Equilibrium Potential Ex where x is an ion

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

Experiment 1 + K 0 m. V Outside Inside - + + + - - + + - - Concentration + gradient - - + + Electrical gradient + + - +

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

EK+ = -90 m. V + K -90 Experiment 1 Another moment later. . . Equilibrium! m. V Outside Inside - - + + + + - - + - + - Concentration gradient - + + - Electrical gradient + + - +

Experiment 2 + Na 0 m. V X X Inside Outside - + + + - - + + - - Concentration + gradient - - + + Electrical gradient + + - +

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

ENa+ = +60 m. V + Na +60 Experiment 2 Another moment later. . . Equilibrium! m. V Inside Outside - - + + + + - - + - + - Concentration gradient - + + - Electrical gradient + + - +

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