Cell membranes water permeability

As mentioned above, impaired fluid absorption in kidney proximal tubule in AQPl deficiency indicates the need for high cell membrane water permeability for rapid, near-isosmolar fluid transport. The involvement of AQPs in fluid secretion by glands (salivary, submucosal, sweat, lacrimal), and by the choroid plexus and the ciliary body has been investigated using appropriate knockout mouse models. The general conclusion is that AQPs facilitate active fluid (secretion and absorption) when sufficiently rapid, in which case AQP deletion is associated with reduced volume and increased ion/solute content of secreted fluid. AQPs appear not to be needed when fluid secretion rate (per unit epithelial surface area) is low, as AQP-independent water permeability is high enough to support slow fluid secretion (or absorption). 

Helpem, J., R. Ordidige, and R. Knight. The effect of cell membrane water permeability on the apparent diffusion of water, in SMRM. 1992. Berlin. 

Krane CM, Melvin JE, Nguyen HV, Richardson L, Towne JE, Doetschman T, Menon AG (2001) Salivary acinar cells from aquaporin 5-deficient mice have decreased membrane water permeability and altered cell volume regulation. J Biol Chem 276 23413-23420... 

A key determinant of the final urine concentration is antidiuretic hormone (ADH also called vasopressin). In the absence of ADH, the collecting tubule (and duct) is impermeable to water, and dilute urine is produced. However, membrane water permeability of principal cells can be increased by ADH-induced fusion of vesicles containing preformed water channels with the apical membranes (Figure 15-6). ADH secretion is regulated by serum osmolality and by volume status. 

Osmotic lysis The plasma membranes of cells are water-permeable but are impermeable to large molecules and some ions. Thus, if cells are placed into water or dilute buffer, they swell owing to the osmotically driven influx of water. Since the plasma membrane is not able to stretch very much (the red blood cell membrane can stretch only up to 15 percent of its normal area before disruption), the cells burst. The method is effective for isolated cells but is not so effective for tissues. 

The presence of water channels has been demonstrated by successful cloning of proteins that increase membrane water permeability. These have been expressed in erythrocytes and in cells of the renal tubule. 

The cell membrane is not a true semipermeable membrane—that is to say, permeable to water and impermeable to solute. The cell membrane is permeable to ions more to anions than to cations. This is particularly true under nonphysiological conditions, such as anoxia or prolonged exposure to cold. Nonelectrolytes like urea and glucose traverse the cell membrane either by a simple diffusion process or by active transport. 

Water Permeation and Solute Separation through the Membrane. The measurements of water permeability of the 67 membranes prepared under different conditions were carried out by using an Amicon Diaflo Cell (effective membrane area, 13.9 cm2) under a pressure of 3 kg/cm2 at 25 °C. Some results are listed in Table 1067. It is apparent that much higher water absorption and permeability than the cellulosic membrane are characteristic of the 67 membranes prepared by both the casting polymerization and conventional casting. 

There are aspects of cell membranes other than their permeability to water and solutes that also play a critical role in the responses of cells to freezing. The structure of the plasma membrane allows cells to supercool and probably determines their ice-nucleation temperature. The nucleation temperature along with the permeability of membranes to water are the chief determinants of whether cells cooled at... 

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. 

The importance of lipids in membrane structure was established early in the 20th century when pioneering biophysicists established positive correlations between cell membrane permeabilities to small non-electrolytes and the oil/water partition coefficients of these molecules. Contemporary measurements of the electrical impedance of cell suspensions suggested that cells are surrounded by a hydrocarbon barrier, which was first estimated to be about 3.3 nm thick. This was originally thought to be a lipid monolayer. Among the pioneering biophysical experiments were those that established that the ratio of the area of a monolayer formed from erythrocyte... 

Cell membranes or synthetic lipid vesicles with normal low permeability to water will, if reconstituted with AQP1, absorb water, swell and burst upon exposure to hypo-osmotic solutions. The water permeability of membranes containing AQP 1 can be about 100 times greater than that of membranes without aquaporins. The water permeability conferred by AQP1 (about 3 billion water molecules per subunit per second) is reversibly inhibited by Hg2+, exhibits low activation energy and is not accompanied by ionic currents or translocation of any other solutes, ions or protons. Thus, the movement of water through aquaporins is an example of facilitated diffusion, in this case driven by osmotic gradients. 

The major route for bioaccumulation of hydrophobic organic compounds in aquatic animals is passive diffusion over cell membranes. In fish, the gill epithelia are the predominant port of entry, with less than 40% of uptake across the skin [181]. Since permeability of the membrane is a direct function of the membrane-water partition coefficient and the diffusion coefficient across the membrane interior [182], the bioconcentration factor (logBCF) can be directly correlated with log K0Vl. or log Km%v for compounds with intermediate hydro-phobicity [183,184],... 

Mercury can influence ion, water, and nonelectrolyte transport in different cells [ 14, 77]. The cell membrane is believed to be the first point of attack by heavy metals however, intracellular enzymes and metabolic processes may also be inhibited [70, 78, 79]. The attachment of heavy metals to ligands in or on the plasma membrane may result in changes in passive permeability or selective blockage of specific transport processes. Many membrane transport systems are known to be sensitive to sulphydryl-group modification [ 14, 80, 81]. 

---