Equil. potential

[chiddler]

Answer: B.

Not understanding this very well. Doesn't K+ have a tendency to leave the cell?

thanks.

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[syoung]

Is this resting membrane potential?
If so, then maybe the reasoning is that although the tendency is there to leave the cell, the Na+/K+ ATP transporter is actively pumping them back into the cell?

 

[syoung]

And they want equilibrium, so maybe the question wants to see what it will look at equilibrium, where the potential is 0?

 

[chiddler]

boo

i googled it. turns out my understanding of the term is wrong:

"In a biological membrane, the reversal potential (also known as the Nernst potential [and equilibrium potential]) of an ion is the membrane potential at which there is no net (overall) flow of that particular ion from one side of the membrane to the other."

 

[syoung]

google ftw! i have so much to catch up next week. I hope my books come on time...

 

[SaintJude]

Yeah, regardless of the process, I think equilibrium means that rate of forward rx= rate of reverse rx. So the arrows should always be of "same length" by definition. So there are no net changes in the concentrations of reactant and product.

 

[pfaction]

Wait, I don't understand. Why isn't it A? K+ is large inside and low outside so letting it freely equilibriate is A.

 

[syoung]

We're looking at one species, so I believe that is at the beginning (that there is HIGH K+ inside and low outside), and we want equilibrium which is resting membrane potential. High K+ goes out and very very little comes in. However this is NOT equillibrium, as this scenario results in a potential across the membrane. So, the K+ must be actively transported in.

EDIT:
[st]Plus, at the end of the day, I think this is testing a basic gchem question: What is equilibrium? That is when the forward reaction and the reverse reaction are at the equal rates (not necessarily zero).
[/st]
THIS was stupid of me!

I think A would only be true if A- was going out just like K+, then the membrane potential would be at equilibrium

 

[chiddler]

Now that i've slept on it, I don't understand this anymore .

Equilibrium potential is the potential that would be achieved if the ion could freely diffuse. For K+, we know that it is negative which is why it tends to leave the cell. Therefore, to represent the equilibrium potential, it should tend to leave. The arrow should be bigger going out.

Syoung I think you have to differentiate between equilibrium and equilibrium potential.

 

[syoung]

Now that i've slept on it, I don't understand this anymore .

Equilibrium potential is the potential that would be achieved if the ion could freely diffuse. For K+, we know that it is negative which is why it tends to leave the cell. Therefore, to represent the equilibrium potential, it should tend to leave. The arrow should be bigger going out.

Syoung I think you have to differentiate between equilibrium and equilibrium potential.

Maybe this will help? describes the same situation as your questionL:
http://en.wikipedia.org/wiki/Resting_potential

"Put another way, the tendency of potassium to leave the cell by running down its concentration gradient is now matched by the tendency of the membrane voltage to pull potassium ions back into the cell. K+ continues to move across the membrane, but the rate at which it enters and leaves the cell are the same, thus, there is no net potassium current. Because the K+ is at equilibrium, membrane potential is stable, or "resting"."

 

[chiddler]

Maybe this will help? describes the same situation as your questionL:
http://en.wikipedia.org/wiki/Resting_potential

"Put another way, the tendency of potassium to leave the cell by running down its concentration gradient is now matched by the tendency of the membrane voltage to pull potassium ions back into the cell. K+ continues to move across the membrane, but the rate at which it enters and leaves the cell are the same, thus, there is no net potassium current. Because the K+ is at equilibrium, membrane potential is stable, or "resting"."

i understand this, but i don't understand how it makes what I wrote incorrect.

 

[syoung]

I dont think what you wrote is incorrect, the K+ ion is freely diffusing down its gradient. At the beginning it is diffusing out more than "in" due to permeability. But the cell doesn't want that? So it uses K+ to pump back in K+, bringing it to an equilbrium potential.

