1. Equilibrium Potential E x (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

2. Electrical gradient Concentration gradient + + + + + + + + + ++ + + + + + + + + + - - - - - - - - - - - - - - - - - - - - K+K+ mV 0 Inside Outside Experiment 1

3. Electrical gradient Concentration gradient + + + + + + + + + + + + + + + + + + + + - - - - - - - - - - - - - - - - - - - - K+K+ Inside Outside Experiment 1 A moment later...

4. Electrical gradient Concentration gradient + + + + + + + + + + + + + + + + + + + + - - - - - - - - - - - - - - - - - - - - K+K+ Inside Outside Experiment 1 Another moment later... Equilibrium! mV -90 E K + = -90mV

5. Electrical gradient Concentration gradient + + + + + + + + + ++ + + + + + + + + + - - - - - - - - - - - - - - - - - - - - Na + mV 0 Inside Outside Experiment 2 Inside Outside X X

6. Electrical gradient Concentration gradient + + + + + + + + + + + + + + + + + + + + - - - - - - - - - - - - - - - - - - - - Na + Outside Inside Experiment 2 A moment later...

7. Electrical gradient Concentration gradient + + + + + + + + + + + + + + + + + + + + - - - - - - - - - - - - - - - - - - - - Na + Outside Inside Experiment 2 Another moment later... Equilibrium! mV E Na + = +60mV +60

8. Equilibrium Potential E x (where x is an ion) –E Na + E K + 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