Talbot - Membrane Biophysics and Membrane Potential Flashcards
what are the typical levels of K+, Na+, Cl- and A- intracellularly
K+ =120mM, Na+ = 15mM, Cl- = 20mM and A- =»_space;0mM
what are the typical ion levels outside the cell
Na+ (140mM), Ca2+ (1.2mM), Cl- (116mM)
what area of a cell is negative and what area is positive
slight negative inside cell and positive outside
what is considered the ground substance or 0mV when measuring a membrane potential
the ECF - compare voltage to ICF
what is a typical resting potential for a cell
-65mV to -75mV
what 2 things is the actual voltage influenced by
ionic concentration gradients and membrane permeability to those ions
what 2 things influence the driving force for passive transport
concentration gradient and membrane potential (electrical gradient) across membrane
when does a cell have net flux
when a solute is at equilibrium (net flux = 0)
what state are ions in when the net flux = 0
electrochemical equilibrium (there can still be a concentration gradient)
when does voltage develop across a membrane
when there is unequal flow (current) of charge/ions across membrane - typically from selectivity for specific solute
which directions are Na+ and K+ always moving
Na+ = into cell and K+ = out of cell
what direction is Cl- moving: into or out of cell
direction depends on cell type and intracellular concentrations of Cl-
what is the TEPD: transepithelial potential difference
apical and basolateral maintain their own Vm (membrane potential) -> TEPD = Vb- Va
what is resistance (R)
measure of how difficult it is for current to pass - refers to ionic fluxes and is inversely proportional to the density of open ion channels
what is capacitance (Cm)
stores charge separated across membrane; influences rate of voltage charge
what is conductance (Gx)
conductance of an ion x - refers to ease of movement of an ion across the membrane (inverse of resistance)
what is current (Ix)
(amps) across the membrane, carried by ion “x” as the ions flow through an ion channel
what is the length constant (lamda)
the distance needed before the Vm decays to 1/e or (37%) of its peak value and is a way to characterize how far passive current flow spreads before it leaks across membrane
what happens with a large length constant
the further the initial change in membrane potential will spread down membrane before the voltage decays back to resting potential
what is the time constant (tau)
time it takes membrane to reach 63% of final voltage - the longer the time constant the longer the potential will take to return to resting
what is reversal potential
membrane potential where there is no net current and the current reverses direction with an increase in voltage
what happens to a membrane that is a pure resistor
will not show any lag time in change of membrane potential in response to current pulse (Vm would mirror current pulse)
why does a membrane have resistance to ion flow
based on the opening/closing of ion channels
why does a membrane have capacitance
based on insulating properties (lipid bilayer - enables charged ions to accumulate along inner and outer surfaces)
what happens to a membrane that is a pure capacitor
slowly increases voltage deflection in response to a fixed current pulse - Vm would increase linearly as membrane charged in response to current pulse
what properties does a membrane actually have
mix of resistive and capacitive properties -> current pulse will induce voltage change that has properties of both
what does the Nernst equation find
predicts the equilibrium potential at which a given ion will be at equilibrium (can also predict which direction the ion will move)
what do you do with the result from the Nernst equation
use the calculated (theoretical) voltage and compare it to the actual/measured membrane potential/voltage
what does it mean if the calculated voltage is close to the equilibrium potential
there will be a lower driving force
Nernst example: If we calculate Na+ = +61mV with Nernst equation and the Vm inside cell = -60mV what can we conclude
Na will move into cell (down electro gradient) because actual is -60mV and it wants to be +61mV. There is also a large difference (61 - -60 = 121mV) so the driving force will be large
what is the Goldman-Hodgkin-Katz (GHK) equation used for
finds the membrane potential based on: 1. permeability of various ions and 2. concentration gradients of those ions across the membrane
why would small changes in [K+] out of the cell induce a greater change in equilibrium potential of K+ and Vm than change in [Na+] out
ECF [K+] is slow and and normally low concentration (about 4) - small changes are proportionally larger ** changes above 7mM are life threatening
what is the difference between membrane potential (Vm) and equilibrium potential (Ex)
Vm = real value that can be measured and Ex = theoretical value based on a single ion at equilibrium
