The membrane potential of a cell membrane is -70mV. This means that the electric
ID: 1527868 • Letter: T
Question
The membrane potential of a cell membrane is -70mV. This means that the electric potential is – 70 mV inside the cell and 0V outside
a) A K+ ion moves from the outside into the cell. What is the change in the ion’s electric potential energy? Does it increase or decrease?
b) The cell membrane has a thickness of 5nm. Calculate the electric field inside the membrane. (Think of the membrane as a parallel plate capacitor.)
c) An ion of charge -8e is embedded in the membrane. Calculate the electric force on the ion. In which direction is the force?
Explanation / Answer
(a)potential differnece= -70 mV= -0.07 V
charge of ion= + 1.6 x10^-19 c
change in eletric potential eenrgy= qV= 1.6x10^-19 (-0.07)
U=1.12x10^-20 j. the change in potential energy is negative, so it decreases.
(b) thickness= 5 nm= 5x10^-9 m
eletric field= E= V/d= -70/(5x10^-9)
E=-1.4x10^10 N/c .
(c) charge on ion= -8e= -8(1.6x10^-19)=-1.28x10^-18 c
force= q E
F= (-1.28x10^-18)(-1.4x10^10)
F=1.792x10^-8 N..the directionof force inwards.