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Correction to sodium-potassium pump video

Correction to "Sodium-potassium pump" video. Created by Sal Khan.

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  • blobby green style avatar for user Yuki Mai
    At , the video says that the membrane is highly permeable for potassium to pump out than sodium to pump in. Why is it easier to pump out potassium?
    (31 votes)
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    • blobby green style avatar for user Andrew Busch
      That's a great question! But to clarify, highly permeable does not really mean "easier to pump" because the word "pump" usually implies an active process that requires energy like the Na-K ATPase. Here, K is passively "leaking" out of a channel that is basically always open. It only gets out by bumping into this channel while bouncing around randomly inside the cell. The reason the cell is more "permeable" to K is because there are more of these types of K channels in the membrane!!
      (97 votes)
  • blobby green style avatar for user mollymvora
    Please explain again why the inside is less positive than the outside. Also,does that mean that the inside is not negative it is just less positive than the outside? I do not understand what Sal meant in this video and the edits he made. If the outside is more positive then why would K+ ions from the inside of the cell membrane want to go to the outside through the channels? Wouldn't it be the other way around where ions from the outside want to come into the inside due to the gradient?
    (24 votes)
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    • blobby green style avatar for user jwadleigh00
      Saying the inside is less positive is the equivalent of saying it is more negative.

      To answer your other question, recall that there are two kind of gradients here. There's the electrical gradient you first mentioned (the outside is more positive), and there's the chemical gradient (there's more K+ inside). You're absolutely right in saying that K+ ions want to stay inside because it's negative inside, but they also want to diffuse outside because they have such a high concentration inside. These forces (so to speak) work against each other and establish an equilibrium. This electrochemical equilibrium is between the electrical equilibrium and the chemical equilibrium, which is why K+ appears to be "happy" even though there's a net positive charge outside and a high concentration of K+ inside. The key to all of this is remembering that there's a high membrane permeability to K+, but not to Na+. This allows K+ ions to move down their concentration gradient just enough make the inside negative (remember, less positive equals more negative), but Na+ ions can't cross the membrane fast enough to try and neutralize the charge.
      (92 votes)
  • blobby green style avatar for user Shruti Kumar
    is the main cause of this membrane potential is actually the channel and not the sodium potassium pump, then what is the purpose of the pump? since the pump actually uses ATP to function, wouldn't it be a waste of energy?
    (38 votes)
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    • blobby green style avatar for user Andrew Busch
      Yes, you are right that the pump (Na-K ATPase) is VERY expensive in terms of energy! But that is because it is one of the most important processes in the UNIVERSE!!! The "DIRECT" cause of the "negative resting potential" of the cell is the "leak" of K, but why does it leak from inside to outside? Because it is higher inside and wants to be balanced through diffusion. But why is it higher inside? Because of the Na-K pump that pumps it in!! So the pump ESTABLISHES the "gradient" that makes everything possible!
      (70 votes)
  • blobby green style avatar for user Tony Arena
    Interesting, yet how is this possible? Wouldn't this require active transport of K+? This does not make sense. Passive transport against an electrochemical gradient does not make sense to me. Does this hold up with it is a 1 to 2 ratio and a 0 to 2 ratio? Could it be that the pump works faster?
    (5 votes)
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    • blobby green style avatar for user derek_furutani
      I guess it depends on how strong the electromagnetic attraction of the protein-anions to the K ions is compared to the effect of the K concentration gradient. If it is not strong enough to counter the effect of the gradient than both explanations would contribute to the membrane voltage difference.
      (6 votes)
  • blobby green style avatar for user Davide Gislon
    Hi, i have a question about the potassium membrane permeability .. if this is a passive channel why would potassium want to "leak" out of the cell if the outside is more positively charged? Wouldn't it make more sense for the potassium to stay inside the cell as per the diffusion principle? I guess there are other things (a lot probably) i am not considering.
    (5 votes)
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    • blobby green style avatar for user philotimo6
      Hi, here's the answer I thought of: There is more potassium on the inside because that's where the pump is dumping it. Therefore, the potassium would want to travel to the outside where there is less potassium. Potassium can do this because the neuron's membrane is highly permeable to it and only partially permeable to sodium. Sodium would do the same thing if it could get through the membrane as easily but it can't, so only a little of it leaks into the cell. Hope that helped!
      (9 votes)
  • blobby green style avatar for user Houria Javed
    isn't it that the potential difference occur due to more k+ going inside the cell due to its greater permeability to the membrane, and less of it going outside the membrane making high conc. of k+ inside and high conc. of Na+ outside, rather than jamming of k+ on the inside???
    (5 votes)
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  • blobby green style avatar for user Gokul Rajan
    But aren't the K+ ions larger than Na+ ions which should actually make them difficult to pass through the channels? And make the passage of Na+ much easier! No?
    (3 votes)
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    • blobby green style avatar for user wilder.vet
      Na+ is smaller than K+. the K+ channel is smaller than Na+ in diameter (0.3 vs 0.5 nm), never the less the size of the K+ allow to interact with O2 and H2O to a greater extent than Na+ and thats create an environment to pass faster and in consecuence more moles through their channel (even though is smaller in diameter). To compare, both channels are highly selective: Na+ channels transport Na+ ions 11 times faster than K+ ions, K+ channels are 100 times faster to transport K+ ions than Na+ ions
      (7 votes)
  • piceratops tree style avatar for user Ermias Merine
    What is the name of the channel that K+ and N+ leak from?
    (3 votes)
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  • blobby green style avatar for user Jake
    at university here in the UK, we were taught that the charge inside the neuron at resting state is -60mV compared to the outside the cell. Where in these videos it is said to be around -80mV at its "normal value". Can anyone explain this difference to me?
    (2 votes)
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  • leaf green style avatar for user wannabeDoc
    Did I understood this correctly?
    - Potassium [K] pumps in the cell membrane and then goes back out because it is highly permeable?
    - Sodium [Na] pumps out the cell membrane and it hardly goes back in because it is slightly permeable?
    (3 votes)
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Video transcript

