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Beng Final 2007

# Beng Final 2007 - 6a Consider a red blood cell as a sphere...

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6a) Consider a red blood cell as a sphere with a radius of 7 μm. We have modified the cell membrane so that it is only permeable to sodium ions. If this cell is immersed in a 0.15M NaCL solution, a 10 mV potential is produced across the cell membrane (the inside of the cell is positive). If the cell is immersed in a 0.3M NaCL solution, what potential would be measured (magnitude and polarity)? V = -58log(C in /C out )mV (4pts) 10 = -58log{[Na] in /[0.15]} mV [Na] in = 0.101 M When the cell is in immersed into the 0.3M NaCl soln, the membrane potential becomes V = -58log{[0.101]/[0.3]} = 27.42 mV (positive inside) b) Charged ions have to move across the membrane to cause this change in potential. Using the definition of capacitance C=Q/V where C is the capacitance of the interface between the two concentrations in farad, Q is the amount of charge in coulombs required to charge up the interface to a voltage V, how many moles of sodium ion must move across the membrane to produce this new potential assuming the capacitance of the red blood cell membrane is 1 μF/cm 2 . F=96,500 Cs/mol. (4 pts) Q = CV = 10 -6 x 4 x 3.1416 x (7x10 -4 ) 2 x (27.42-10) x 10 -3 / 96500 = 1.11x10 -18 moles c) In the discussion of the electrical properties of the glass microelectrode we talked about a spreading resistance. What is the spreading resistance? (2 pts)

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