Electrical Properties of Cells: Membrane Potentials Flashcards
List the 3 ways electrical events in cells are measured
1) IC
2) EC
3) Patch clamping
Describe IC recordings
- 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
Describe EC recordings and give e.g.s
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
Describe patch clamping
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
Give 3 reasons why electrochem gradients are established
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
Describe the cell when mem imperm to ions
Inside:
Higher conc of KCl due to Na+/K+ pump bringing in 2K+
Outside:
Lower conc of KCl
Why is the sol inside cell electrically neutral despite having more particles?
– same no. of K+ and Cl- both sides
Describe the cell when mem perm to K+ ions
- 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
Why does mem pot dec as more K+ ions diffuse out?
- more Cl- ions stuck to inside of mem
Why is inside of mem -ve charged compared to outside?
-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
Compare sep of ions in sol compared to set at mem
- 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
What is voltage and what is it measure in?
measure of the electrical work done in separating opp charges across the membrane
Joules/coulomb (energy/charge)
Describe wha types of work are done
- Work done at sep charges at mem (voltage)
- osmotic/conc work done by the concentration gradient - pump does work and creates gradient
Where does the pump get energy from and what does it use energy for?
hydrolysis of ATP and uses this to create concentration
gradients
What is the gradient?
- [C]out / [C]in (for +ve ions) [C] = conc of ion
- For –ve ions: [C]in/[C]out
What is the equ pot and how does it come about?
- 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
If the concentration gradient is known, how can equilibrium potential be determined
from the Nernst equation
What is the Nernst equation and how does it differ at diff temps?
E = 58 (mV) x log [C] out / [C] in
E = equ pot
58mV if working at 25 degrees
61mV for body temp (37 degrees)
What is the conc of Na+ inside and outside cell and why?
- 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
What is the conc of K+ inside and outside cell and why?
Inside: high (140mM)
Outside: low (4mM)
Gradient trying to push K+ out but most doesn’t go through and remains inside cell
What is resting mem pot and what is it determined by?
typically around -70 mV
movement of Na+ and K+
What is the equ pot for an ion?
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
What happens when inside cell v. +ve/-ve?
- 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
What are the values for the equ pot of K+ and Na+ for phys conc?
EK = -90 mV ENa = +50 mV
What do the EK and ENa values tell you?
- Cell needs to be at -90 mV to stop K+ leaving and
+50 to stop Na+ entering
Why is there no net movement of ions at EK and ENa?
as work done by conc gradient balanced by electrical gradient in opp direction as mem sep charges
Why is Vm much closer to EK than ENa?
- membrane about 50x more permeable to K+ than Na+
- many more open K+ channels at rest than Na+ channels
At constant Vm, why is net flow of ions zero?
passive (as movement of ion through open ion channel is passive movement) leak of K+ out is matched by leak of Na+ in
How does EK dominate Vm?
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
What is driving force on an ion?
Vm - Eeq (equ pot of that ion)
What is driving force for K+ and what does it mean?
-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
What is driving force for Na+ and what does it mean?
- 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+
Even though diff values and unequal forces for Na+ and K+, why is amount of Na+ in and K+ out same?
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
Why is RMP constant?
- 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
What is permeability?
- ease with which u can move ions across mem determined by no. of open ion channels
– more open ion channels – more perm the mem
What is conductance?
amount of charge that gets across mem
What is conductance dependent on and why?
- 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
