origin of membrane potentials Flashcards
what is membrane potential?
- it’s the separation of opposite charges across the membrane. (refers to the difference in charge between the thin layers of ECF and ICF located next to the outside & inside of membrane respectively)
- plasma membranes of all cells are polarised electrically
is the membrane charged?
NO - it’s not the membrane itself that is charged
when is resting membrane potential in:
a) non-excitable cells?
b) excitable cells?
a) constant
b) resting membrane potential only when they’re at rest
where does sodium and chloride ions have the highest concentration?
more Na+ and Cl- outside compared to inside
where do potassium ions have the highest concentration?
greater K+ ions inside compared to outside
how many more times permeable is K+ compared to Na+?
K+ ions are 100 times more permeable than Na+
what direction do K+ ions travel?
K+ move outwards (think opposite of SOPI as SOPI is active transport)
what direction do Na+ ions travel?
Na+ move inwards (think opposite of SOPI as SOPI is active transport)
what is plasma membrane impermeable to?
the large negatively charged (anionic) intracellular proteins (A-)
describe the movement of K+ and it’s effect on Em?
- more K+ inside over outside means potassium ions move from inside → outside meaning positively charged outside and negatively charged inside = membrane potential difference
- however know some K+ ions are attracted back into cell due to negative charge inside the cell
- now we have 2 forces acting across lipid bilayer
what is potassium equilibrium potential?
movement (of 2 forces acting against each other) will occur until both forces are at balance
the point of balance = potassium equilibrium potential
value = -90 mV
what are the 2 opposing forces acting on K+?
- The concentration gradient (tending to move K+ out of the cell)
- The electrical gradient (tending to move K+ into the cell)
what does the value of potassium equilibrium membrane potential mean?
-90 mV
= Important: the sign (+ or -) is the polarity of the excess charge on the inside of the membrane
- so at equilibrium the excess charge on inside of membrane is K+
what is the nernst equation?
the equation that can be used to calculate the equilibrium potential for any given ion (e.g. -90 mV for K+ and +60 mV for Na+)
E(ion) = RT/zF ln [ion] outside/[ion] inside
it’s is the (gas constant x temp divided by valency x faradays constant) x log of (outside ion concentration divided by inside ion concentration)
simplified to 61 log10 x (outside ion concentration divided by inside ion concentration)
what is the movement of Na + effect on membrane potential?
- more outside than inside so some Na+ move inside cell making inside more positive and make outside more negative = concentration gradient
- again means some want to move back out = electrical gradient
what is the sodium equilibrium potential?
+ 60 mV when balanced
- means polarity of excess charge on inside membrane is +ve
what is equilibrium membrane potential of typical nerve cell at rest and why?
- -70mV (so - tells us that inside of cell more negative than outside of cell)
- this is because membrane more permeable to K+ than Na+ so more K+ going out than Na+ going in which makes overall -ve inside
why is K+ equilibrium potential is closer to resting membrane potential?
simply due to plasma membrane being more permeable to K+ over Na+
what is goldman-hodgkin-katz equation?
equation used to calculate membrane potential
= 61 log 10 x (relative permeability of K+ x conc of K+ outside cell) + (relative permeability of Na+ x conc of Na+ outside cell) divided by the same but with concentration of inside
what is value of Pk+ and PNa+ in GHK equation?
Pk =1
PNa = 0.01
(as PK is 100x PNa)
what is the effect of Na/K pump?
- The Na+-K+ pump helps maintain the K+ and Na+ gradients across the membrane.
- Since its stoichiometry is 3Na+ exported for every 2K+ entering, the pump will generate a hyperpolarizing current (SOPI)
- The amount that this current adds to Em is variable as some cells have more Na/K pumps than others
what is effect on membrane potential if we make membrane more permeable to a particular ion?
membrane potential will move toward that ions equilibrium potential → essentially what happens in excitable cell
what results in fluctuations of membrane potential
- Specialised use of Em in nerve and muscle cells:
- Rapidly and transiently alter their membrane permeabilities in response to appropriate stimulation