Two corrections I want to make to the video on the sodium potassium pump. One very minor one-- and I don't think it would trip too many of you guys up, but near the end of the video, as we learned, we have potassium getting pumped into the cell by the sodium potassium pump. Let me draw the membrane. It'll actually be useful in the more significant correction I'd like to make. So let me draw a cross section of a cell membrane. And let me draw the sodium potassium pump right here. We saw it pumps out three sodiums for every two potassiums that it pumps in. It definitely doesn't look like that, but it gives the idea. And we're pumping potassium ions in-- so K plus-- and we're pumping sodium ions out-- and that's what the whole point of that video was. When this thing changes shape with ATP, it pumps the sodium ions out. Now the minor correction I want to make-- and I don't think it would have tripped you up too much-- is near the end of that video, I drew the potassium ions-- and I wrote a K plus, but a few times near the end of the video, I referred to them as sodium ions-- and I don't want that to confuse you at all. It is potassium ions that are getting pumped in. Two potassium ions get pumped in for every three sodium ions that get pumped out. So I don't want-- even thought I drew a K plus, sometimes I said sodium by accident. Don't want that to confuse you. That is the minor error. The more significant error is that I said that the main reason that we had this potential difference-- why it is more positive on the outside than the inside-- so this is less positive. I said that the main reason was because of this ratio. We're pumping out three sodium ions for every two potassium ions that we pump in. And I just got a very nice letter from a professor of physiology, Steven Baylor at University of Pennsylvania, and he wrote a very interesting email and it corrects me. And it's a very interesting thing to think about in general. So here's what he wrote and let's think about what he's saying. He says: Here at Penn Medical School, we have a nice teaching program that stimulates the ion fluxes across a generic cell, --So the ion flux is just the movement of the ions across the membrane-- including that due to the sodium potassium pump and that which arises from the resting permeabilities of the membrane. So the resting permeabilities is how easy it is for these ions to go through the membrane. And we'll talk more about that in a second. And the resting permeabilities of the membrane to sodium, potassium, chloride, et cetera. One option our program gives students is to change the pump stoichiometry from three to two. So when he's talking about pump stoichiometry from three to two, he's just talking about they're changing the ratios. So they change it from 3:2 to 2:2. So what that means is, they have a simulation program that says, well, what if the sodium potassium pump, instead of pumping three sodiums out for every two potassium it pumps in, what if it was even? What if it was two sodiums and two potassiums? And based on my explanation of why we have this potential difference, that should not lead to a potential difference if the main reason was the stoichiometry-- the ratio of sodium being pumped to the potassium being pumped in. But he goes on to say: They could change it to 2:2 in the simulation. As a result of this maneuver, the membrane potential changes from its normal value of about -80 millivolts-- and they measure that. They take the voltage here minus the voltage there so that you get a negative number. This is more positive. It's a larger number. So it changes from -80 millivolts to about -78 millivolts. So what he's saying is, if you change this from three and two-- three sodiums for every two potassiums that get pumped in-- if you change that to 2:2, it actually doesn't change the potential that much. You still have a more positive environment outside than you have inside. So that leads to the question-- then why do we have the potential if the stoichiometry of this ratio is not the main cause? So it says, it changes a little bit. The potential difference becomes a little bit less. The cell swells a few percentage and then everything stabilizes. So then he goes on to write: So while it is true that the normal stoichiometry of the pump does have a slight negative influence on the membrane potential-- that's just the membrane potential, the voltage across the membrane-- the imbalance in the pump stoichiometry is not the main reason for the large negative membrane potential of the cell. Rather, the main-- let me underline this-- the main reason is the concentration gradients established by the pump in combination with the fact that the resting cell membrane is highly permeable to potassium and only slightly permeable to sodium. So we said in the last video-- or the first video on the sodium potassium pump-- we said there were channels that the sodium could go through and there's also channels that the potassium could go through. And now what he's saying is that the main cause of the potential difference isn't this ratio, it's the fact that the membrane is highly permeable to potassium. So this is very permeable. Potassium can get out if it wants to, much easier than it is for sodium to get in. So what that happens-- even if this was a 2:2 ratio-- it's actually a 3:2, but even if this was a 2:2 ratio, even though this environment is more positive, you're just more likely to have to potassium ions down here bump in just the right way to get across and get to the other side, go against its chemical gradient, right, because you have a higher concentration of potassium here than over here. So you're more likely to have a potassium bump in just the right way to get through this channel and get out-- than you are to have a sodium be able to go the opposite direction. And that's what makes this environment. So you have more potassium coming outside because of this permeability than sodium coming inside-- and that's the main cause of the potential difference between the outside and the inside. And so thank you, Steven Baylor, for that correction. Very interesting.