Hayley - Week 3 key - TA Hayley Cliatt [email protected]

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TA: Hayley Cliatt Section: Wed 1-1:50 pm @ HSS 2150 [email protected] OH: Fri 3-4 pm @ PC Theatre Lobby BIPN100 WEEK 3 WORKSHEET TOPICS COVERED: -­‐ Nernst Equation + Equilibrium Potentials (E ION ) -­‐ Ionic Current + Revisiting Ohm’s Law -­‐ GHK Equation -­‐ Graded potentials -­‐ Action potentials 1. NERNST EQUATIONS + EQUILIBRIUM POTENTIALS (a) The cell membrane is __ polarized __ meaning that there are different concentrations of ions between the outside and the inside of the cell. This allows for the movement of ions by providing a concentration gradient. Draw a picture detailing the concentrations of the following ions inside and outside of the cell (Note: You should memorize this!!): Na + , K + , Cl - , Ca 2+ . (b) The Nernst equation describes how much voltage can be generated for a given concentration gradient. Give the Nernst Equation. Be sure to define any variables you give. E ION = (61/Z)(log ([ion] OUT /[ion] IN ) Where Z = valance (aka charge)
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TA: Hayley Cliatt Section: Wed 1-1:50 pm @ HSS 2150 [email protected] OH: Fri 3-4 pm @ PC Theatre Lobby (c) Using the concentrations you gave in part (a), use the Nernst equation you gave in part (b) to calculate the equilibrium potential (E ION ) for Na + , K + , Ca 2+ , and Cl - . E Na+ = (61 mV/Z Na+ )(log([Na + ] OUT /[Na + ] IN ) E Na+ = (61 mV/1)(log([140 mM]/[14 mM]) E Na+ = (61 mV)(log(10)) = (61 mV)(1) = 61 mV For Na + , this means that the 10 fold concentration gradient across the membrane will generate a potential of 61 mV. E K+ = (61 mV/Z K+ )(log([K + ] OUT /[K + ] IN ) E K+ = (61 mV/1)(log([5 mM]/[150 mM]) E K+ = (61 mV)(log(1/30)) = (61 mV)(-1.5) = (approximately) -90 mV E Cl- = (61 mV/Z Cli )(log([Cl - ] OUT /[Cl - ] IN ) E Cl- = (61 mV/-1)(log([100 mM]/[10 mM]) E Cl- = (-61 mV)(log(10)) = (-61 mV)(1) = -61 mV E Ca2+ = (61 mV/Z Ca2+ )(log([Ca 2+ ] OUT /[Ca 2+ ] IN ) E Ca2+ = (61 mV/2)(log([10 -3 M]/[10 -7 M]) E Ca2+ = (30.5 mV)(log(10 4 )) = (30.5 mV)(4) = 122 mV 2. IONIC CURRENT + OHM’S LAW (again) (a) Ionic current, the movement of ions requires two things: a __ driving force ___ and a __ pathway __. Give the revised Ohm’s Law equation in regards to ionic current. Make sure to define each variable you give. I ION = (V M – E ION )(G ION ) (Where I = current of ion, V M = membrane potential, E ION = equilibrium potential of said ion, and G ION = conductance for that ion) Note that the conductance depends on the number of open channels for that ion. (b) Which of the term(s) in the equation you gave describe the driving force ? Which term(s) describe the pathway ?
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