Boundary Conditions for Maintaining a Steady State Intracellular and extracellular solutions must be electrically neutralIntracellular and extracellular solutions must be electrically neutral The cell must be in osmotic balanceThe cell must be in osmotic balance There must be no net flux for any ion into and out of the cellThere must be no net flux for any ion into and out of the cell

Intracellular and Extracellular Ion Concentrations for a “typical” cell K + = 90 mM Na + = 30 mM An - = 116 mM Cl - = 4 mM Cell K + = 3 mM Na + = 117 mM An - = 0 Cl - = 120 mM Outside

A Cell Exclusively Permeable to K + K+K+ Na + K+K+

A Cell Exclusively Permeable to K + K+K+ Na + K+K+

A.B. “A” has a steeper gradient, so you’d need to exert more force to balance it. Balancing the Gradient

E K = k (ln [K] out - ln [K] in ) E K = (RT/zF) (ln [K] out - ln [K] in ) Nernst Equation

A Cell Permeable to K + and Na + K+K+ Na + K+K

GHK (constant field) equation note: GHK is based on the assumption that the sum of all transmembrane currents = 0

If there’s no net chloride current, then: where “b” = p Na /p K

Ohm’s Ohm’s law: i = V/R = Vg i Na =g Na (V m -E Na ) i K =g K (V m -E K ) If V m is stable, then i Na = - i K so, if g K >> g Na, driving force on Na must be greater then the driving force on K and in the opposite direction

PUMP + ATP PUMP-P + ADP PUMP-P + H 2 O P i +PUMP Na K 3 Na + out for 2 K + in, so the pump is electrogenic Prevents establishment of a Donnan equilibrium