Lecture 3 The Membrane at &acirc;€œRest&acirc;€

Rate they are going in at equilibrium ions flow at

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rate they are going in) at equilibrium ions flow at equal rates need to specify ions we are talking about b/c they have diff equilibrium points –> means that don't have multiple ions at equilibrium at the same time potassium and sodium are major cations that control membrane potentials both at rest and action potentials. Potassium found largely concentrated inside the cell sodium found large concentrated outside the cell A- represents a net charge of proteins which never cross the membrane ions across a membrane potential is determined by selective permeability and the direction of the concentration gradient the permeability and initial concentrations determine the direction of ion flow resulting

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in a relative charge distribution across the membrane Principles of Movement of Ions Across a Selective Membrane 1. large changes in membrane potential are caused by very small changes in ionic concentration the reason that this occurs is because per particle, the electrical force is much, much greater than the diffusions factor [therefore, changes in concentrations are negligible in determining equilibrium] 2. the net difference in electrical charge occurs at the inside and outside surfaces of the membrane outside the membrane is more positive than inside. Attraction across membranes 3. ions are driven across the membrane at a rate proportional to the difference between the membrane potential and the equilibrium potential. V diff = Ig (Ohm's law) Vdiff (voltage difference) is the “driving force” on a given ion is determined by the difference in membrane potential (V m ) and its equilibrium potential (E ion ) g (conductance) is determined by the net capacity of ions to move through a channel; inverse of resistance (1/R) I (current) is the rate of flow of charge across the membrane we will focus on voltage difference → the driving force on a single ion 4. If the concentration difference across the membrane is known an equilibrium potential can be calculated for any ion E ion = 2.303 * RT/zF * log [ion]outside/[[ion]inside → Nernst equation R=gas constant T=temperature increasing T, increases KE in a cellular system, weights equilibrium potential more towards concentration radiance and less from separation of charge z=valency of ion F=Faraday's constant Take out 3 factors –> R, F, T 61.54 * 1/z * log [ion]outside/[ion]inside
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• Fall '08
• Kippin,T
• Electric charge, Selective Membrane

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