 

[chiddler]

I dont think what you wrote is incorrect, the K+ ion is freely diffusing down its gradient. At the beginning it is diffusing out more than "in" due to permeability. But the cell doesn't want that? So it uses K+ to pump back in K+, bringing it to an equilbrium potential.

hmm but does that answer the original question? specifically, which image was an accurate depiction of equilibrium potential.

you're saying that we have to take Na+/K+ pumps into account. i don't think that's required, is it?

or at least i'm not understanding why it should be required.

 

[typicalindian]

Without reading any of the other posts in this thread (because im lazy). Here's my go at the question.
In my neurobio class we learned that equilibrium potential (or nernest (sp?) equillibrium ) is when the net force due to an electrostatic force is balanced out by the concentration force. Potassium is highly concentrated in nerve cells so the concentration force wants to push potassium out. At the same time, the resting membrane potential is something like -75mV which strongly pulls potassium in, since potassium is a positively charged ion. Essentially I think the question was just testing to see if you understood what "equilibrium potential means".

edit:
and now that i looked at some of the responses above I see some talk of sodium potassium pumps. I really don't think it's necessary to include that into your thought process because those just maintain the already existing gradient rather than shift it greatly.

 

[syoung]

hmm but does that answer the original question? specifically, which image was an accurate depiction of equilibrium potential.

you're saying that we have to take Na+/K+ pumps into account. i don't think that's required, is it?

or at least i'm not understanding why it should be required.

No I'm saying you can only take into account the K+ pump, since that's all it shows, the K+ conc.

And for Indian, isn't the resting membrane potential of one ion, just the equilibrium between the conc and electrostatic forces? So if the conc forces K+ out, the electrostatic would pull it in via pumps, and so the picture would be B at equilibrium potential.

 

[chiddler]

Without reading any of the other posts in this thread (because im lazy). Here's my go at the question.
In my neurobio class we learned that equilibrium potential (or nernest (sp?) equillibrium ) is when the net force due to an electrostatic force is balanced out by the concentration force. Potassium is highly concentrated in nerve cells so the concentration force wants to push potassium out. At the same time, the resting membrane potential is something like -75mV which strongly pulls potassium in, since potassium is a positively charged ion. Essentially I think the question was just testing to see if you understood what "equilibrium potential means".

edit:
and now that i looked at some of the responses above I see some talk of sodium potassium pumps. I really don't think it's necessary to include that into your thought process because those just maintain the already existing gradient rather than shift it greatly.

Bear with me: normally, when in = out, the ion is at equilibrium. In a RMP things change a bit such that ion may be favored on one side of the membrane rather than the other. The question says what is an accurate representation of this equilibrium?

Your response is that it's when in = out. But when this occurs, then potassium has left the cell and the K+ outside should be greater than the K+ inside. This is not what the correct answer represents.

No I'm saying you can only take into account the K+ pump, since that's all it shows, the K+ conc.

And for Indian, isn't the resting membrane potential of one ion, just the equilibrium between the conc and electrostatic forces? So if the conc forces K+ out, the electrostatic would pull it in via pumps, and so the picture would be B at equilibrium potential.

Ok then i don't understand. Can you please explain?

sorry :<

 

[syoung]

Ok, let's try explanation again:
I think what may be confusing you is that, yes while the resting membrane potential of a neuron is negative (relative to outside), the picture doesn't involve other ions other than K+. (I'm assuming this because it looks like you drew the picture from the question stem).
Now, by itself, yes rate of ion flow out is high (the picture shows High K+ inside and low outside), so the gradient pushes out K+. However, the cell also wants to resist that change through K+ pumps, which pump back in the K+. But the membrane is permeable to K+ to flow out, not in, remember.

When we look at a normal neuron cell it has many other ions, and these affect the membrane potential as well, thus giving us our -70mV, most especially when we are looking at the Na+/K+/ATP pump and their ions together. This picture doesn't have Na+.

 

[chiddler]

Ok, let's try explanation again:
I think what may be confusing you is that, yes while the resting membrane potential of a neuron is negative (relative to outside), the picture doesn't involve other ions other than K+. (I'm assuming this because it looks like you drew the picture from the question stem).
Now, by itself, yes rate of ion flow out is high (the picture shows High K+ inside and low outside), so the gradient pushes out K+. However, the cell also wants to resist that change through K+ pumps, which pump back in the K+. But the membrane is permeable to K+ to flow out, not in, remember.

