Membrane Potential Flashcards
- Compare qualitatively the relative strengths of electric and osmotic forces.
Electric forces are extremely stronger than osmotic forces, which is why just a few excess ions are needed to counter large concentration differences.
- Describe the two forces acting on an ion moving across a membrane.
1) Concentration difference (chemical gradient)
2) Electrical potential difference across the membrane (i.e. membrane potential, produced by imbalance in number of cations and anions in a cell).
These two forces = an electrochemical gradient.
- Define equilibrium potential.
For a given ion, the equilibrium potential is the potential across the membrane that the ion would like to see exist in order to be in equilibrium. If the MP isn’t at the equilibrium potential, either the ion isn’t able to permeate the membrane, or there’s some mechanism that keeps the ions from crossing the membrane effectively (i.e. it’s being pumped). Measured in in mV.
Equilibrium potential relates the concentration gradient to the electrical force and the electrical potential difference across the membrane that must exist if the ion is to be at equilibrium.
If MP = equilibrium, ion is distributed at its electrochemical equilibrium.
Equilibrium potential is the voltage difference across a membrane at which all the individial charged components are at dynamic state. No net movement.
- Describe the difference between an equilibrium potential and a recorded membrane potential.
Membrane potential = the actual electrical difference across the cell membrane. It’s the ‘real’ voltage; measured by impaling cell with microelectrode and measuring the real voltage difference between in and out of cell.
Equilibrium potential = calculated from the Nernst equation. Arbitrarily defined as the potential inside the cell with respect to the outside (so if E = -40 mV, the inside is 40 mV negative to the outside). Not a ‘real’ voltage.
- Recognize that each and every ion species has its own, independent equilibrium potential.
E is dependent on the properties of the ion. For a non-pumped, permeating ion species, the E is equal to the MP for that membrane. But cell only has one membrane potential, so you compare these values to figure out if a pump exists.
- Answer correctly that the number of excess anions in a typical cell is small compared to the total number of anions.
Membrane potentials arise from an excess of one type of ion in the cell. Doesn’t need to be a big difference to keep the gradients in place because of the ‘awesomely powerful’ electric force – excess number of ions is very small compared to total number of ions. Ex: 100,000 cations in the cell need 100,001 anions to have a negative resting membrane potential of -80 mV.
- Answer correctly that bulk solutions are always electrically neutral.
This is the principle of electrical neutrality. Bulk solutions in and out of the cell have to be electrically neutral; the total cation concentration in the ECF must equal the total anion concentration in the ECF (and this holds for the ICF too).
- Describe how an artificial cell can be in a state of equilibrium even though the concentrations of ions are not the same inside and out.
Specific ions do not have to be equal inside and out – the cell must follow the Donnon rule, which states that the products of the ion concentrations in the cell must equal the product of the ion concentrations outside the cell. You balance the electrical equilibrium with the concentration equilibrium.
Ex of Donnan: [K]o[Cl]o = [K]i[Cl]i
- Apply osmotic balance, charge neutrality, and the Nernst equation to calculate ion concentrations and the membrane potential of an artificial cell.
Osmotic balance = mOsm will be equal in ICF and ECF.
Charge neutrality = can assume an equal number of cations and ions inside the cell and outside the cell (not that inside is equal to outside)
Nernst = E = 62/z * log ([ion]o/[ion]i) where E is proportional to the concentration
Donnan equation = [K]o[Cl]o = [K]i[Cl]i
DO THE PRACTICE PROBLEMS.
- Differentiate between equilibrium and steady state.
Real cells are in a steady state, so like cell at equilibrium, the ion concentrations aren’t changing over time. Unlike the cell at equilibrium, a constant input of energy is needed in the real cell (like in form of ATP to drive Na/K pump). ypu are at dynamic eq in both energy is used.
- Describe how membrane potential depends on relative, not absolute, permeabilities to ions.
A membrane that is very permeable to sodium, not potassium, has a MP closer to that of the ENa (+60). If membrane is more permeable to potassium, Vm approaches Ek (-80). If the permeabilities are equal, Vm doesn’t change.
Vm can also be dependent on ion concentrations, which really only matters clinically when you have too much K in the ECF.
Vm depends on Gr, which is relative conductance of Gna to Gk. G = conductance = # of ion channels open. More conductance = more current; more driving force = more current. I = G(Vm-E)
- Describe how the primary short term determinant of membrane potential is not the Na/K pump, but relative membrane permeabilities to the different ions.
Membrane potential is determined by relative membrane potentials in the short term. Depends on the size of cell and number of channels in the membranes (larger cells with more channels are more quickly vulnerable to Na/K pump failure).
If you block the pump, cells with larger surface area see Na and K reaching equilibrium quicker = depolarization; but other cells may not get there for a while, which is why relative permeabilities matter more in the short term.
- Define Driving Force on an ion.
The ion wants to have membrane potential approach its own E. It will drive the membrane potential towards that equilibrium, aided or counteracted by other ions. The driving force, at any instant, is the difference between Vm and Eion.
Postive Driving force when pos things leave cell
Positive driving force when neg things enter cell.
- Describe why, in neurons and other excitable cells, membrane potential is sensitive to small changes in [K+]o, but not [Na+]o.
Because the MP of cell is so close to Ek anyway, the loss of Na from ECF (which brings ENa closer to 0) makes the Vm slightly hyperpolarized (closer to -80), but not have much other effect. ECF levels of Na+ have little effect on the membrane potential of this cell, simply because the membrane is relatively impermeable to Na+.
Sensitivity to changes in [K]o is higher because it’s starting concentration outside the cell are much smaller than Na. Plasma membrane is so much more permeable to K that wherever Ek goes, Vm follows. If you have too much K in ECF, MP depolarizes a lot and Vm will follow.
Hyperkalemia = excess of K in ECF changes MP of cell because alters concentration gradient on which electrical gradient is based. Raises the Ek, makes MP rise too (even if just for a 2% change). Can be fatal because it messes up action potential propagation, especially in heart arrhythmia.
Fix with CBIGK = Calcium (stabilizes cardiac conduction), Bicarb (helps cell take up K), Glucose and Insulin (helps power Na/K ATP pump), Kayexalate (anion that is given as a sodium salt, but has higher affinity for K binds up K ions).
Can also clean blood via dialysis blood passed through tubing that allows free exchange of ions between blood and dialysate. Less [K] in dialysate K leaves blood.
- Describe two treatments for hyperkalemia by which cells can be encouraged to take up potassium from the ECF.
Encourage cells to take up potassium from ECF by giving glucose and insulin, or giving bicarbonate.
