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Equilibrium Potential
• Ex (where x is an ion)
• Membrane potential with an electrical driving force equal but opposite to the driving force of the concentration gradient– Applies to a single type of ion
• Opposite ion is relatively impermeant
Electrical gradient
Concentrationgradient
+
+
+
+
+
+
+
+
++ +
+ ++
++
+
+
+
+-
-
-
-
-
-
-
-
-
-
-
--
-
-
-
-
-
-
-
K+
mV
0
Inside Outside
Experiment 1
Electrical gradient
Concentrationgradient
+
++
+
+
+
+
+
+ +
+
+ +
+
+ +
+
+
+
+-
-
-
- -
-
-
-
-
-
-
--
-
-
-
-
-
-
-
K+
Inside Outside
Experiment 1
A moment later . . .
Electrical gradient
Concentrationgradient
+
++
+
+
+
+
+
+ +
+
++
+
+ +
+
+
+
+-
-
-
--
-
-
-
-
-
-
--
-
-
-
-
-
-
-
K+
Inside Outside
Experiment 1
Another moment later . . .
Equilibrium!mV
-90
EK+ = -90mV
Electrical gradient
Concentrationgradient
+
+
+
+
+
+
+
+
++ +
+ ++
++
+
+
+
+-
-
-
-
-
-
-
-
-
-
-
--
-
-
-
-
-
-
-
Na+
mV
0
Inside Outside
Experiment 2
InsideOutsideX X
Electrical gradient
Concentrationgradient
+
++
+
+
+
+
+
+ +
+
+ +
+
+ +
+
+
+
+-
-
-
- -
-
-
-
-
-
-
--
-
-
-
-
-
-
-
Na+
Outside Inside
Experiment 2
A moment later . . .
Electrical gradient
Concentrationgradient
+
++
+
+
+
+
+
+ +
+
++
+
+ +
+
+
+
+-
-
-
--
-
-
-
-
-
-
--
-
-
-
-
-
-
-
Na+
Outside Inside
Experiment 2
Another moment later . . .
Equilibrium!mV
ENa+ = +60mV
+60
Equilibrium Potential• Ex (where x is an ion)
– ENa+ EK+
• Membrane potential with an electrical driving force equal but opposite to the driving force of the concentration gradient– Applies to a single type
of ion• Opposite ion is relatively
impermeant
Electrical gradient
Concentrationgradient
+
++
+
+
+
+
+
+ +
+
++
+
+ +
+
+
+
+-
-
-
--
-
-
-
-
-
-
--
-
-
-
-
-
-
-
K+
Inside OutsidemV
-90
EK+= -90mV
