Membrane Potential

Question 1

relatively which is stronger, the electric or osmotic force?

Answer 1

electric! (~10^18 more powerful)

Question 2

What are the 2 forces acting on an ion moving across a membrane?

Answer 2

1. concentration difference
2. electrical potential difference (membrane potential)

=>electrochemical gradient

Question 3

What produces a membrane potential?

Answer 3

an imbalance of cations and anions within the cell

Question 4

What is the typical charge of a bulk solution?

Answer 4

Bulk solutions can safely be considered neutral

Question 5

What is the Nernst equation (original and at body conditions)

Answer 5

BODY:
E(mV) = (60/z)log(Cout/Cin)

ORIGINAL:
E = (RT/zF)ln(Cout/Cin)

Question 6

What is equilibrium potential?

Answer 6

electrical potential difference across a membrane that must exist if the ion is to be at equilibrium at a given concentration

Question 7

What is the difference between equilibrium potential and recorded membrane potential?

Answer 7

Equilibrium potential is a theoretical value (electromotive force) for a single ion

Membrane potential is measure with a micro electrode and is the voltage difference measured between the tip of a micro-electrode place inside a cell and a micro-electrode outside the cell

Question 8

How many equilibrium potentials are there?

Answer 8

One for each ion species
# equilibrium potentials = # ion species

Question 9

What does it mean if membrane potential does not equal equilibrium potential for an ion?

Answer 9

one of two things:

1. the membrane is impermeable to that ion
2. the ion must have a pump

Question 10

What is the principle of electrical neutrality?

Answer 10

Bulk solutions inside and out must be electrically neutral
Total cations in cell = total anions incell
same for outside of cell

Question 11

How does can membrane potential exist without contradicting the principle of electrical neutrality?

Answer 11

The excess anions or cations in a cell is very small compared to the total number of ions:

a cell with a resting membrane potential of -80mV:
has 100,000 cations
100,001 anions

Question 12

What is the Donnan rule?

Answer 12

[K+out][Cl-out]=[K+in][Cl-in]

ions are distributed at equilibrium

Question 13

What does the general charge balance equation state?

Answer 13

[K]i + [Na]i = [Cl]i + n[A]i
n[A] concentration of proteins, SO4, and HPO4

And [K]o + [Na]o = [Cl]o

Question 14

What does the general osmotic balance equation look like?

Answer 14

[K]i + [Na]i + [Cl]i + [A]i = [K]o + [Cl]o + [Na]o

Question 15

How many ions, and in which direction, are moved with each cycle of the potassium/sodium pump?

Answer 15

3Na+ out
2K+ in

uses ATP

Question 16

Describe 1 cycle of the Na/K pump

Answer 16

A) both gates closed, 2 K+ ions inside
B) ATP binds -> inner gate opens-> affinity changes to Na+ -> K+ leaves and Na+ enters
C) ATP hydrolyzed -> Inner gate closes
D) Outer gate opens spontaneously -> affinity changes from Na+ to K+ -> Na+ leaves and K+ enters
D->A: pump loses phosphate group, outer gate closes => cycle is complete

Question 17

What does steady state mean?

Answer 17

ion concentrations are not changing over time, but unlike model cel, constan input of energy is needed (in the form of ATP) to drive the Na/K pump

v. equillibrium of ideal cell which required no energy

Question 18

What does relative permeability mean?

Answer 18

Different cells, depending on the concentration of K+ and Na+ channels have different relative permeability to those ions

Question 19

What is the primary short term determinant of membrane potential?

Answer 19

relative permeability

Question 20

A small change in the concentration of which ion in the extracellular fluid can have important clinical implications?

Answer 20

K+

Question 21

How does the Na+/K+ pump relate to membrane potential?

Answer 21

less impact on short term membrane potential
BUT
If non-functional:
- cell gradually depolarizes
- speed of depolarization depends on surface area (faster over larger surface area)

Question 22

What is the goldman equation?

Answer 22

Vm =

60log {Ko +Pr[Na]o…}/{Ki +Pr[Na]i…}

Question 23

What is the effect of a change in the ECF level of Na+ on the membrane potential of a cell?

Answer 23

Relatively little:

• membrane is relatively impermable to Na
• & Na is a major component of ECF- major reduction would be accompanied by large volume of fluid loss

Question 24

What is the effect of a change in the ECF level of K+ on the membrane potential of a cell?

Answer 24

HUGE

• reduces eflux of K+ from cell-> cell gradually depolarizes
• Ek moves closer to 0 (-60mV from -90mV)

WHEREVER EK GOES, VM FOLLOWS B/C MEMBRANE IS RELATIVELY MORE PERMEABLE TO K

Question 25

What is C BIG K used for and what do the letters stand for?

Answer 25

Treat hyperkalemia

C: Ca2+ (usually IV to treat cardia arrythmias_
B: Bicarb (to encourage K+ uptake)
I: Insulin (Energy -ATP- for Na+/K+ pump)
G: Glucose (Energy -ATP- for Na+/K+ pump)
K: Kayexalate (ion exchanger oral/enema removes K)

Question 26

What is a long term treatment for chronic hyperkalemia?

Answer 26

Dialysis