Lectures 5 & 6: Ionic Equilibria & Membrane Potentials Flashcards
Electrochemical gradient/potential
- Combination of forces acting to move a charged particle/ion
Concentration gradient/chemical potential
- The driving force of concentration differences on the two sides of the membrane
The Nernst equation is valid only for
- An ion at equilibrium
OR - When the membrane is permeable to that ion only
Electrochemical equilibrium conditions
- When the two forces act on an ion such that there is no net movement of the ion
- The electrical force and the concentration force, are equal and opposite
- Only under this condition can the Nernst equation be applied
The Nernst equation allows us to calculate
- Magnitude of the electrical gradient (mV) that balances the chemical gradient, usually given in molarity (mM)
Electrochemical equilibrium definition
- Balance achieved when diffusion of Na from side A down its concentration is opposed by the buildup if negativity on side A
- At this point no more net movement of Na+ ions occurs
At electrochemical equilibrium there will be a
- Separation of charge
- Equal numbers of charged particles will not exist on each side of the membrane
The size of the separation of charge will depend on
- Concentrations of Na+ ions on side A and B at the end
- Can be determined using the Nernst equation if the [Na+]A and [Na+]B are known
In an excitable cell, such as a skeletal muscle cell, the resting measured membrane potential, Vm, is
- (-90 mV)
Net driving force
- NDF = electrical gradient - concentration gradient
- NDF = membrane potential - equilibrium potential
- NDF = Vm - Eion
For [Ko] > 10 mM, Vm behaves as though
- Membrane is permeable only to K+
For [K+] o < 10 mM, Vm
- Deviates from this simple relationship
- Some other ions must be contributing to determination of Vm
The Gibbs-Donnan equilibrium describes
- The steady-state properties of a mixture of permeant and impermeant ions as exists in the cytoplasm of cells
- This also encompasses the principle of electroneutrality
Electroneutrality states that
- Any microscopic region of a solution must have an equal number of positive and negative charges
The presence of an impermeant ion on one side of a membrane leads to
- An unequal distribution of the permeable ions
- Results in the generation of an electrical potential across the membrane
In cells, the permeant K+ and Cl- are nearly in
- Electrochemical equilibrium across the plasma membrane - These two ions essentially obey the Gibbs-Donnan equilibrium
Sodium ions do not obey
- Gibbs-Donnan equilibrium
- Their movement causes changes in volume of cell to occur when the ion concentrations change either inside or outside the cell
NDF on Na+
- Very large
- Usually over 100 mV
- Normally the cell is very impermeable to this ion
Normally, the NDF on K+ and Cl- ions
- Is very small
- Although the membrane is relatively permeable to these ions, the amount of these ions crossing the membrane is very small
Na+ enters the cell by
- Diffusion down its electrochemical gradient
- Usually pumped out of the cell, preventing the cell volume and the Gibbs-Donnan equilibrium from being disturbed
Inhibition of the Na+/K+ ATPase causes
- Disequilibration of the Gibbs-Donnan equilibrium
- Results to Na+ entry and accumulation (causes cell swelling)
Ions in solution move across membranes under the influence of
- An electrochemical gradient made up of two forces (concentration and electrical)
Ionic diffusion ceases if
- The concentration force can be balanced by the electrical force
The Nernst equation allows the calculation of
- Electrical force/equilibrium potential (Eion) that must be applied to oppose the concentration force
- It only applies to one ion at a time
Goldman-Hodgkin-Katz equation allows the calculation of
- Membrane potential (Vm) based on ionic permeabilities
Net driving forces can be calculated using
- Vm and Eion
- These identify the nature, direction and size of the net force acting on the ion of interest
The Gibbs-Donnan equilibrium identifies
- How a membrane potential is generated when a solution containing some impermeant ions is separated form a solution containing only permeable ions
The impermeant ions of major interest are
- Proteins inside cells
- Some of the larger inorganic anions
Two forces to consider with charged particles
- Concentration/chemical (acts downhill)
- Electrical (attracting or repelling)
Impermeable membrane characteristics
- Equal numbers of charged particles on one side of membrane
- No separation of charge
- No Em recordable (Vm= 0)
Non-selectively permeable membrane characteristics
- Flux (J)(+) = Flux (J)(-)
- Charged particles move equally easily in both directions until reach equilibrium
- No separation of charge
- No Vm recordable
Selectively permeable membrane characteristics
- Flux (J)(+)»_space; Flux (J)(-)
- (+) charged particles move until concentration force is equally opposed by electrical force
- Electrochemical Equilibrium
Electrochemical equilibrium is a form of
- Diffusion equilibrium
- Considers both charge and concentration of solute
- Solute concentrations may not be equal on both of membrane
Electrochemical equilibrium is achieved when
- Movement of a charged solute down its concentration gradient is equally opposed by the increasing electrical gradient being generated due to its movement
Nernst equation
- Allows calculation of the equilibrium potential for an ion
- This is the electrical force acting to stop NET diffusional movement of an ion across a membrane
- The units of this force are millivolts (mV)
Driving force
- The resting membrane potential (Vm) of a cell does not equal any one ion’s equilibrium potential
- This means that NONE of the ions are at equilibrium
Vm is greatly influenced by
- The ion with the largest permeability
Gibbs-Donnan Equilibrium
- The equilibrium reached by two solutions separated by a permeable barrier when one of the solutions contains impermeant ions
According to Gibbs-Donnan equilibrium,
- Regardless of the solution composition,
- Equal numbers of (+) and (-) ions inside
- Equal numbers of (+) and (-) ions outside
- K+ and Cl- ions obey this principle across plasma membranes
Na ions do not obey G-DE and in fact upset it because
- NDF for K+ and Cl- in resting cells is quite small
- NDF for Na+ in resting cells is very large
- Na+ moves into cells driven by this NDF
In G-DE, Na movement is accompanied by
- Water
- Hence, a volume change occurs
- The cell counters this Na+ movement by pumping Na+ out, using the Na+/K+- ATPase (sodium pump)
Charged particles, ions, in solution move
- Across membrane driven by electrochemical gradient
- This gradient is made up of chemical and electrical forces
The magnitude of the chemical force acting on an ion making it move down its concentration gradient can be calculated using
- Nernst equation
- It is called Eion
In order to look at the concentration forces acting on multiple ion simultaneously,
- The GHK or Chord conductance equation is used
Nernst and GHK equations yield
- Membrane potential difference, Vm, acting across a cell membrane
The NDF
- Causes ions to diffuse down their electrochemical gradient
- Is used by cells for signaling purposes
Gibbs-Donnan equilibrium describes a scenario when
- Two solutions (one of which contains an impermeant ion) are separated by a membrane
- Leads to the development of electrical potentials across membranes
