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Sodium-potassium pump

How a sodium potassium pump can maintain a voltage gradient across a cell or neuron's membrane.

The sodium-potassium pump goes through cycles of shape changes to help maintain a negative membrane potential. In each cycle, three sodium ions exit the cell, while two potassium ions enter the cell. These ions travel against the concentration gradient, so this process requires ATP.
Created by Sal Khan.

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  • blobby green style avatar for user Leroy Coles
    does the sodium potassaum pump use any energy if so how much does it recive any energy back
    (12 votes)
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  • aqualine ultimate style avatar for user Patrick
    I am confused about what ATP is, it was said that it was mentioned in the previous videos on pulmonary and circulatory but i don't recall that. All this information pertaining to ATP is getting confusing. What is ATP?
    (10 votes)
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  • aqualine ultimate style avatar for user ricfra0117
    Ok, so why is ATP(adesine triphosphate) more energetic than TDP(adesine diphosphate) the only differens is that ATP har one more phosphate, and how does it get transported to the brain. I there capillaires inside the brain or what?
    PS. sorry for not being on topic
    (8 votes)
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    • piceratops ultimate style avatar for user ILoveToLearn
      Great question!
      The answer is very straightforward because the energy comes from the unstable high-energy bonds between the phosphates. Therefore, with more phosphates, more high-energy bonds, more energy is released when those bonds are broken.
      Blood glucose goes to the brain. Neurons depend on a constant supply of glucose for energyfrom which to make ATP!
      There are many blood vessels of the brain, including the vertebrobasilar system, the anterior, middle, and posterior arteries, the choroidal arteries, and more. You can Google this if you're curious or I will happily provide more information.
      There are also many veins draining deoxygenated (used) blood from the brain, including but not limited to the superior sagittal sinus, vein of Galen, internal cerebral vein, superior and inferior anastomotic veins, the basal vein of Rosenthal, and the anterior, middle, and posterior cerebral veins.
      I voted up your question.
      You are on topic - everyone is here to learn and to teach. Questions are more than welcome, they are encouraged.
      Any further questions? I'd love to help!
      (6 votes)
  • piceratops ultimate style avatar for user Tushar Pal
    Well, why is even this necessary? Why suddenly would the Na-K pump start using energy to do work against concentration gradient? Isn't that against the fundamental rule that things want to be in a more stable state??
    Also, why is it mandatory of the outside of a cell to be more positive?
    (5 votes)
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  • purple pi purple style avatar for user Daniel Fu
    At sal says "some of this sodium gets shoved through. Would that release energy? Common sense says that would give of some energy.
    (5 votes)
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    • leaf blue style avatar for user dysmnemonic
      Most of your cells use an inward gradient for Na+ to power lots of secondary active transport. Because the Na+ wants to flow down its gradient and into the cell, the energy provided by the inward electrochemical gradient can be used to transport larger molecules like sugars and amino acids.

      If the sodium ions enter the cell through 'leak channels', as Sal describes, there is some energy released but it isn't used by anything. Instead it reduced the electrochemical potential across the membrane, ensuring the cell can function normally.
      (3 votes)
  • duskpin ultimate style avatar for user siena.duncan
    Is there another video on YouTube that could explain this to a 6th grader? Sal does a good job explaining this concept, no doubt about it, but I still don't quite understand it. Does anybody want to share a good video that I could watch on this?
    (2 votes)
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    • winston default style avatar for user David
      I watched this video last year too, in Grade 6, actually at this time of year. What I did was watched every video 3-5 times, made sure I thoroughly understood them, and then moved. Also, I took notes on the videos just in case I forgot something, I wouldn't have to go back and watch the video again, I would just look at my noted (which I still do now).
      Hope this helped! :D
      (5 votes)
  • aqualine seed style avatar for user Will Phillips
    Are neurons all over the body or just in the brain?
    (4 votes)
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    • winston baby style avatar for user Ivana - Science trainee
      The previous answer is incomplete.

