Lecture_8_Jan_22_

Lecture_8_Jan_22_ - Major topics to be discussed:...

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1 Major topics to be discussed: Determination of ECG for a transported ion How is the transport direction of an ion determined? he origin of membrane potential The origin of membrane potential How is the transport specificity of an ion channel achieved? How is the opening/closing of an ion channel controlled?
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2 Method 1: to determine the transport direction favored by the electrical and chemical driving forces separately electrical gradient: positive ions move toward the negative potential egative ions move toward the positive potential negative ions move toward the positive potential chemical gradient: always from high conc to low conc gy g No math is required. ne can only predict the direction but not the size of driving force One can only predict the direction but not the size of driving force
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3 What is the transport directions for Na + in a resting cell? electrical gradient: simple case Method 1: chemical gradient: ECG direction: ECG driving force: ? Vm=-70 mV (Na + ) O = 145 mM (K + ) O = 5 mM (Na + ) i = 10 mM (K + ) i = 140 mM
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4 What is the transport directions for K + in a resting cell? electrical gradient: simple case complex case Method 1: chemical gradient: ? ECG direction: ECG driving force: ? ? Vm=-70 mV (Na + ) O = 145 mM (K + ) O = 5 mM (Na + ) i = 10 mM (K + ) i = 140 mM
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5 Method 2: to calculate Vx, then compare Vx to Vm Vy= log 61.5 Z Co Ci Unit : mV at 37 o C Z : charge of ion X C 0 : outside conc. of ion X inside conc of ion X C i : inside conc. of ion X for K + ion, V K+ = 61.5 log (5/140) = -89 mV for Na + ion, V Na+ = 61.5 log (145/10) = 71 mV Math is required. ne can predict both the direction and the size of driving force One can predict both the direction and the size of driving force
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6 How can the comparison of Vx and Vm allow one to determine the transport direction Δ G ECG = -RTln(Co/Ci)+ zFVm 0 = -RTln(Co/Ci)+ zFVx At Vx, the driving force from electrical and chemical gradients cancel each other. Thus, there is no net flow of ion X for K + ion, V K+ = -89 mV: there is no net flow of K + when the Vm is -89 mV (see the draw on the board) for Na + ion, V Na+ = 71 mV: there is no net flow of Na + when the Vm is 71 mV What would happen when the Vm is -70 mV?
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7 What are the transport directions for Na + or K + in a resting cell? V Na+ = 71 mV V K+ = -89 mV Method 2: Vm=-70 mV (Na + ) O = 145 mM (K + ) O = 5 mM (Na + ) i = 10 mM (K + ) i = 140 mM ansport direction ut- - - - ut transport direction out to in in to out
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8 What are the sizes of driving forces for Na + or K + in a resting cell?
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Lecture_8_Jan_22_ - Major topics to be discussed:...

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