Biology: Active Transport: Sodium-Potassium Pump

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One of the coolest cells out there, in terms of pumping materials across concentration gradients and looking at facilitated diffusion and all these other things, is this idea of the sodium-potassium ATP pump, which occurs in nerve cells. Now there's a mouthful for you - not nerve cells, but the sodium-potassium ATP pump - and it occurs in neurons and nerve cells. I want to tell you a little bit about this, because every once in a while we biology teachers rejoice, because we come across some phenomena that pulls it all together. And this sodium-potassium ATP pump is something that not only pulls stuff together, but starts to give you a little inkling as to what's to come. What more could you ask for?
Let's talk about what the sodium ATP pump is, but first I need to tell you about what nerves do. Well, you might know that nerves conduct so-called electrical impulses. And you probably also know that cells have ways of generating their own electricity. You know that cells, by several different ways, can have protons, or, excuse me, have charges on one side of a membrane or the other. And I said the word "protons" there because quite often that's the way they do it, they get like pluses - protons on one side and not protons on the other. However, in the sodium-potassium ATP pump, protons aren't going to be so much of a function as ions. So let's talk about a neuron then, let's take a look at a nerve cell.
Here is a nerve cell, and I'm going to say that this is a section, a little segment of a nerve cell, and I'm going to put it at rest. There's a lot more that I'm not going to tell you about when I tell you about this nerve cell right now, but that just gives you something to look forward to in life. Generally speaking, what we have here in this nerve cell is an overall negative interior, and an overall positive exterior. Now there's a watchword there; it's called "overall." Let me show you what I mean. See, outside the nerve cell, there will be many, many, many sodium ions. Now sodium is a positively charged ion. And I want you to understand something, that the membrane is impermeable to sodium. What that means is sodium cannot diffuse back and forth across there, so you're not going to have any kind of like "equilibrium diffusion kind of passes transport" thing happening here. But, in addition to sodium out here, you do have potassium, and potassium is another positive ion. Now potassium also has a - like I said, it's a positive ion. But the membranes are permeable to potassium, so you're going to have an equilibrium of potassium inside.
So I want you to understand some things here. Number one, when we talk about positive and negative, we're talking about relative amounts. It's much more positive out here than in here, so it's a relative thing.
And the other thing I've not told you about is the fact that there are negative proteins in here. So we have these very large molecules that the membrane is definitely not permeable to on the inside of that nerve cell. That is a resting nerve cell. I want to tell you what happens when a nerve impulse moves. And I'm only telling you a smidgen of this. Why? Because I really don't want to talk much about nerve cells, believe it or not, I want to talk about what happens after a nerve impulse comes along. But to tell you what happens after, I've got to kind of tell you a little bit about what happens first.
First of all, you see we have a voltage here, and let's just say, along comes a nerve impulse - boom - and it stimulates this section. Well, something begins to happen. The first thing that happens is - ooh, here comes number one. Remember gated channels? Remember the fact that there were some proteins in the cell membrane - and this has a cell membrane - that would not open unless they were stimulated? And then you could have an active passage - not active transport - but a very quick diffusion of materials through an ion channel? Welcome to your first ion channel besides the one I showed you with the hydrogen.
The first thing that happens here - and I'm going to not throw this away, we'll see if we have to come back to it in a second - but the very first thing that happens here is, after the stimulus, gated channels open. And guess what? When those gated channels open, sodium rushes in. We call this a depolarization. Why is this a depolarization? Well, once again, let's go back to my original diagram here - and the sodium rushes in because the gated channel's open, so all the sodium ions are going to move in there, which is going to make this much more positive inside. So it goes from negative to positive in milliseconds - in milliseconds, that's thousandths of seconds - really quickly. And that's where I want to start my story, because now, if we were to stop here, you could never have an impulse move along there again. You have to somehow reestablish, and get these sodiums back out. And if you think about the dilemma, how are you going to do that? Because the sodiums are in there, and it's impermeable to sodium, and the gates - yes, you could have the gates, but it's only going to - if you're dependent on the fusion, what, you're going to end up with equilibrium. You want all the sodiums out of there as quickly as you can get them.
So now, we can get rid of this, and now that the sodium has rushed in, we can get rid of this. But now we can bring in the big guns; we can take a look at what really happens inside of this thing to reestablish the positivity outside. So here's what's going to happen: Holy mackerel, look who's coming, ATP. I've been warning you guys about ATP. ATP has something to do with energy, and so we have ATP going to be involved with this, and since ATP is involved, it must be active transport, and that's going to be the first step.
Step number one: Here's the sodium inside; we've got a whole bunch of sodium in here, remember, rushed in through the gated channels. And now the sodium is going to bond to here, here, and here. In other words, what we have is a membrane protein that has some sites that can bond sodium. So the first thing that happened is the sodium bonds there. Then what's going to happen is ATP is going to get rid of one of its phosphate groups.
Now ATP - I'm going to give you a little clue now. This is killing me to do this, but ATP has three phosphate groups on it. It's going to take one of these phosphate groups, and it's going to bond it right here. So the ATP is going to bond there, and now we just saw the input of energy. You just saw a way active transport is going to work. You just saw, in this little diagram right here, the release of energy by ATP. Are you guys excited? Wait, there's more.
So now that the ATP is bound there, we're going to be able to go to our next step, which is going to change the shape of this protein. And because the phosphate group bonds there, the protein changes its shape, and it almost looks a little bit like facilitated diffusion, doesn't it? Remember facilitated diffusion where we had the thing open like this, the thing's bound in there, and then it changed its shape and it dumped them inside? I have a question for you: Why isn't this facilitated diffusion, then? Why isn't this the fact that just because it's changed its shape? The answer: Right there. It's an energy-dependent reaction. You need ATP.
So now the shape changes and look, we just pumped three sodiums out. But that's not all. Now we can't get any more sodiums out there unless we reopen this another way. And golly, gee whiz, here's what's going to happen now: As soon as the sodiums leave, the phosphate breaks off. And so now, this thing - and look, we're calling that a PI, which stands for inorganic phosphate; it's just breaking off there, it's no longer a part of anything. It's a phosphate PO[4] just kind of floating around, right?
Now look what's going to happen: When this thing comes out here, remember we have some potassium out here. So we are going to bond 2 potassiums in there, and then that's going to cause it to change its shape back. And so it goes: sodium bonds; ATP bonds; boom, out come the sodium; the sodium come out, and in come the potassium; boom, out go the potassium - what's going to happen next? Now that we're open to the inside of the cell again, the potassiums will come back into the cell, and the sodium, more can get pumped out.
Picture this: We in essence have a series of revolving doors here; that's in essence what we have, driven by an ATP. For every 3 sodiums you kick out, you bring 2 potassiums in - 3 sodiums, 2 potassiums; 3 sodiums, 2 potassiums. How quickly do you think you're going to reestablish that gradient? - because you have a lot of these throughout the cell membrane. This is using energy - active transport - to reestablish the gradient so that you can send another impulse along that - so that when we're done, once again, we will be loaded with sodium on the outside, we'll still have some potassium out here, but we've reestablished our potassium concentration fairly quickly, and we also - but even most importantly - we have put our sodium to the outside, and there's no more sodium in there. That is the sodium-potassium ATP pump, you guys. You've got a lot of nerve, and let me tell you, your nerves do a great job.
Cell Biology
Cell Transport
Active Transport: The Sodium-Potassium Pump Page [1 of 2]