The Nernst Equation Protein Protein DNA DNA Protein Na Cl Protein K K K K K K Na Na Na Na Cl Cl Cl Na Cl K Na Cl Cl Cl Inside of the cell How many negative charges are there inside the cell How many positive charges are there inside the cell Outside of the cell How many negative charges are there outside the cell How many positive charges are there outside the cell Is the inside of the cell positive or negative compared to the outside The Nernst Equation Protein Protein DNA DNA Protein Na Cl Protein K K K K K K Na Na Na Na Cl Cl Cl Na Cl K Na Cl Cl Cl potassium channel What will happen if we add a potassium ion channel to the membrane The Nernst Equation K K K Protein Protein DNA DNA Protein Na Cl Protein Na Na Cl Cl Cl Cl Na Na K K Na K K Na Cl Cl Cl potassium channel Inside of the cell How many negative charges are there inside the cell now How many positive charges are there inside the cell now Outside of the cell How many negative charges are there outside the cell now How many positive charges are there outside the cell now So now is the inside of the cell positive or negative compared to the outside The Nernst Equation K K K Protein Protein DNA DNA Protein Na Cl Protein Na Na Cl Cl Cl Cl Na Na K K Na K K Na Cl Cl Cl potassium channel Is potassium at its chemical equilibrium are intracellular and extracellular concentrations equal Is there anything else happening here that may affect how potassium diffuses The Nernst Equation K K K Protein Protein DNA DNA Protein Na Cl Protein Na Na Cl Cl Cl Cl Na Na K K Na K K Na Cl Cl Cl potassium channel Potassium diffusion will stop when the chemical concentration gradient driving potassium out of the cell is exactly balanced by the electrical gradient pulling potassium back into the cell The cell s membrane voltage at this balance point is called the equilbrium potential remember that the word potential means the same as voltage The equilibrium potential for any ion at 37OC can be calculated using the Nernst equation The Nernst Equation K K K Protein Protein DNA DNA Protein Na Cl Protein Na Na Cl Cl Cl Cl Na Na K K Na K K Na Cl Cl Cl potassium channel The equilibrium potential for any ion Eion at 37OC can be calculated using the Nernst equation Eion 61 z log ion out ion in Where the number 61 is the constant 2 303 RT F at 37OC and z is the electrical charge of the ion 1 for K ion out and ion in are ion concentration outside and inside the cell The Nernst Equation K K K Protein Protein DNA DNA Protein Na Cl Protein Na Na Cl Cl Cl Cl Na Na K K Na K K Na Cl Cl Cl potassium channel Eion 61 z log ion out ion in Base on the figure above will the equilibrium potential for potassium be positive or negative Keep in mind that membrane potentials always compare the inside of the cell to the outside The Nernst Equation Protein Protein DNA DNA Protein Na Cl Protein K K K K K K Na Na Na Na Cl Cl Cl Na Cl K Na Cl Cl Cl Sodium channel Now let s start over again What happens if we add a sodium ion channel to the membrane So now is the inside of the cell positive or negative compared to the outside Identify two factors that will determine how much sodium diffuses across the cell membrane Sodium diffusion will stop when the chemical concentration gradient driving sodium into the cell is exactly balanced by the electrical gradient pulling sodium back out of the cell The Nernst Equation Protein K K K Protein DNA DNA Protein Na Na Na Cl Protein K K K Cl Cl Na Na Cl Na Cl K Na Cl Cl Cl Sodium channel Eion 61 z log ion out ion in Based on the figure above will the equilibrium potential for sodium be positive or negative Keep in mind that membrane potentials always compare the inside of the cell to the outside The Nernst Equation Protein Protein DNA DNA Protein Na Cl Protein K K K K K K Na Na Na Na Cl Cl Cl Na Cl K Na Cl Cl Cl What value did we discuss as a typical resting membrane potential of a cell If EK 61 1 log 5 mM out 150 mM in 90 mV and ENa 61 1 log 150 mM out 15 mM in 60 mV then which equilibrium potential is closer to the resting membrane potential of a cell Which ion Na or K has a greater role in determining the resting membrane potential of a cell The Nernst Equation Protein Protein DNA DNA Protein Na Cl Protein K K K K K K Na Na Na Na Cl Cl Cl Na Cl K Na Cl Cl Cl Eion 61 z log ion out ion in The Nernst equation only applies to a theoretical cell whose membrane is permeable to only ONE ion Real life is much more complicated Cells at rest are typically permeable to potassium sodium and chloride The Nernst Equation Protein Protein DNA DNA Protein Na Cl Protein K K K K K K Na Na Na Na Cl Cl Cl Na Cl K Na Cl Cl Cl Sodium channel potassium channel chloride channel Real life is much more complicated Cells at rest are typically permeable to potassium sodium and chloride The Goldman Hodgson Katz GHK equation is used to calculate the resting membrane potential that results from the contribution of all ions that can cross the cell membrane Vm 61 log PK K out PNa Na out PCl Cl in PK K in PNa Na in PCl Cl out The Nernst Equation Sodium channel potassium channel Protein Protein DNA DNA Protein Na Cl Protein K K K K K K Na Na Na Na Cl Cl Cl Na Cl K Na Cl Cl Cl chloride channel The Goldman Hodgson Katz GHK equation is used to calculate the resting membrane potential that results from the contribution of all ions that can cross the cell membrane Vm 61 log PK K out PNa Na out PCl Cl in PK K in PNa Na in PCl Cl out Permeability of the membrane to each ion Pion is a new term here Resting membrane potential Vm is determined by each ion s concentration on either side of the membrane and how easily it crosses the membrane permeability
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