Model proposed by Singer and Nicolson
Cell Membrane
Functionally the cell membrane behaves as a semipermeable barrier: Some water-soluble and fat-soluble substances of low molecular weight readily cross cell membranes, while electrically charged molecules or large (protein) do not. Transmembrane movement of drugs are influenced by the composition and structure of the cell membrane. This is thin (70 - 100 Armstrong) and composed of phospholipids and carbohydrates as well as scattered groups of proteins comprehensive act as receptors, channels, pumps, enzymes or simply structural. The theory of the lipid bilayer or membrane unit proposed by Davson and Danielly (1952), considers that the cell membrane is composed of two layers of phospholipids between two layers of proteins, with the "heads" hydrophilic phospholipids oriented outside while the "tails" are aligned towards the hydrophobic interior. This theory explains the observation that lipid-soluble drugs tend to more easily penetrate the membrane polar substances. However, this model does not take into account the diffusion of water, the passage of low molecular weight molecules (urea) and certain ions.
fluid mosaic model proposed by Singer and Nicolson (1972), explains the transcellular diffusion of polar molecules. According to this model represented in the graph, the membrane globular protein is embedded in a fluid dynamics in a matrix of a lipid bilayer. These proteins provide a route for the selective transfer of ions and polar molecules through the lipid barrier and pore forming two types: The first about 10 nanometers and the other between 50-70 nm. The first act as channels for the diffusion of water or ions (Na +, K +, Cl-, etc).
taken from: http://www.virtual.unal.edu.co/cursos/ciencias/12161/lecciones/02_01_01.htm
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