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Understanding Ion Equilibrium and Membrane Mechanics
Sep 12, 2024
Lecture Notes on Ion Equilibrium and Membrane Permeability
Initial Setup
Two compartments with sodium chloride (NaCl) initially in equal amounts on both sides.
Both sides are electrically neutral.
Membrane is permeable to both sodium (Na) and chloride (Cl).
Expectation
: Movement of Na and Cl to reach diffusional equilibrium (equal concentration on both sides).
Modified Setup
NaCl initially only on the left side.
Membrane only permeable to Na, not to Cl.
Reflection Coefficient
:
Na: 0 (membrane permeable)
Cl: 1 (membrane impermeable)
Chemical and Electrical Gradients
Chemical gradient for Na and Cl: From left to right, Cl cannot move.
Movement Outcome
:
Na moves right leaving negative charge behind (due to immobile Cl).
Development of diffusion potential (charge separation).
Electrochemical Gradient
As Na moves, an electrical gradient develops opposing further Na movement.
Equilibrium
: Electrical force equals chemical gradient.
Equilibrium Potential
:
At equilibrium, the electrochemical gradient is zero.
Nernst Equation
Used to calculate the equilibrium potential (Nernst Potential) for an ion.
Formula:
Equilibrium potential = Z (valence) × 60 mV × log(concentration outside/concentration inside)
Z: Valence of the ion.
Relative Ion Concentrations
Sodium (Na): More concentrated outside the cell.
Potassium (K): More concentrated inside the cell.
Equilibrium Potentials for Ions
Sodium (Na): Approx. +60 mV
Potassium (K): Approx. -90 mV
These are reference values for simplicity.
Key Takeaways
Equilibrium Potential
:
Represents the electrical force that counterbalances the chemical force for an ion.
Specific to individual ions.
Magnitude of electrical force equals the chemical gradient.
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