Review session #2 questions and answer key
20. A selectively permeable membrane is separated into compartment 1 & 2. Cl can
diffuse down its concentration gradient but Na+ can’t
cross the membrane.
A) What is the driving force of Cl?
Driving force=EmEion, so first we need to
calculate the Em.
Em=0, since there is equal charge on each side of the membrane
at the inital state.
Ecl=58/valence *log (out/in), or 58/1 * log (1.0/.1) =58, so the
driving force is 58 (or 58 driving chloride to compartment 2)
B) What is the electromotive force (Em) acting on Cl?
This will be the same answer as
above, electromotive force is the force driving the Cl to compartment 2.
C) What is the state when there is no net flux of Cl across membrane?
When Em=58
(Nernst potential for Cl, since it’s the only permeable ion), this is also the
equilibrium potential since there is no net flux of Cl.
21. What would be the direction of the ion movement (flux) for the
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following values of Em?
For all values of Em, Na flux is 0 since it’s not permeable.
At 58, the net flux of Cl is zero, since 58 is it’s Nernst Potential.
At 0mV, Cl is going to go from 2
1.
This is since 0mV is more positive then Cl
Nernst potential.
To conceptualize this, try to think of it as “what way would Cl
have to go to get from it’s Nernst potential (58) to 0mV?”
At 100 mV, the value is more negative then it’s Nernst potential, so Cl would flow
from 1
2.
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 Spring '08
 SimonHalegoua
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