Resting potential

Question 1

What is diffusion useful for

Answer 1

useful for transport over short distances

Question 2

Describe diffusion

Answer 2

Movement of solute down a concentration gradient across a selectively permeable membrane until dynamic equilibrium is reached.
It is spontaneous, no energy input needed

Question 3

Describe dynamic equilibrium

Answer 3

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.

Question 4

Why do cells need an electric potential

Answer 4

Cell need an electrical potential to transmit information reliably and quickly over large distances.

Question 5

What is meant by flux

Answer 5

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.

Question 6

Describe some of the properties of ions

Answer 6

Charged molecules

Opposite charges attract

Like charges repel

Question 7

Describe voltage/potential difference

Answer 7

Unit: Volts

Generated by ions to produce a charge gradient (i.e. like a chemical battery)

Question 8

Describe current

Answer 8

Unit: Amps

Movement of ions due to a potential difference

Question 9

Describe resistance

Answer 9

Unit: Ohms

Barrier that prevents the movement of ions

Question 10

What is the equation that relates voltage, current and resistance

Answer 10

R= V x I

Question 11

Do all cells have a resting potential

Answer 11

Almost all cells have some degree of resting potential- but they will vary in size.

Question 12

When measuring resting membrane potential, where is the reference electrode placed.

Answer 12

To measure membrane potential a reference electrode is placed outside the cell. This is the zero volt level.

Question 13

Where is the other electrode placed

Answer 13

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

Question 14

Hoe do membranes separate charge

Answer 14

1. Membrane is selectively permeable. 2. Concentration of at least one permeant ion is different on the two sides of the membrane.

Question 15

Explain how the membrane is a barrier to ion movements

Answer 15

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.

Question 16

Explain how the membrane has selective permeability to different ions

Answer 16

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

Question 17

What does the selectivity of the ion channel depend on

Answer 17

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.

Question 18

What is meant by the fact that the ion channels are gated

Answer 18

A specific stimulus triggers them to switch between a closed and open state by a change in their conformation.

Question 19

Describe the two different types of ion channel

Answer 19

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).

Question 20

Describe what happens to the membrane potential when there are no channels in the membrane

Answer 20

No channels in the membrane
So… no diffusion across the membrane despite concentration gradients
No separation of charge
Membrane potential = 0 mV

Question 21

Describe what happens to the membrane potential when the membrane is permeable to K+ but not Na+

Answer 21

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

Question 22

What is meant by electrochemical equilibrium

Answer 22

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.

Question 23

Describe what happens in terms of the membrane potential when the membrane is permeable to Na+ but not K+

Answer 23

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+

Question 24

What is the difference between case 2 and case 3

Answer 24

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

Question 25

What is meant by equilibrium potential

Answer 25

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

Question 26

List the variables in the Nernst equation

Answer 26

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

Question 27

What is the Nernst equation

Answer 27

E= RT/zF x ln X2/X1

Simplified: E= -61/Z x logX2/X1.

Question 28

What does the Nernst equation calculate

Answer 28

Relates the size of the equilibrium potential of an ion to the size of it’s concentration gradient.

Question 29

Real membrane potentials (Em) do not rest at EK (–90 mV) or ENa (+72 mV)
Typical Em is -70 mV
Why?

Answer 29

Membranes have mixed K+ and Na+ permeability (but at rest K+&raquo_space; Na+) hence why it lies closer to the Ek.

Question 30

Describe the GHK voltage equation

Answer 30

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

Question 31

What is the GHK voltage equation

Answer 31

E= -61log x Pk[K]In/Pk[K]out + PNa[Na]in/PNa[Na]out + PCl[Cl]out/PCl[Cl]in