When we look at a normal neuron cell it has many other ions, and these affect the membrane potential as well, thus giving us our -70mV, most especially when we are looking at the Na+/K+/ATP pump and their ions together. This picture doesn't have Na+.

No my friend I don't think that's accurate. Nuerons have K+ leak channels that allow ions in OR out. The reason they flow out is because the RMP is at -70 mV while the equilibrium potential is around -80 mV. So they leave the cell to try and reach their equi potential by making the cell less positive. If we flood the cell surrounding with an excess of K+, then they will surely go in.

This question says "what represents the equil potential of K+?" First, let me redefine equilibrium potential nicely. I know you probably know this but just so we're looking at the same thing:

"The negative charge across the membrane that would be necessary to oppose the movement of K+ down its concentration gradient is termed the equilibrium potential for K."

So we can either change voltage of the cell to achieve equilibrium for K, or we can simply let concentration shift. In this question, we're assuming RMP is intact (-70 mV) and we are changing concentration. Since K+ tends to leave the cell, then equilibrium is established when K+ is higher outside than inside. But we don't see that. We see high inside, low outside. Because equilibrium is not established, K+ still has a tendency to leave the cell and therefore the arrow should be pointing outwards not in.



i feel like i'm running on a treadmill, explaining the wrong answer

edit: i'd like to add that the answer says that "Only B shows the resting membrane potential for K" which makes perfect sense and goes with what you are writing if the question was asking for RMP. But is asking for equil pot.

 

[typicalindian]

No my friend I don't think that's accurate. Nuerons have K+ leak channels that allow ions in OR out. The reason they flow out is because the RMP is at -70 mV while the equilibrium potential is around -80 mV. So they leave the cell to try and reach their equi potential by making the cell less positive. If we flood the cell surrounding with an excess of K+, then they will surely go in.

This question says "what represents the equil potential of K+?" First, let me redefine equilibrium potential nicely. I know you probably know this but just so we're looking at the same thing:

"The negative charge across the membrane that would be necessary to oppose the movement of K+ down its concentration gradient is termed the equilibrium potential for K."

So we can either change voltage of the cell to achieve equilibrium for K, or we can simply let concentration shift. In this question, we're assuming RMP is intact (-70 mV) and we are changing concentration. Since K+ tends to leave the cell, then equilibrium is established when K+ is higher outside than inside. But we don't see that. We see high inside, low outside. Because equilibrium is not established, K+ still has a tendency to leave the cell and therefore the arrow should be pointing outwards not in.



i feel like i'm running on a treadmill, explaining the wrong answer

edit: i'd like to add that the answer says that "Only B shows the resting membrane potential for K" which makes perfect sense and goes with what you are writing if the question was asking for RMP. But is asking for equil pot.

equilibrium potential is not established when the concentrations are equal, take a look at the nernest equation. Very tiny tiny tiny tiny tiny tiny changes in concentration cause big changes in the equilibrium potential. You can still have a very unbalanced concentration on the inside vs outside, the equilibrium potential just means that the cell has enough of a charge so that the flow of ions due to electrical current can balance out.

Also here's one fact you should know. Resting membrane potential is around -75mV, equilibrium potential for potassium is around -80mV, the two are virtually the same.

 

[chiddler]

equilibrium potential is not established when the concentrations are equal, take a look at the nernest equation. Very tiny tiny tiny tiny tiny tiny changes in concentration cause big changes in the equilibrium potential. You can still have a very unbalanced concentration on the inside vs outside, the equilibrium potential just means that the cell has enough of a charge so that the flow of ions due to electrical current can balance out.

Also here's one fact you should know. Resting membrane potential is around -75mV, equilibrium potential for potassium is around -80mV, the two are virtually the same.

yup yup I understand the difference and the nernst equation. And that fact. I even mentioned both in the post you responded to lol

So because equilibrium potential pulls K+ out, and that small concentrations make huge differences, a high K+ inside the cell and low K+ outside the cell can still achieve potassium's equilibrium potential. Because the change in K+ is small enough to keep concentrations the same, but significant enough to change the voltage of the cell.

