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I've got kidney disease > Lecture 6: Membrane potentials > Flashcards

Lecture 6: Membrane potentials Flashcards

1
Q

How can we calculate the equilibrium potential of a cell?

A

add up all the membrane potentials

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2
Q

At equilibrium, the chemical potential equals the ________ potential but they are operating in __________ directions

A

electrical

opposite

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3
Q

Given that at equilibrium, the chemical potential equals the electrical potential but they are operating in opposite directions, what does this mean for the electrochemical gradient?

A

it means that the electrochemical gradient is zero

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4
Q

If the chemical potential = the electrical potential, the cell is in __________ and the driving force is ______

A

equilibrium

0

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5
Q

Why are we interested in electrical potential?

A

because we can measure it, predict it from our concentration gradients and get a feel for which way the ions want to move

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6
Q

What is the equation for the difference in electrical potential between one side and another?

A

Δφ = (RT/zF)Ln([C1]/[C2])

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7
Q

What does Δφ tell us?

A

If there is a distribution of ions across the membrane, it tells us what the electrical driving force would be in order for equilibrium to occur

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8
Q

If there is an ion gradient across the membrane and the membrane is permeable to that ion, either what two things could happen?

A
  • the membrane potential will change to the equilibrium potential for that ion if that ion, and that ion alone is responsible for membrane potential
    OR
  • if the potential is held at the equilibrium potential for that ion, there will be no net movement of the ion across the membrane
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9
Q

What does the Nernst equation calculate?

A

the diffusion potential for an ion

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10
Q

What is the diffusion potential?

A

the is the membrane potential that occurs when the electrical and chemical gradients are equal and opposite (and diffusion can occur freely)

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11
Q

If the cell is only permeable to one ion, what will the membrane potential be equal to?

A

the equilibrium potential

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12
Q

Cells are 10,000 times more permeable to _______ than they are to _______

A

K+ than they are to Na+

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13
Q

Which ion drives membrane potential?

A

K+

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14
Q

What determines the membrane potential?

A

all the different ions equilibrium potentials

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15
Q

What do you expect the resting membrane potential for a normal cell?

A

-60mV to -70mV

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16
Q

What is the Nernst equation?

What do each of the components stand for?

A

Ex = (RT/zF)Ln([Xo]/[Xi])

Ex = equilibrium potential for ion x
R = gas constant
T = temperature (K)
z = valency
F = Faraday's constant
[X]o = concentration of ion outside
[X]i = concentration of ion inside
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17
Q

How do cations and anions differ in the way you write the Nernst equation?

A

for cations, the outside-of-the-cell concentration goes on the top ie.
Ex = (RT/zF)Ln([Xo]/[Xi])
but for anions, the outside-of-the-cell concentration goes on the bottom ie.
Ex = (RT/zF)Ln([Xi]/[Xo])

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18
Q

What does the equilibrium potential mean?

A

the point at which Cl- enter the cell and the concentration gradient is match by Cl- leaving the cell as determined by the electrical gradient

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19
Q

Even if the membrane potential equals the equilibrium potential for Cl-, the resting potential for the cell is not determined by Cl-. True or false?

A

True

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20
Q

The membrane potential is dominated by which ions?

A

K+ and Na+

21
Q

If K+ and Na+ determine the resting membrane potential, what does this mean for Cl-?

A

the equilibrium potentials for K+ and Na+ determine the membrane potential for the cell and this is close to the equilibrium potential for Cl- so that there is no net movement of Cl-

22
Q

How can we calculate driving force?

A

membrane potential - equilibrium potential

23
Q

Given that Na+ is positively charged and the driving force for Na+ is -154mV, where is Na+ going to go?

A

there is going to be a huge influx of Na+ into the cell

24
Q

Given that K+ is positively charged and the driving force for K+ is +8mV, where is K+ going to go?

A

there is going to be a small efflux of K+ out of the cell

25
Q

Given that Cl- is negatively charged and the driving force is 0, where is Cl- going to go?

A

there is going to be no net movement of Cl-

26
Q

Cl- is at equilibrium so there is no _______ movement so Cl- remains ________. Na+ diffuses ______ the cell and K+ diffuses ______ the cell

A

net
constant
into
out of

27
Q

Why can K+ and Na+ not reach equilibrium if they are diffusing out of and into the cell respectively?

A

because Na+/K+ ATPase takes 3Na+ out and puts 2K+ into the cell

28
Q

What is the difference between an equilibrium and a steady state? Give an example

A

The cell is in a steady state but is not in equilibrium because the cell membrane potential can change at any time. If they were in equilibrium, there would be no driving force for the ions to move

29
Q

What is the membrane potential?

A

The combination of diffusion potentials for all the different ions

30
Q

K+ drive the membrane potential towards its equilibrium potential of what?

A

-90mV

31
Q

Na+ drives the membrane potential towards its equilibrium potential of what?

A

+72mV

32
Q

The contribution of the membrane potentials for both Na+ and K+ depend on the what?

A

permeability of the membrane to K+ and Na+

33
Q

Why don’t we want the membrane potential to be at the equilibrium potential for either K+ and Na+?

A

because this would stop the movement of these ions across the membrane

34
Q

What does the Goldman equation tell us?

A

what the membrane potential is

35
Q

What do the components of the Goldman equation mean?

A

Px = permeability of ion x
[X] = concentration of ion X
i and o = inside and outside of the cell
R,T and F have their usual meanings

36
Q

How do we account for a negative valence in the Goldman equation?

A

by inverting the Cl- ratio (ie. [Cl(i)]/[Cl(o)]

37
Q

Why does the membrane potential approach the equilibrium potential for K+ (towards -90mV)?

A

because the membrane is more permeable to K+

38
Q

The gradients for Na+ and K+ determine the membrane potential if what?

A

if Cl- is at equilibrium

39
Q

Is membrane potential an equilibrium potential?

A

no, it is a steady state potential

40
Q

It is difficult to measure _________ permeability but it is easy to measure _______ permeability

A

absolute

relative

41
Q

How is relative permeability expressed?

A

as a ratio of how permeable an ion is compared to the permeability of K+

42
Q

In most cells, Cl- is held at equilibrium but it is not in which type of cells?

A

secretory epithelia

43
Q

Secretory epithelia want Cl- to be above/below its equilbrium potential

A

above

44
Q

In epithelial cells, only things that absorb and excrete are expressed on the __________ membrane and there is housekeeping stuff kept on the ________ membrane

A

apical

basolateral

45
Q

Describe how the secretion of Cl- is maintained

A

There is a NKCC on the basolateral membrane which uses the Na+ gradient bringing it into the cell to also bring in K+ and 2Cl- ions into the cell too. Na+ gets pumped out the basolateral membrane via the ATPase. K+ comes in and is removed from the basolateral membrane via leak channel to keep the concentration of K+ balanced. Cl- is high in the cell and there is an electrical gradient for Cl- to leave. The CFTR on the apical membrane allows Cl- to leave. Na+ follows paracellularly to maintain charge neutrality and then H20 follows Na+

46
Q

The effect of Cl- depend on the size of what?

A

the permeability of Cl- relative to the permeability of K+ and Na+

47
Q

What does activating CFTR do to the apical membrane permeability of Cl-?

A

it increases it

48
Q

What are the units for membrane potential?

A

mV (so multiply the final answer by 1000)

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