      Neurons are all over your body, letting you use all of your senses plus helping you react with every single part fo your body. (not just having afferent but efferent pathways are as important!) We have also motor neurons, not just sensory neurons.

      There are parts of the body where you have relatively less concentrated neurons or thicker skin (such as the back) and those parts that are hypersensitive (such as lips).
      (2 votes)
  • leafers sapling style avatar for user Max Shine
    Is there an evolutionary hypothesis as to why we evolved to have a gradient with the inside of the cell being more negative rather than reversing the pump and using a more positive gradient? Nature would rush to equalise it either way.
    (3 votes)
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    • leaf blue style avatar for user dysmnemonic
      I don't know if there's a hypothesis for why the specific ions are involved the way they are, except "it worked so nature kept it and found new ways to use it".

      On a more practical level, the cell has a negative resting potential because that's how we named charges and currents before we understood how they work. There's nothing intrinsically 'negative' about the charge gradient, it's just what humans called it and it would be much too confusing to change the name now.
      (3 votes)
  • old spice man green style avatar for user Kris Mb
    So, basically what are negative charges?
    And why do they have to escape ?? why don't the positive charge repel ?
    How can a +ve charge move to an area where there is more of it ? Why can't the +ve charge get binded with -ve charge ?
    (3 votes)
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    • winston baby style avatar for user Ivana - Science trainee
      Because of the selective permeability of the cell.

      Forst, the cell is generated negative charge thanks to potassium ions which leave the cell leaving a negative charge balance inside the cell.

      When the neuronal membrane is at rest, the resting potential is negative due to the accumulation of more sodium ions outside the cell than potassium ions inside the cell.

      And for an action potential to get generated, first depolarization must occur. For that to happen, the negative charge is required.

      Hypothetically, it could have been a positive charge, but then, other ions would have played a critical role.
      (1 vote)
  • leaf blue style avatar for user alexanderkorol20
    How fast is this process? How many of these pumps happen in one second?
    (2 votes)
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    • leaf blue style avatar for user dysmnemonic
      It's difficult to quantify exactly for a single pump, because the pump and the ions are too small to directly observe as they work and because they depend on optimal conditions to work at their fastest rate. The best estimate I could find is about 135 times per second for a single pump, based on ATP usage. One paper I read estimates that all of the sodium-potassium pumps from all of your muscle cells, working together under ideal conditions, could clear all of the potassium in your body in about 25 seconds.
      (3 votes)

