05-Membrane_potential - BSCI330 Cell biology and physiology...

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BSCI330 – Cell biology and physiology Fall 2009 LAB MANUAL – Lab Exercise #5 Carpenter et al. 2009. BSCI330 Laboratory Manual. University of Maryland, College Park. 1 EXERCISE 5: THE MEMBRANE POTENTIAL A steady electrical potential exists between the interior and exterior of all living cells across the cell membrane. What is the origin or basis of such a potential? In this lab exercise, the ionic basis of the transmembrane potential in cells will be demonstrated by using artificial membranes with defined permeability properties. The specific objectives are: 1. To generate an electrical potential across a membrane simply by adding solutions differing in ion concentration on the two sides of the membrane; 2. To see how the magnitude of the potential depends on the ion concentration difference (or gradient); 3. To see how the membrane's permeability properties influence the sign of the potential The Membranes and Their Properties Artificial membranes will be used because their properties are simple, well understood and clearly defined. They consist of porous sheets composed of plastic polymers containing either positive, negative, or no charged groups. Ions which have the same charge as the group on the polymer will be selectively prevented from passing through the pores of the membrane. Thus anions (negatively charged ions) will be excluded by an artificial membrane containing negatively charged groups, and cations (positively charged ions) will be excluded from membranes having positively charged pores. Membranes without charged groups are permeable to both cations and anions. In a selectively permeable membrane (so called ion exchange membrane) which allows only one type of ion to pass through, the flow of the permeant ions will rapidly result in the generation of an electrical potential across the membrane which will prevent further net ion movement. The Nernst equation allows one to calculate the potential which must be generated to exactly balance the ion's chemical gradient. V = (RT/zF) ln (Conc 1 /Conc 2 ) where V = the equilibrium potential, the potential which exactly opposes the ion's chemical gradient. It is measured as V 2 -V 1 . R = the gas constant, 8.314 volt-coulombs/mole/ o K T = absolute temperature ( o K), which at room temperature (20 o C) is 293 o K F = Faraday's constant, equal to 96,485 coulombs/mole z = the valence (charge) of an ion. For potassium it is equal to +1, for chloride it is equal to -1 Conc 1 /Conc 2 = the ratio of the concentration of the permeant ion on side 1 relative to side 2, i.e., the
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This note was uploaded on 12/15/2009 for the course BSCI 330 taught by Professor Payne during the Spring '08 term at Maryland.

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05-Membrane_potential - BSCI330 Cell biology and physiology...

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