Electrical Properties of Cells: Membrane Potentials

Question 1

List the 3 ways electrical events in cells are measured

Answer 1

1) IC
2) EC
3) Patch clamping

Question 2

Describe IC recordings

Answer 2

• electrode inside cell
• Record with Voltmeter with ref to outside
  measure peak to peak displacement of AP
• Inject fluorescent dyes into cells – see morphology as well as allow it to run into other connected cells – see behaviour of network of cells

Question 3

Describe EC recordings and give e.g.s

Answer 3

electrode outside cell
recordings smaller even though waveforms of act pot same as IC
Smaller pot – due to outside pot, not one occurring inside mem
e.g. electrocardiogram, electroencephalogram, Electromyography

Question 4

Describe patch clamping

Answer 4

electrode sealed to cell surface
Mem fuses with glass – all current goes through mem and doesn’t escape through sides of electrode
Investigate opening and closing of ion channels in mem
Pulses in recording as Na+ go through

Question 5

Give 3 reasons why electrochem gradients are established

Answer 5

1) The Na+/K+ pump (ATP-ase) with sub ATP – releases energy - moves ions against their concentration gradients
2) There is restricted ion movement through channels
3) The membrane stores ionic charges on its inner and outer surfaces – it is a capacitor – where charges immobilised

Question 6

Describe the cell when mem imperm to ions

Answer 6

Inside:
Higher conc of KCl due to Na+/K+ pump bringing in 2K+
Outside:
Lower conc of KCl

Question 7

Why is the sol inside cell electrically neutral despite having more particles?

Answer 7

– same no. of K+ and Cl- both sides

Question 8

Describe the cell when mem perm to K+ ions

Answer 8

• Passive diffusion of K+ down conc gradient out of cell
• K+ +ve charged, drags –ve charged Cl- which is trying to attract K+ back towards it but Cl- can’t get out
• mem is capacitor so both ions stick to it
• Must exist in ion pairs – can’t just have 1 ion shoot from 1 side to other or get net +ve charge outside

Question 9

Why does mem pot dec as more K+ ions diffuse out?

Answer 9

• more Cl- ions stuck to inside of mem

Question 10

Why is inside of mem -ve charged compared to outside?

Answer 10

-effect of conc gradient driving K+ through open ion channels and electrostatic attraction between K+ and Cl- on mem - shell of charge on mem with K+ stick to outside and Cl- stuck to inside

Question 11

Compare sep of ions in sol compared to set at mem

Answer 11

• Across mem – much bigger av distance than in sol

– ions want to be attracted to each but mem pulling them apart as well as pot energy from pump stored in gradient

Question 12

What is voltage and what is it measure in?

Answer 12

measure of the electrical work done in separating opp charges across the membrane
Joules/coulomb (energy/charge)

Question 13

Describe wha types of work are done

Answer 13

• Work done at sep charges at mem (voltage)

- osmotic/conc work done by the concentration gradient - pump does work and creates gradient

Question 14

Where does the pump get energy from and what does it use energy for?

Answer 14

hydrolysis of ATP and uses this to create concentration

gradients

Question 15

What is the gradient?

Answer 15

• [C]out / [C]in (for +ve ions) [C] = conc of ion

- For –ve ions: [C]in/[C]out

Question 16

What is the equ pot and how does it come about?

Answer 16

• point when force of conc gradient pushing K+ out of cell matched by electrical force pulling K+ back into cell (Cl- trying to pull K+ back into mem and K+ trying to attract Cl- back towards them)
• Ions trying to attract each other but at same time, conc gradient trying to push more K+ out and as it coats more outside in more +ve charge, that’s going to bring more –ve charge on inside of mem and opp charges immobilised at mem and work against conc gradient
• At this point there’s no net movement of K+ and electrical force exactly balances osmotic force

Question 17

If the concentration gradient is known, how can equilibrium potential be determined

Answer 17

from the Nernst equation

Question 18

What is the Nernst equation and how does it differ at diff temps?

Answer 18

E = 58 (mV) x log [C] out / [C] in
E = equ pot
58mV if working at 25 degrees
61mV for body temp (37 degrees)

Question 19

What is the conc of Na+ inside and outside cell and why?

