How the the sodium-potassium (Na+/K+) pump works. Its role in establishing resting membrane potentials. Created by Sal Khan.
- [Voiceover] What I hope to do in this video is give ourselves an appreciation for the sodium-potassium pump, and as the name implies, it pumps sodium and potassium, but it does it in different directions. So this little depiction right over here, this is my drawing, my rendition of the sodium-potassium pump, it's a trans-membrane, I guess you could say protein complex right over here. And in this resting state, it is open to the inside of the cell and it has an affinity for sodium ions. And so the sodium ions, you see three sodium ions depicted here in blue. They're going to bind to the pump. And once they bind to it, then it's going to want to be phosphorylated by an ATP, and we see that right over here. This is ATP, adenosine triphosphate. And when it gets phosphorylated, it's a release of energy and it allows the confirmation of the actual protein to change. So the new confirmation of the protein, it's now going to open up to the outside, close off to the inside, and now it's no longer going to have an affinity for sodium ions, but an affinity for potassium ions, and this is fascinating, that release of energy, change of confirmation, that these proteins really are these molecular machines, these fascinating molecular machines. But once that happens, this change of confirmation, the sodium ions are going to be released outside of the cell. And then you're going to have potassium ions that are going to bind from the outside. And then once that happens, the change in confirmation, we're going to have a, it's going to get dephosphorylated and then you're going to go back to your original confirmation, your original confirmation right over here. Where you no longer have an affinity for potassium ions, they're going to be released, and then you're going to be back in the original phase. So this is fascinating. By using ATP, by using energy, this is active transport, it takes energy to do this. Let me write this down. This is active, this is active transport that we are talking about right over here. We're able to pump, using an ATP, we're able to pump three sodium ions out, three sodium ions out, so let me write that down. Three sodium ions out. And in the process, we pump two potassium ions in. So we pump two potassium ions in. Now you might say, okay, the outside, since these both have positive charge, but I have three sodium going out, two potassium going in. That must make the outside more positive than the inside, and that actually is true. But that by itself isn't fully responsible. It's actually only partially responsible for the electric potential difference between the inside of the membrane and the outside of the membrane. What really sets that up is that you actually have channel proteins that allow potassium ions to move down, to diffuse down their concentration gradient. So let's think about what happens before I even talk about these channel proteins right over here. Because of the sodium-potassium pump, what is sodium's concentration gradient? Well, it has a higher concentration on the outside, it has a higher concentration on the outside and it has a lower concentration on the inside. This is sodium's concentration gradient. What is potassium's concentration gradient? Well, potassium is getting pumped in from the outside into the cell. So potassium has the opposite concentration gradient. It has a high concentration inside and it has a low concentration outside. Now if we let potassium go through, we've talked in previous videos about ions just not being that, that just the general membrane, if it's not facilitated in some way, isn't that permeable to things like ions, like sodium and potassium ions. But if you have channel proteins right over here that let the potassium get out, what's going to happen? Well, you might have one of two answers. You might say, oh, well, look. Hey, you know, things diffuse down their concentration gradient, you have a higher probability since you have more potassium here than up here, higher probability of them going in the right direction on this side and moving from this side to that side than you have them going from that side to that side. And so you would have a net outflow of potassium. And some of you might say, well, okay, that makes sense if you only care about the concentration gradient. But what happens if we look at the charge? Because we're saying that the inside of the cell is going to be less positive and the outside of the cell is going to be more positive because it has more, that we have the net ion change, so it's gonna be more positive out here. So positive ions, they don't like, you want to move away from charges that are the same as you. You want to move to the places that are a more negative, so you'd say, well, these potassium ions are positively charged, why would they wanna go from a less positive place to a more positive place? And if you are saying either one of these things, talking about the concentration gradient or talking about the electric potential difference, you are actually going to be right in both cases. These are going to be balancing forces. The concentration gradient is going to allow some of these potassium ions to pour out, but the concentrations of potassium ions aren't going to fully equalize because of the electric potential difference, because, hey, it's more positive out here, it's less positive here. When they're moving out, they're going against what their charge wants to do. They're going with the concentration gradient, but at some point, that is going to balance out. And by going through this process, by pumping sodium out and with that larger ratio than what you're pumping potassium in, and then you're further allowing more positive charge to go out. You're establishing what's called the resting membrane potential for a cell. And this is super important for all cells, but especially neuron cells or neural cells or neurons. And those are gonna spend two-thirds of their energy just to establish or to keep their resting membrane potential. And as we'll see in the videos on neurons, that's because they keep leveraging that potential to send signals down the neuron. But the resting membrane potential, it's less positive here and more positive there. If you measure this relative to, let me make that a little bit neater, relative to this right over here, this difference is, depending on what estimates you look at, approximately negative 70 millivolts. I've seen estimates negative 60, negative 80. Negative 70 millivolts. And this is key for neurons, but it's key for all cells. Now the sodium-potassium pump isn't just about establishing the resting membrane potential. Having this higher sodium concentration on the outside can also be used later on for other forms of active transport. When they move down their gradient, you can do things like co-transport glucose molecules. So these biological systems are far more complicated than I often give credit for in these videos, but I want to give you a full appreciation for this. And just so you know how big of a deal the sodium-potassium pump was, it was discovered in the 1950s, but the 1997 Nobel Prize was awarded for the discovery of the sodium-potassium pump and how it works.