Talbot - Membrane Biophysics and Membrane Potential

Question 1

what are the typical levels of K+, Na+, Cl- and A- intracellularly

Answer 1

K+ =120mM, Na+ = 15mM, Cl- = 20mM and A- =&raquo_space;0mM

Question 2

what are the typical ion levels outside the cell

Answer 2

Na+ (140mM), Ca2+ (1.2mM), Cl- (116mM)

Question 3

what area of a cell is negative and what area is positive

Answer 3

slight negative inside cell and positive outside

Question 4

what is considered the ground substance or 0mV when measuring a membrane potential

Answer 4

the ECF - compare voltage to ICF

Question 5

what is a typical resting potential for a cell

Answer 5

-65mV to -75mV

Question 6

what 2 things is the actual voltage influenced by

Answer 6

ionic concentration gradients and membrane permeability to those ions

Question 7

what 2 things influence the driving force for passive transport

Answer 7

concentration gradient and membrane potential (electrical gradient) across membrane

Question 8

when does a cell have net flux

Answer 8

when a solute is at equilibrium (net flux = 0)

Question 9

what state are ions in when the net flux = 0

Answer 9

electrochemical equilibrium (there can still be a concentration gradient)

Question 10

when does voltage develop across a membrane

Answer 10

when there is unequal flow (current) of charge/ions across membrane - typically from selectivity for specific solute

Question 11

which directions are Na+ and K+ always moving

Answer 11

Na+ = into cell and K+ = out of cell

Question 12

what direction is Cl- moving: into or out of cell

Answer 12

direction depends on cell type and intracellular concentrations of Cl-

Question 13

what is the TEPD: transepithelial potential difference

Answer 13

apical and basolateral maintain their own Vm (membrane potential) -> TEPD = Vb- Va

Question 14

what is resistance (R)

Answer 14

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

Question 15

what is capacitance (Cm)

Answer 15

stores charge separated across membrane; influences rate of voltage charge

Question 16

what is conductance (Gx)

Answer 16

conductance of an ion x - refers to ease of movement of an ion across the membrane (inverse of resistance)

Question 17

what is current (Ix)

Answer 17

(amps) across the membrane, carried by ion “x” as the ions flow through an ion channel

Question 18

what is the length constant (lamda)

Answer 18

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

Question 19

what happens with a large length constant

Answer 19

the further the initial change in membrane potential will spread down membrane before the voltage decays back to resting potential

Question 20

what is the time constant (tau)

Answer 20

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

Question 21

what is reversal potential

Answer 21

membrane potential where there is no net current and the current reverses direction with an increase in voltage

Question 22

what happens to a membrane that is a pure resistor

Answer 22

will not show any lag time in change of membrane potential in response to current pulse (Vm would mirror current pulse)

Question 23

why does a membrane have resistance to ion flow

Answer 23

based on the opening/closing of ion channels

Question 24

why does a membrane have capacitance

Answer 24

based on insulating properties (lipid bilayer - enables charged ions to accumulate along inner and outer surfaces)

Question 25

what happens to a membrane that is a pure capacitor

Answer 25

slowly increases voltage deflection in response to a fixed current pulse - Vm would increase linearly as membrane charged in response to current pulse

Question 26

what properties does a membrane actually have

Answer 26

mix of resistive and capacitive properties -> current pulse will induce voltage change that has properties of both

Question 27

what does the Nernst equation find

Answer 27

predicts the equilibrium potential at which a given ion will be at equilibrium (can also predict which direction the ion will move)

Question 28

what do you do with the result from the Nernst equation

Answer 28

use the calculated (theoretical) voltage and compare it to the actual/measured membrane potential/voltage

Question 29

what does it mean if the calculated voltage is close to the equilibrium potential

Answer 29

there will be a lower driving force

Question 30

Nernst example: If we calculate Na+ = +61mV with Nernst equation and the Vm inside cell = -60mV what can we conclude

Answer 30

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

Question 31

what is the Goldman-Hodgkin-Katz (GHK) equation used for

Answer 31

finds the membrane potential based on: 1. permeability of various ions and 2. concentration gradients of those ions across the membrane

Question 32

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

Answer 32

ECF [K+] is slow and and normally low concentration (about 4) - small changes are proportionally larger ** changes above 7mM are life threatening

Question 33

what is the difference between membrane potential (Vm) and equilibrium potential (Ex)

Answer 33

Vm = real value that can be measured and Ex = theoretical value based on a single ion at equilibrium