Lect 03 F10 - I on T r anspor t thr ough Cell M embr anes...

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Molecular structure of a K+ channel. From Doyle et al. (1998). Science 280: 69-77 I on Transport through Cell M embranes
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Some permeability » small nonpolar molecules Lipid bilayer are highly impermeable to charged molecules, even if they are small.
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The movement of ions and small polar molecules across cell membranes is mediated by transport proteins that form aqueous channels. Transport proteins are highly specific for particular molecules. The distribution of transporters is a primary determinant of the solute composition of membrane- bounded compartment.
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Energetics of ion transport. 1) Like other molecules, ions diffuse down their concentration gradient, from high concentration to low concentration. 2) Ions are also charged, and the sum of different ion concentrations on either side of a membrane results in a membrane potential. The transport of ions across a membrane is therefore also influenced by their electric charge. 3) The net movement of an ion across the membrane is determined by it’s Electrochemical Gradient = Ion Concentration Gradient + Electric Potential
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Representative electrochemical gradients of four important ions OUTSIDE INSIDE 145 mM 5-15 mM + + + + + + + + + + + + - - - - - - - - - - - - + + + + + + + + + + + + - - - - - - - - - - - - Na + Ca ++ Cl - K + 10 -4 mM 5-15 mM 140 mM 1-2 mM 110 mM 5 mM Strong tendency for both Na + and Ca ++ to influx into cells down their steep electrochemical gradients. The influx of Cl - into the cell and the efflux of K + out of the cell down their concentration gradient is partially offset by membrane potential. Membrane potential tends to be slightly negative on inside of cell. This potential is largely determined by the transport of K + as we’ll see a bit later.
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Channel proteins Form narrow hydrophilic pore that allows diffusion of specific ions along their electrochemical gradient. Carrier proteins
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This note was uploaded on 11/10/2010 for the course CBNS 101 taught by Professor - during the Spring '08 term at UC Riverside.

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Lect 03 F10 - I on T r anspor t thr ough Cell M embr anes...

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