1000-5000 kPa), which are 5-10 times higher than those used in UF [11,36]. Unlike UF, the separation by RO is achieved not by the size of the solute but due to a pressure-driven solution-diffusion process [36]. Like UF membranes, RO membranes are uniquely stmctured films from synthetic organic polymers and consist of an ultrathin skin layer superimposed on a coarsely porous matrix [3]. The skin layer of the RO membrane is nonporous, which may be treated as a water-swollen gel, and water is transported across membrane by dissolving in this gel and diffusing to the low-pressure side... 

The numerator term is not equal to zero because of volume changes that occur in cell chambers caused by water transport across the membrane during the experiment. The selectivity of the membrane for one acid over another can be described as... 

At least 11 different mammalian AQP have now been identified, of which seven (AQPl, -2, -3, -4, -6, -7, -8) are expressed in the Iddney. Many of these also have extra-renal expression sites (e.g, AQPl may be unportant in fluid removal across the peritoneal membrane). Two asparagine-prohne-alanine sequences in the molecule are thought to interact in the membrane to form a pathway for water translocation. AQPl is found in the proximal tubule and descending thin limb of the loop of Henle and constitutes almost 3% of total membrane protein in the kidney. It appears to be constitutively expressed and is present in both the apical and basolateral plasma membranes, representing the entry and exit ports for. water transport across the cell, respectively. Approximately 70% of water reabsorption occurs at this site, predominantly via a transcellular (i.e., AQPl) rather than a paracellular route. Water reabsorption in the proximal tubule passively follows sodium reabsorption, so that the fluid entering the loop of Henle is still almost isosmotic with plasma. 

All of the authors imply that separation of water phases probably occurs at the cellular level. The semi-permeable nature of the cell membrane towards ions and solutes which are capable of relaxing water protons provides compartments in which relaxation rates can be significantly different, even when water transport across the membrane is very rapid. Indeed this property of whole tissue has been used in the development of an NMR method of determining water transport across erythrocyte membranes (11). 

R. Kiyono, Y. Asai, Y. Yamada, A. Kishihara and M. Tasaka, Anomalous water transport across cation-exchange membranes under an osmotic pressure difference in mixed aqueous solutions of hydrochloric acid and alkali metallic halide, Seni Gakkaishi, 2000, 56, 298-301 M. Tasaka, T. Okano and T. Fujimoto, Mass transport through charge-mosaic membranes, J. Membrane Sci., 1984,19, 273-288. 

Studies of permeability characteristics of the cell membrane have been of considerable interest to cell physiologists, since these characteristics help to define functional and structural properties of the plasma membrane and help elucidate the factors that determine the rate of movement of different substances into and out of various tissues in the body. Much of our present understanding of the cell membrane structure has been derived from the early work of Overton [1] on the movement of water and nonelectrolytes across cell membranes. Aside from being of considerable theoretical importance, the process of water transport across biological membranes and the effect of certain hormones on this process in some tissues is of practical importance. 

In certain cases such as toad urinary bladder and mammalian kidney, the movement of small polar molecules, especially urea, are significantly increased by antidiuretic hormones [52], Recently, it has been found that urea and water transport across the toad bladder can be separately activated by low concentration of vasopressin or 8 Br-cAMP [57], Based on these studies it was concluded that membrane channels for water and small polar nonelectrolytes differ significantly in both their dimensions and densities. The solute channels are limited in number, have relatively large radii and carry only a small fraction of water flow. On the other hand, the water channels have small radii. These findings provide strong experimental support for the concept of membrane pores which we have been advocating (see Fig. 5 in [4]). In this respect it is not unreasonable to expect that PCMBS and phloretin would also inhibit the ADH-sensitive increase in the permeability of these systems to urea and other small polar nonelectrolytes. 

B. Simulation of Water Transport Across a Lipid Membrane... 

Aquoporins are water channels that mediate rapid osmotically driven water transport across ceU membranes (Preston et al., 1992 Agre et al., 2002 Badaut et al., 2002). Brain tissues, including microvessels, contain relatively high levels of aquaporins. In particular, Aquaporin 4 (AQP4) is abundantly expressed in the brain, mainly in astrocytes, which are commonly observed to be swollen during cerebral edema (Jung et al., 1994 Kimelberg, 1995 Nielsen et al., 1997). 

FIGURE 4.8.30. Dependence of NaCl in caustic on the water transport across Nafion membrane [96]. (Reproduced with permission of the Society of Chemical Industry.)... 

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