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Electrochemical gradients and secondary active transport

How a cell can use a molecule's electrochemical gradient to power secondary active transport.

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Video transcript

- [Voiceover] In the video on the sodium-potassum pump, we talk about how it helps a cell establish its resting membrane potential. And it does that by pumping, actively pumping, three sodium ions out for every two potassium ions it pumps in, and that by itself, that ratio of three to two by itself doesn't establish the full resting membrane potential, but then the potassium ions are allowed to start diffusing down their concentration gradient from the inside back to the outside. And of course there's a balancing force there, or a balancing factor, and that's the charge. Because if the outside is more positive than the inside, a positively-charged ion, which the potassium ions are, well, they're not gonna want to go up here so much because of their charge. It's more positive here than it is over here. They'd actually want to go back, but their concentration gradient, they're going to be bumping into the bottom of this channel more than the top, and so you're going to have a balance. They're going to start diffusing through, but you're not going to have equal concentrations because the charge is going to keep them back here. But what about the sodium ions? The sodium ions are getting more and more concentrated up here, and up here is getting more and more positive. If the sodium ions were left to their own devices, if there was no membrane over here, they would naturally, if we just looked at the concentration gradient, they would naturally want to diffuse down. We have a high concentration over here, we have a low concentration over there. So if there was no membrane, then they would just naturally diffuse from high to low. That's their concentration gradient. And also if there was no membrane, we've already talked about it being much more positive on this side than it is on this side. Or you could say we have a positive potential difference between here and here. So the positively charged ions, like the sodiums up here, would want to go down because of their charge. And so there's two reasons why they would want to go from this side of the membrane to that side of the membrane. Their concentration gradient and their charge, the electric potential. There's this potential energy of them wanting to get away from all the positive charges. And so that combined motivation for the sodium ions to go in that direction, we call that the electrochemical gradient. Electrochemical gradient. And I already said it once, but I'll say it again. It's a combination of the electric gradient and the chemical gradient. The chemical gradient, you have higher concentration here, lower here, you would want to diffuse down, more things are going to bump on this side than on this side, so you're going to have a net flow down, if you didn't have this membrane here. And then when you think about the electric potential, more positive on this side than on this side, so positive ions would want to go down. And so you could view this gradient as a source of potential energy. And cells, in fact, use this gradient, in fact, the sodium electrochemical gradient as a source of energy. And so let's say this protein right over here, this is what we're going to call a symporter. This is a symporter. And what it does is, it uses the electrochemical gradient of one ion, in this case sodium, so it uses the fact that sodium really wants to go through the membrane, and it uses that energy. Imagine like water falling down a waterfall and it can turn a turbine or turn a water mill type of thing. And so it uses that energy of the sodium flowing down its electrochemical gradient, it wants to go in this direction for two reasons, concentration and electric potential, or I guess you'd say its electrostatic charge, and then it uses that energy to transport other things. And the most famous symporter with sodium is glucose. It's going to use that, the sodium and the glucose are going to go together. And the glucose is being transported against its concentration gradient. And so if you're going to transport something against its concentration gradient, you're going to have to use active transport. So this concentration gradient, so let me be clear on glucose's concentration gradient, it looks like this. You have high concentration over here and you have low right over here. And the cell might not want to waste all this glucose, it wants to get as much glucose into the cell, or across the membrane, as possible. And so it's going to have to do some active transport to go against its concentration gradient. To go in this direction. And over here, the source of energy to go against the concentration gradient, is the stored potential energy from the electrochemical gradient of the sodium. And so this type of active transport, where you're using the energy that was stored up through another form of active transport, the sodium-potassium pump, we call this Secondary Active Transport. So what's going over here, this sodium-glucose symporter, this is Secondary Active Transport. Secondary Active Transport. It's using the stored energy from the electrochemical gradient of one molecule, it's using that stored energy to drive the active transport of another molecule, glucose, going against its concentration gradient.