Abstract

The plasma membrane of mammalian cells is formed by two layers of lipids (lipid bilayer), primarily phospholipids, glycolipids and cholesterol, in which many different proteins are embedded. Phospholipid consists of a glycerol backbone esterified to fatty acids (the "lipid tail") and, via a phosphate group, to either choline, serine, inositol or ethanolamine (the "head group"). Whereas the head group is hydrophilic and oriented towards the outer surface of the membrane, the lipid tail is hydrophobic and pointed towards the inner part. The plasma membrane is impermeable to large molecules like carbohydrates and proteins but readily permeable to small uncharged molecules like oxygen, water, and carbon dioxide. Molecules can move through the membrane either by direct diffusion or through specialized channels or transport proteins (facilitated diffusion). In an isolated system, as stated by the Second Law of Thermodynamics, all events move spontaneously from a higher energy state to a lower energy state and are driven by the tendency to increase the entropy (degree for randomness/ disorder). When molecules are evenly distributed throughout the available space, the entropy is at its maximum. Therefore, free moving molecules and atoms (i.e. not part of a crystal structure and not restrained by additional forces) tend to distribute themselves over an as large as possible area. As a consequence of the Second Law of Thermodynamics, differences in the concentrations of non-permeable solutes between the cell and its surrounding medium will result in a redistribution of the solvent (e.g. water) to maintain the lowest energy possible, a phenomenon known as osmosis. Therefore, in response to an osmotic imbalance, water will move across a semi-permeable membrane until the water molecules are equally distributed, resulting in swelling or shrinkage. The Gibbs-Don nan effect refers to the observation that under certain conditions charged molecules or ions fail to distribute evenly across a semi-permeable membrane. At the Gibbs-Donnan equilibrium, the total number of positive charges balanced the total number of negative charges (bulk electroneutrality). Due to the presence of charged membrane-impermeable macromolecules and the electrogenic Na+ /K+ pump, however, the Gibbs Donnan equilibrium will never be obtained in an intact cell, leading to an asymmetric distribution of permeable ions and the generation of the membrane potential. A consequence of the presence of negatively charged membrane-impermeable proteins is the constant tendency of cells to accumulate water. To counteract a potential increase in volume, sodium is removed from the cells by the Na+ /K+ pump on the expenditure of metabolic energy. Because alterations in cell volume may affect many biochemical and physiological processes, almost all cell types have additional mechanisms to regulate their volume and to limit or prevent swelling and shrinkage.

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C.P. Verrijzer (Peter)
Erasmus University Rotterdam , Erasmus University Rotterdam
hdl.handle.net/1765/77011
Erasmus MC: University Medical Center Rotterdam

Lim, C. (2008, December 12). Osmoregulation of chloride channels in epithelial cells. Retrieved from http://hdl.handle.net/1765/77011