Right?!

Please?! i'm sick of this question :'(

 

[syoung]

I dunno. I thought I was explaining it all nice and stuff>_<
For the EQ potential I didn't mean that rate itself was = but just that the conc gradient out for K+ is balanced by the electrochemical gradient for K+ in via the pump.

 

[typicalindian]

yup yup I understand the difference and the nernst equation. And that fact. I even mentioned both in the post you responded to lol

So because equilibrium potential pulls K+ out, and that small concentrations make huge differences, a high K+ inside the cell and low K+ outside the cell can still achieve potassium's equilibrium potential. Because the change in K+ is small enough to keep concentrations the same, but significant enough to change the voltage of the cell.

Right?!

Please?! i'm sick of this question :'(

at the bold:

Potassium is moving out of the cell because of its concentration gradient. By leaking out of the cell, this causes a negative charge to build up on the inside which will slowly become more and more negative. When the rate of potassium flowing out has reached the "equilibrium" concentration ( I put equilibrium in quotes because it doesn't mean equilibrium in the sense that the concentrations are equal) there will be a strong enough electrical force pulling the potassium ions back into the cell from the outside. Thus at the equilibrium potential you still have potassium flowing out of the cell due to its concentration gradient, but an equal amount of potassium ions are moving back into the cell due to the electrical gradient. Therefore you have no net flow of potassium movement.

Basically what you said is right, but I don't think saying "the equilibrium potential pulls K+ out" is the right way to say it. Considering the equilibrium potential, if anything, pulls potassium back into the cell.

edit: also @ syoung, I have no idea why you're bringing the na+/k+ pump into things here, I see no reason for it. The membrane has potassium channels where potassium can freely leak in or out at a slow rate. The Na+/K+ pump is mainly used to keep resting potential consistent when changes occur in the resting membrane potential. ie the undershot phase of an action potential.

 

[chiddler]

at the bold:

Potassium is moving out of the cell because of its concentration gradient. By leaking out of the cell, this causes a negative charge to build up on the inside which will slowly become more and more negative. When the rate of potassium flowing out has reached the "equilibrium" concentration ( I put equilibrium in quotes because it doesn't mean equilibrium in the sense that the concentrations are equal) there will be a strong enough electrical force pulling the potassium ions back into the cell from the outside. Thus at the equilibrium potential you still have potassium flowing out of the cell due to its concentration gradient, but an equal amount of potassium ions are moving back into the cell due to the electrical gradient. Therefore you have no net flow of potassium movement.

Basically what you said is right, but I don't think saying "the equilibrium potential pulls K+ out" is the right way to say it. Considering the equilibrium potential, if anything, pulls potassium back into the cell.

edit: also @ syoung, I have no idea why you're bringing the na+/k+ pump into things here, I see no reason for it. The membrane has potassium channels where potassium can freely leak in or out at a slow rate. The Na+/K+ pump is mainly used to keep resting potential consistent when changes occur in the resting membrane potential. ie the undershot phase of an action potential.

very cool, I understand what the original error and the one I made above.

thank you.

 

[chiddler]

I dunno. I thought I was explaining it all nice and stuff>_<
For the EQ potential I didn't mean that rate itself was = but just that the conc gradient out for K+ is balanced by the electrochemical gradient for K+ in via the pump.

lol

thanks i appreciate the effort but I think you made some errors in your explanation. i hope you learned as much as I did, anyway.

 

[syoung]

Yeah, could we get the summary here? I feel like I'm on a circular track and don't know what direction I'm facing.

 

[typicalindian]

Yeah, could we get the summary here? I feel like I'm on a circular track and don't know what direction I'm facing.

see my last post, I explained it pretty well in that post I think.

 

[syoung]

Cool thanks.

I brought it up mostly on a side note, not really for the problem at hand.