Video transcript

In the last video, I showed you what a neuron looked like and we talked about the different parts of a neuron, and I gave you the general idea what a neuron does. It gets stimulated at the dendrites-- and the stimulation we'll talk about in future videos on what exactly that means-- and that that impulse, that information, that signal gets added up. If there's multiple stimulation points on various dendrites, it gets added up and if it meets some threshold level, it's going to create this action potential or signal that travels across the axon and maybe stimulates other neurons or muscles because these terminal points of the axons might be connected to dendrites of other neurons or to muscle cells or who knows what. But what I want to do in this video is kind of lay the building blocks for exactly what this signal is or how does a neuron actually transmit this information across the axon-- or really, how does it go from the dendrite all the way to the axon? Before I actually even talk about that, we need to kind of lay the ground rules-- or a ground understanding of the actual voltage potential across the membrane of a neuron. And, actually, all cells have some voltage potential difference, but it's especially relevant when we talk about a neuron and its ability to send signals. Let's zoom in on a neuron's cell. I could zoom in on any point on this cell that's not covered by a myelin sheath. I'm going to zoom in on its membrane. So let's say that this is the membrane of the neuron, just like that. That's the membrane. This is outside the neuron or the cell. And then this is inside the neuron or the cell. Now, you have sodium and potassium ions floating around. I'm going to draw sodium like this. Sodium's going to be a circle. So that's sodium and their positively charged ions have a plus one charge and then potassium, I'll draw them as little triangles. So let's say that's potassium-- symbol for potassium is K. It's also positively charged. And you have them just lying around. Let's say we start off both inside and outside of the cell. They're all positively charged. Sodium inside, some sodium outside. Now it turns out that cells have more positive charge outside of their membranes than inside of their membranes. So there's actually a potential difference that if the membrane wasn't there, negative charges would want to escape or positive charges or positive ions would want to get in. The outside ends up being more positive, and we're going to talk about why. So this is an electrical potential gradient, right? If this is less positive than that-- if I have a positive charge here, it's going to want to go to the less positive side. It's going to want to go away from the other positive charges. It's repelled by the other positive charges. Likewise, if I had a negative charge here, it'd want to go the other side-- or a positive charge, I guess, would be happier being here than over here. But the question is, how does that happen? Because left to their own devices, the charges would disperse so you wouldn't have this potential gradient. Somehow we have to put energy into the system in order to produce this state where we have more positive on the charge of the outside than we do on the inside. And that's done by sodium potassium pumps. I'm going to draw then a certain way. This is obviously not how the protein actually looks, but it'll give you a sense of how it actually pumps things out. I'll draw that side of the protein. Maybe it looks like this and you'll have a sense of why I drew it like this. So that side of the protein or the enzyme-- and then the other side, I'll draw it like this. It looks something like this, and of course the real protein doesn't look like this. You've seen me show you what proteins really look like. They look like big clusters of things, hugely complex. Different parts of the proteins can bond to different things and when things bond to proteins, they change shape. But I'm doing a very simple diagram here and what I want to show you is, this is our sodium potassium pump in its inactivated state. And what happens in this situation is that we have these nice places where our sodium can bind to. So in this situation, sodium can bind to these locations on our enzyme or on our protein. And if we just had the sodiums bind and we didn't have any energy going into the system, nothing would happen. It would just stay in this situation. The actual protein might look like something crazy. The actual protein might be this big cloud of protein and then your sodiums bond there, there, and there. Maybe it's inside the protein somehow, but still, nothing's going to happen just when the sodium bonds on this side of the protein. In order for it to do anything, in order for it to pump anything out, it uses the energy from ATP. So we had all those videos on respiration and I told you that ATP was the currency of energy in the cell-- well, this is something useful for ATP to do. ATP-- that's adenosine triphosphate-- it might go to some other part of our enzyme, but in this diagram maybe it goes to this part of the enzyme. And this enzyme, it's a type of ATPase. When I say ATPase, it breaks off a phosphate from the ATP-- and that's just by virtue of its shape. It's able to plunk it off. When it plunks off the phosphate, it changes shape. So step one, we have sodium ions-- and actually, let's keep count of them. We have three sodium-- these are the actual ratios-- three sodium ions from inside the cell or the neuron. They bond to pump, which is really a protein that crosses our membrane. Now, step two, we have also ATP. ATP gets broken into ADP plus phosphate on the actual protein and that changes the shape. So that also provides energy to change pump's shape. Now this is when the pump was before. Now after, our pump might look something like this. Let me clear out some space right here. I'll draw the after pump right there. And so this is before. After the phosphate gets split off of the ATP, it might look something like this. Instead of being in that configuration, it