Boundary Conditions for Maintaining a Steady State

1
Boundary Conditions for Maintaining a Steady State

• Intracellular and extracellular solutions must
  be electrically neutral
• The cell must be in osmotic balance
• There must be no net flux for any ion into and
  out of the cell

2
Intracellular and Extracellular Ion
Concentrations for a typical cell


K
90 mM
K
3 mM


Na
30 mM
Na
117 mM
-
-
An
116 mM
An
0
-
-
Cl
4 mM
120 mM
Cl
Cell
Outside
3
A Cell Exclusively Permeable to K
K
K
Na
4
A Cell Exclusively Permeable to K
K
K
Na
5
Balancing the Gradient
A.
B.
A has a steeper gradient, so youd need to
exert more force to balance it.
6
Nernst Equation
EK k (ln Kout - ln Kin)
EK (RT/zF) (ln Kout - ln Kin)
7
A Cell Permeable to K and Na

---- - ---
K
K
Na
Na
8
GHK (constant field) equation
note GHK is based on the assumption that the sum
of all transmembrane currents 0
9
If theres no net chloride current, then
where b pNa/pK
10
Ohms law i V/R Vg
iNagNa(Vm-ENa)
iKgK(Vm-EK)
If Vm is stable, then iNa - iK
so, if gK gtgt gNa, driving force on Na must be
greater then the driving force on K and in the
opposite direction
11
Na
PUMP ATP PUMP-P
ADP PUMP-P H2O PiPUMP
K
3 Na out for 2 K in, so the pump is electrogenic
Prevents establishment of a Donnan equilibrium