Membrane Potentials Review:

Answer Key

#1. See your Textbook!!

#2. The Nernst Equation is :

Vm = (RT/zF) ln [Ion]side1/[Ion]side2

where R = gas constant (8.314 joules/deg mole)

T = degrees Kelvin;

z = valence or charge;

F = Faraday's constant (96,500 coulombs/mole)

At a body temperature of 37oC, and plugging in the constants and converting to log10, the equation becomes:

Vm (in mV) = (61.5/z) log10 [Ion]1/[Ion]2

So ENa+ = +61.5 mV ( inside of cell positive since Na+ moves in, down ÆC)

EK+ = -61.5 mV,

ECa++ = +123 mV (did you forget to divide by z which is 2 for calcium?),

ECl- = - 61.5 mV

#3a. increase P of sodium or calcium or decrease P of potassium or chloride

b. increase P of potassium or chloride (if Vm did not = ECl-) or decrease P of sodium or calcium.

c. increase potassium inside or decrease it outside

decrease sodium outside or increase it inside

increase chloride outside or decrease it inside

d. reverse of "c" above

(Parts of question 3 assumes that each ion is somewhat permeable and that Vm does NOT = the equilibrium potential for that ion.)

#4a. At threshold (depolarization caused by stimulus or signal molecule), voltage-gated sodium channels open and membrane depolarizes, opening more channels. Sodium channels inactivate (close). This, combined with the opening of voltage-gated potassium channels (caused by depolarization due to entry into cell of sodium), causes repolarization. Cell may briefly hyperpolarize due to Vm going to equilibrium potential of potassium which is more negative than Vm. You should understand why this is so!

b. See your text!!!




QUESTIONS