Resting potential Flashcards
What is diffusion useful for
useful for transport over short distances
Describe diffusion
Movement of solute down a concentration gradient across a selectively permeable membrane until dynamic equilibrium is reached.
It is spontaneous, no energy input needed
Describe dynamic equilibrium
In dynamic equilibrium there is no net flux of substances. There is a uniform concentration of solutes. However the solutes will still move in Brownian motion.
Why do cells need an electric potential
Cell need an electrical potential to transmit information reliably and quickly over large distances.
What is meant by flux
The number of molecules that cross a unit area per unit of time (number of particles). ie molecules.m−2.s−1
It is essentially the rate of transfer of molecules.
Describe some of the properties of ions
Charged molecules
Opposite charges attract
Like charges repel
Describe voltage/potential difference
Unit: Volts
Generated by ions to produce a charge gradient (i.e. like a chemical battery)
Describe current
Unit: Amps
Movement of ions due to a potential difference
Describe resistance
Unit: Ohms
Barrier that prevents the movement of ions
What is the equation that relates voltage, current and resistance
R= V x I
Do all cells have a resting potential
Almost all cells have some degree of resting potential- but they will vary in size.
When measuring resting membrane potential, where is the reference electrode placed.
To measure membrane potential a reference electrode is placed outside the cell. This is the zero volt level.
Where is the other electrode placed
Another electrode is placed inside the cell (axon). It measures a voltage difference that is negative compared with the outside (i.e. reference).
All cells have a membrane potential
Hoe do membranes separate charge
- Membrane is selectively permeable. 2. Concentration of at least one permeant ion is different on the two sides of the membrane.
Explain how the membrane is a barrier to ion movements
Lipid (hydrophobic) cell membrane is a barrier to ion movement and separates ionic environments.
Ions are hydrophilic and so cannot freely diffuse through the plasma membrane.
Explain how the membrane has selective permeability to different ions
But membrane can selectively allow ions to cross the barrier by changing its permeability.
Permeable pores in the membrane (ion channels) open and close depending on trans-membrane voltage, presence of activating ligands or mechanical forces.
Ion channels can be selective for different types of ion (K+, Na+, Cl-, Ca2+) and movement across the membrane will occur when the concentration of the permeant species is different on one side of the membrane
What does the selectivity of the ion channel depend on
Ion selectivity depends on the diameter and shape of the ion channel and on the distribution of the charged amino acids that line it. Each ion in aqueous solution is surrounded by a small shell of water molecules, most of which have to be shed for the ions to pass through single file, through the selectivity filter in the narrowest part of the channel. An ion channel is narrow enough in places to force ions into contact with the wall of the channel, so that only those ions of the appropriate size and charge are able to pass.
What is meant by the fact that the ion channels are gated
A specific stimulus triggers them to switch between a closed and open state by a change in their conformation.
Describe the two different types of ion channel
o Voltage-Dependant = open by a change in membrane potential. o Voltage-Independent = open all the time (there are the channels responsible for producing the resting potential of a cell).
Describe what happens to the membrane potential when there are no channels in the membrane
No channels in the membrane
So… no diffusion across the membrane despite concentration gradients
No separation of charge
Membrane potential = 0 mV
Describe what happens to the membrane potential when the membrane is permeable to K+ but not Na+
K+ crosses the membrane and the direction of flux is dictated by its concentration gradient
+ve charge accumulates in compartment 1 preventing further influx of K+
This is the state of electrochemical equilibrium: electrical forces balance diffusion forces
A stable trans-membrane potential is achieved
The fluxes are equal in the two compartments (so no net diffusion) however the K+ concentration in C1 is still less than in C2. As C1 gains positive charge, C2 loses positive charge
What is meant by electrochemical equilibrium
Electrochemical equilibrium is achieved when electrical force prevents further diffusion across the membrane
Prevents net diffusion of the ion, but the ions will still be able to move back and forth.
Describe what happens in terms of the membrane potential when the membrane is permeable to Na+ but not K+
Na+ crosses the membrane and the direction of flux is driven by its concentration gradient
Charge separation occurs
Compartment 2 gains +ve charge
Compartment 1 becomes more –ve
Enough +ve charge accumulates in compartment 2 to prevent further net movement of Na+
What is the difference between case 2 and case 3
In both cases a membrane potential exists, but its sign is opposite
The difference in sign arises because of the selectivity of the membrane
Case 2 permeable to K+
Case 3 permeable to Na+
In both cases the electrochemical equilibrium has been reached at which
the concentration gradient exactly balances the electrical gradient
What is meant by equilibrium potential
Equilibrium potential
The potential at which electrochemical equilibrium has been reached. It is the potential that prevents diffusion of the ion down its concentration gradient
List the variables in the Nernst equation
R = gas constant T = Temp. Kelvin Z = charge on ion -1 for Cl-, +2 for Ca2+ F = Faraday’s number charge per mol of ion ln = log to base e X2 = conc mM inside cell X1 = conc mM outside cell
What is the Nernst equation
E= RT/zF x ln X2/X1
Simplified: E= -61/Z x logX2/X1.
What does the Nernst equation calculate
Relates the size of the equilibrium potential of an ion to the size of it’s concentration gradient.
Real membrane potentials (Em) do not rest at EK (–90 mV) or ENa (+72 mV)
Typical Em is -70 mV
Why?
Membranes have mixed K+ and Na+ permeability (but at rest K+»_space; Na+) hence why it lies closer to the Ek.
Describe the GHK voltage equation
K+, Na+ and Cl- concentrations all contribute to the real membrane potential
The size of each ion’s contribution is proportional to how permeable the membrane is to the ion.
The GHK equation describes the resting membrane potential (Em)
P is permeability or channel open probability
(0 = 100% closed, 1 = 100% open, 0.5 = open 50% of time)
Subscript indicates the ion
[K], [Na] and [Cl] represent concentration and the subscript indicates inside or outside the cell
What is the GHK voltage equation
E= -61log x Pk[K]In/Pk[K]out + PNa[Na]in/PNa[Na]out + PCl[Cl]out/PCl[Cl]in
