Resting potential

Hello!

Can someone please help me, I think I am struggling with my exam technique on this question. I know the knowledge but I would like someone to guide me where to pin point....


The question is, Explain how the resting potential is establishing in an axon.

Thanks! The idea of it being established is confusing me...

Established just means "how did it come about?". So let's say our resting membrane potential is -70mV, how does it end up like that in the axon?

There's not much 'technique' involved in this question, it's just asking you to describe and explain what happens: so write the question like you're explaining it to me! Want to give it a go? From start to finish?

Hello!

Can someone please help me, I think I am struggling with my exam technique on this question. I know the knowledge but I would like someone to guide me where to pin point....


The question is, Explain how the resting potential is establishing in an axon.

Thanks! The idea of it being established is confusing me...

Yeah that would be good!

I suppose that, the sodium ion channels close so no sodium ions can diffuse into the cell membrane. The voltage gated potassium ion channels open, potassium ions diffuse into the cell membrane down their respective concentration gradient and thus repolarising the cell membrane. This occurs until the membrane potential is reached (-70mV) then potassium ion channels close so no more potassium ions diffuse in. Throughout this, the sodium potassium pump is also continually pumping sodium ions out of cell membrane and potassium ions in to help restore membrane potential.

I wouldn't know whether to mention the refractory period or not though?

You'd have to write about sodium potassium ATPase pumps and how they use ATP to actively transport 3 Na+ ions out of the cell for every 2 K+ they pump in. - that answer'd get you 3 marks

How many marks is it out of btw?

I'm not so sure as for some reason it doesn't say but assuming it is 4/5. But surely you would write about potassium ions repolarising the cell? I did think the sodium potassium pump would need to be mentioned but I didn't think it played as big a part in establishing the resting potential, but more maintaining the resting potential.

This describes how a cell returns to the resting membrane potential after depolarisation (which you have gotten a little incorrect, I'll correct in the spoiler), but what the question wants to know is how the resting membrane potential came to be in the first place.


Pretend we've got an axon that has JUST come into existence, so there would be no mention of refractory periods, depolarisation or repolarisation. If a nerve never fires, it just chills out there at -70mV: how does it come to be at that value in the first place, pretending it's never ever fired before? Do you know how that comes about?

Establishing means to create or to start, so my understanding of it would be to talk about the ATPase pump

Aha oh dear, when I have attempted the question on paper I did however say move out of the cell, don't why I put moved in when writing that post lol. But thank you for the correction!

Right so the sodium potassium pump works by pumping out 3 Sodium ions for every 2 potassium ions in the cell and also, sodium and potassium ion channels do leak in/out small numbers of these ions anyway which overall helps to maintain the membrane potential at 70mV?
-Damn! This confuses me, in-out, out in.
I'm stumped now. I know less than what I think lol

Note that the question is asking about ESTABLISHING, and not re-establishing.

Just looked through my text book and can't find anything about the ATPase? There's ATP synthase, but that's unit 4 respiration....

In the textbook it's called sodium potassium pump (but my teacher calls it sodium potassium ATPase pumps, they're apparently the same thing)

Just looked through my text book and can't find anything about the ATPase? There's ATP synthase, but that's unit 4 respiration....

Note that the question is asking about ESTABLISHING, and not re-establishing.



Don't worry about it, super simple mistake to make.
That's exactly how the pump works, but along with the pump: http://www.youtube.com/watch?v=YP_P6bYvEjE

That video doesn't give a full picture, and there's a full explanation here with a super condensed, non-explanatory, summary here.

So I watched the video which was very helpful thanks!

So is it saying it is also to do with a charge imbalance inside of the cell? I am sure now I watched it I remember my teacher saying something about lots of Chloride ions are on the outside which causes some positive ions to be attracted and leave the cell...leaving the inside to be more negative so small amounts of sodium ions diffuse in as there positively charged helping to establish and maintain resting potential?


Maybe I am not right

I'll give you how I would summarise the process. Note that I didn't do A level biology, so I'm not sure what sort of depth that you're expected to know, so I apologise if any of this is superfluous.

So we've got a cell. It's full of sodium, potassium, chloride and protein. The proteins are negatively charged, and sodium and potassium are positively charged.

There's a large concentration of sodium OUTSIDE of the cell, and a small concentration INSIDE.
There's a large concentration of potassium INSIDE of the cell, and a small concentration outside.
This difference is due to the sodium potassium pump.

That means the concentration gradients are pushing potassium out, and pulling sodium in. However, the cell membrane is pretty impermeable to sodium ions (so it can't rush in by itself), but is very permeable to potassium channels, which are always open and facilitate passive transport.

Our potassium gets pushed out of the cell by the concentration gradient, and this outflow of positive charge causes the inside of the cell to become more negative - this is due to the proteins having a negative charge. This negative charge pulls our potassium BACK into cell down an electrical gradient. This process keeps happening, until an equilibrium is established - this is where the concentration gradient pushing potassium out of the cell is balanced out by the electrical gradient pulling it back in, this balance is what creates out resting membrane potential.


There's a fancy equation called the Nernst equation that we can use to calculate what the membrane potential would be if the cell was ONLY permeable to potassium, and I think we end up with a potential that's too negative. This is because in reality, the membrane is a little permeable to other ions like sodium and chloride, so they contribute to the balancing act, too - including through the sodium-potassium pump, which helps with keeping it steady.