Answer 19

• Inside: low (10mM)
• Outside: high (140mM)
• Gradient trying get Na+ in and gets some Na+ in through open ion channels but most doesn’t go in and stays outside cell

Question 20

What is the conc of K+ inside and outside cell and why?

Answer 20

Inside: high (140mM)
Outside: low (4mM)
Gradient trying to push K+ out but most doesn’t go through and remains inside cell

Question 21

What is resting mem pot and what is it determined by?

Answer 21

typically around -70 mV

movement of Na+ and K+

Question 22

What is the equ pot for an ion?

Answer 22

membrane voltage that a cell needs to be at to prevent movement of that ion down its concentration gradient – tells u amount of work u have to do at mem to oppose conc gradient in trying to push ion in/out

Question 23

What happens when inside cell v. +ve/-ve?

Answer 23

• If inside is very negative it will stop K+ from leaving
  – equ pot for K+ is -ve value
• If inside very positive it will stop Na+ from entering
  – equ pot for Na+ is +ve value

Question 24

What are the values for the equ pot of K+ and Na+ for phys conc?

Answer 24

EK  = -90 mV
ENa = +50 mV

Question 25

What do the EK and ENa values tell you?

Answer 25

• Cell needs to be at -90 mV to stop K+ leaving and

+50 to stop Na+ entering

Question 26

Why is there no net movement of ions at EK and ENa?

Answer 26

as work done by conc gradient balanced by electrical gradient in opp direction as mem sep charges

Question 27

Why is Vm much closer to EK than ENa?

Answer 27

• membrane about 50x more permeable to K+ than Na+

- many more open K+ channels at rest than Na+ channels

Question 28

At constant Vm, why is net flow of ions zero?

Answer 28

passive (as movement of ion through open ion channel is passive movement) leak of K+ out is matched by leak of Na+ in

Question 29

How does EK dominate Vm?

Answer 29

If a cell becomes permeable to an ion then that ion will move down its electrochemical gradient and will drive Vm towards the equilibrium potential for that ion
e.g. if got KCl in cell and open up all K+ channels in mem – drive cell to EK
and other way around – if shut all K+ channels and K+ can’t move – won’t achieve EK so easily

Question 30

What is driving force on an ion?

Answer 30

Vm - Eeq (equ pot of that ion)

Question 31

What is driving force for K+ and what does it mean?

Answer 31

-70mV – (-90 mV) = +20 mV
+20 mV Forcing K+ OUT
- K+ leaking out at rest – not just going down conc gradient but being pushed out bc some Na+ entering (some open Na+ channels at rest) – making inside bit more +ve and repelling so pushing out K+
- electrochem (conc + elec gradient) both acting together to push K+ out of cell at rest

Question 32

What is driving force for Na+ and what does it mean?

Answer 32

• 70mV – (+50mV) = -120 mV
• -120 mV forcing Na+ IN
• Electrochem gradient forcing Na+ into axon at rest
• If cell -70 on inside and high Na+ on outside - -ve charge in inside highly attracted to +ve Na+

Question 33

Even though diff values and unequal forces for Na+ and K+, why is amount of Na+ in and K+ out same?

Answer 33

bc mem more perm to K+ so less force is needed to get same amount of K+ across mem than Na+ - still shift same amount of ions

Question 34

Why is RMP constant?

Answer 34

• Leak of K+ out matched by leak of Na+ into cell – everytime some +ve charge leaves, some +ve enters so resting mem pot constant and doesn’t change – no net movement of ions

Question 35

What is permeability?

Answer 35

• ease with which u can move ions across mem determined by no. of open ion channels
  – more open ion channels – more perm the mem

Question 36

What is conductance?

Answer 36

amount of charge that gets across mem

Question 37

What is conductance dependent on and why?

Answer 37

• conc gradient and no.s of open ion channels (perm)
• Close some channels making mem less perm to K+, means cond will fall – less charge getting across mem
• by leaving perm same (still same no. of channels open) but if u ½ conc gradient, then u ½ driving force pushing K+ Across mem