Membrane Potential

Question 1

Electric vs Osmotic force strength

Answer 1

Electric force is way stronger, 10^18 times stronger. Allows for small number of ions to create large membrane potential difference

Question 2

Two forces acting on ions crossing membrane

Answer 2

Concentration gradient
Electrical gradient: based on relative charge of inside and outside
Together form electrochemical gradient
Electrical is stronger so few ions can balance large concentration gradient if electrical gradient is satisfied.

Question 3

Equilibrium Potential

Answer 3

The electrical potential difference over a membrane that must be met to have ions at equilibrium. Basically balances both electric and chemical gradients.
Given by Nernst Equation: (RT/zF)log(Co/Ci)=60/zlog(Co/Ci)

Question 4

Nernst Equation

Answer 4

(RT/zF)log(Co/Ci) = (60/z)log(Co/Ci)

z=charge of ion

Question 5

Equilibrium potential vs membrane potential

Answer 5

Cell has as many equilibrium potentials (E) as it has ions but only has one membrane potential (Vm)
Es are electromotive forces that together create a voltage difference (Vm) across a membrane when all ion species and pumps are considered.

Question 6

Equilibrium vs Steady State

Answer 6

Equilibrium is when ion reaches equilibrium potential, no net movement occurs.
Steady state is a pseudo-equilibrium reached in dynamic state where pumps and passive movements create a constant Vm despite ions not being at equilibrium.
Constant energy input is required

Question 7

Excess number of anions compared to total number of ions

Answer 7

For every 100,000 cations there are only 100,001 within the cell to produce the -50V resting Vm. This allows for charge balance in fluids as a whole but maintenance of a membrane potential due to strong electric forces.
Bulk solutions are always electrically neutral!

Question 8

Sodium/Potassium Pump

Answer 8

Primary active transport of 3 sodium ions out of cell and 2 potassium ions into cell. This is why cell is functionally impermeable to sodium.

Question 9

Relative permeability and membrane potential

Answer 9

Membrane potential depends on relative permeability where the number of one type of ion channel compared to others determines which E the Vm will be closest to.
This is not absolute permeability because it varies based on channel expression and between cell types.

Question 10

Short term determinant of membrane potential

Answer 10

Relative permeability. Channels can quickly and drastically change membrane potential through large changes in ion concentrations so the relative expression of different channels determines Vm.
Pumps are too slow to establish Vm on its own. It only maintains concentrations in long run.

Question 11

Driving Force on ion

Answer 11

V=IR=driving force. I=current carried by ion. R=membrane resistance to ion. Simply, the difference between Vm and E for a specific ion. Driving force is only 0 when Vm=E for that ion.

Question 12

Why are K changes more important than Na changes?

Answer 12

The membrane is relatively impermeable to Na so large changes do not have a big effect on Vm. High permeability to K means that changes in concentrations have higher effect on Vm