Membrane Potential Flashcards
membrane potential
- all living cells have this
- difference in charge inside and outside the cell membrane
electrical potential
when electrical charges of opposite sign are separated they have the potential to do work if allowed to come together
- measured in mV
inside cell
negative
outside cell
positive
resting membrane potential
-70 mV
resistance
- opposition to passage of current
- phospholipid bilayer had a high electrical _____
phospholipid bilayer
increase this = increase resistance = insulator
current
- movement of charges (ions)
- proportional to voltage
- inversely proportional to resistance
conduction
when ion channels are open, this is high and resistance is low; opposite of resistance
Ohm’s law
I=V/R
magnitude of RMP depends on:
- difference in ion concentration between ECF and ICF
- permeability of membrane to various ions
- -> reflects the number of open channels for the different ions
membrane only permeable to K+
- K+ flows out of because the concentration gradient
- K+ out creates imbalance
- negative charge inside attracts K+ back in
membrane only permeable to Na+
- Na+ flows in because of concentration gradient
- Na+ in creates electrical imbalance
- negative charge outside attracts Na+ back out
equilibrium potential (Eion)
voltage (at given concentration) at which there is no net movement of ion
- 2 forces are equal in magnitude but opposite in direction
Nernst equation
used to determine equilibrium potential for a single permeable ion
- electrical gradient concentration gradient
resting membrane potential #1
Na+ / K+ ATPase sets up concentration gradient
- Na+ / K+ ATPase pump is slightly electrogenic: about -4 mV
- –> pump moves net charge across membrane (+3/+2) and contributes directly to membrane potential
K+ “leak channels” open
- permeability increased
- K+ can easily cross membrane
Tiny number of Na+ channels open
- permeability decreased
- Not much Na+ can cross
some Cl- channels open
special cases
Vm
membrane potential is not equal to equilibrium potential so there is always a small but steady leak of Na+ into cell and K+ out of the cell
– Na+ / K+ ATPase balances leak
equilibrium
- no net flux of ion
- concentration may be different
- no energy required to maintain
steady-state
- no net flux of ion (pump balances leak)
- concentrations are different
- energy is required – not at Eion of either ion
If cell does not have Cl- pumps
No Cl- pumps to change concentration gradient
Goldman-Hodgkin-Katz (GHK) equation
expanded Nernst equation that takes into account the relative permeabilities of all ions
K+ typical permeability
1
Na+ typical permeability
0.04
Cl- typical permeability
0.45
current flow at RMP
- at resting Vm Na+ is moving in and K+ is moving out
- the influx of Na+ is roughly equal to the efflux of K+
- –> I k = I Na
- K+ is 100x more permeable than Na+