opens in the other direction. So now it might look something like this. And of course it's carrying these phosphate groups. They have a positive charge. It's open like this. This side now looks like this. So now the phosphates are released to the outside. So they've been pumped to the outside. Remember, this is required energy because it's going against the natural gradient. You're taking positive charge and you're pushing them to an environment that is even more positive and you're also taking it to an environment where there's already a lot of sodium, and you're putting more sodium there. So you're going against the charge gradient and you're going against the sodium gradient. But now-- I guess we call it step three-- the sodium gets released outside the cell. And when this changes shape, it's not so good at bonding with the sodium anymore. So maybe these can become a little bit different too, so that the sodium can't even bond in this configuration now that the protein has changed shape due to the ATP. So step three, the three Na plusses, sodium ions-- are released outside. Now once it's in this configuration, we have all these positive ions out here. These positive ions want to get really as far away from each other as possible. They'd actually probably be attracted to the cell itself because the cell is less positive on the inside. So these positive ions-- and in particular, the potassium-- can bond this side of the protein when it's in this-- I guess we could call it this activated configuration. So now, I guess we could call it step four. We have two sodium ions bond to-- I guess we could call it the activated pump-- or changed pump. Or maybe we could say it's in its open form. So they come here and when they bond, it re-changes the shape of this protein back to this shape, back to that open shape. Now when it goes back to the open shape, these guys aren't here anymore, but we have these two guys sitting here and in this shape right here, all of a sudden these divots-- maybe they're not divots. They're actually things in this big cluster of protein. They're not as good at staying bonded or holding onto these sodiums so these sodiums get released into the cell. So step five, the pump-- this changes shape of pump. So pump changes shape to original. And then once we're in the original, those two sodium ions released inside the cell. We're going to see in the next few videos why it's useful to have those sodium ions on the inside. You might say, well, why don't we just keep pumping things on the outside in order to have a potential difference? But we'll see these sodium ions are actually also very useful. So what's the net effect that's going on? We end up with a lot more sodium ions on the outside and we end up with more potassium ions on the inside, but I told you that the inside is less positive than the outside. But these are both positive. I don't care if I have more potassium or sodium, but if you paid attention to the ratios I talked about, every time we use an ATP, we're pumping out three sodiums and we're only pumping in two potassiums, right? We pumped out three sodiums and two potassiums. Each of them have a plus-1 charge, but every time we do this, we're adding a net-1 charge to the outside, right? 3 on the outside, 2 to the inside. We have a net-1 charge-- we have a plus-1 to the outside. So we're making the outside more positive, especially relative to the inside. And this is what creates that potential difference. If you actually took a voltmeter-- a voltmeter measures electrical potential difference-- and you took the voltage difference between that point and this point-- or more specifically, between this point and that point, if you were to subtract the voltage here from the voltage there, you will get -70 millivolts, which is generally considered the resting voltage difference, the potential difference across the membrane of a neuron when it's in its resting state. So in this video, I kind of laid out the foundation of why and how a cell using ATP, using energy, is able to maintain a potential difference across its membrane where the outside is slightly more positive than the inside. So we actually have a negative potential difference if we're comparing the inside to the outside. Positive charge would want to move in if they were allowed to, and negative charge would want to move out if it was allowed to. Now there might be one last question. You might say, well, if we just kept adding charge out here, our voltage difference would get really negative. This would be much more negative than the outside. Why does it stabilize at -70? To answer that question-- these are going to come into play in a lot more detail in future videos-- you also have channels, which are really protein structures that in their open position will allow sodium to go through them. And there are also channels that are in their open position, would allow potassium to go through them. I'm drawing it in their closed position. And we're going to talk in the next video about what happens when they open. But in their closed position, they're still a little bit leaky. And if, say, the concentration of potassium becomes too high down here-- and too high meaning when they start to reach this threshold of -70 millivolts-- or even better, when the sodium gets too high out there, a few of them will start to leak down. When the concentration gets really high and this is really positive just because of the electrical potential, some of them will just be shoved through. So it'll keep us right around -70 millivolts. And if we go below, maybe some of the potassium gets leaked through the other way. So even though when these are shut-- if it becomes too ridiculous-- if it goes to -80 millivolts or -90 millivolts, all of a sudden, there'd be a huge incentive for some of this stuff to leak through their respective channels. So that's what allows us to stay at that stable voltage potential. In the next video, we're going to see what happens to this voltage potential when the neuron is actually stimulated.