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Uniporters, symporters and antiporters

Passive transport is the movement of substances across a cell membrane without the use of energy. Examples include diffusion and facilitated diffusion. Active transport describes the use of energy to move molecules across a cell membrane, usually against their concentration gradients. Membrane proteins involved in active transport include symporters, antiporters, and the sodium-potassium pump.

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Video transcript

- [Voiceover] So let's talk about all of the different ways that molecules can be transported across a cellular membrane. So perhaps the most basic, the most passive of the passive transports would be straight up diffusion. And if you have a small enough molecule, let's say this is molecular oxygen. It's small, it doesn't have any charge, it has no polarity. That will be able to diffuse down its concentration gradient through the cellular membrane. But as we start to talk about things with more charge or things that might be larger then we're going to need some help. Now the first type of help is just help to allow things to flow down their concentration gradient. And that we call facilitated diffusion. We have a whole video on facilitated diffusion. And one form of facilitated diffusion, Hey, just open up a tunnel and let things flow down their concentration gradient. We saw that with the potassium ion channels, where potassium builds up on the inside of the cell because of the sodium potassium pump, let me just be clear, down here this is the inside, this is the outside of the cell, and then but then these channels allow the potassium to flow down their concentration gradient. It's gonna be put in check because of its charge and its more positive outside and we'll talk about in other videos, but it's just a simple tunnel. Now sometimes that tunnel is gated. It's only going to be open if a certain trigger is hit. And we see that when we talk about signals going down a neuron voltage gated channel. Once the voltage hits a certain amount, then the channel opens, and then the sodium that has a higher concentration outside, can flow down its concentration gradient inside. But both of these, this is considered passive transport, it's facilitated diffusion, passive transport, we're allowing things to flow down their concentration gradient. You can see here, the sodium is going in, I'm sorry, the potassium is going in the direction of its concentration gradient, it's high concentration inside, low concentration outside, so we're allowing it to flow down the concentration gradient. Here the sodium is high concentration outside, low concentration inside, and this happened because of the sodium potassium pump, but we're allowing it to now flow down its concentration gradient. Now let's talk about active transport. So passive transport doesn't require any energy to make this stuff happen. It's just about things flowing down their gradient. In active transport, we're either directly using energy to make something go against its gradient, or we're using some energy from a previous active transport to help facilitate something else going against this gradient. So first let's talk about primary active transport, 'cause this might be a little bit more easy to think about. None of them are that daunting. And the best case of this, if we're talking about animal cells, is the sodium potassium pump. The sodium potassium pump, super important for establishing resting membrane voltage, I guess you could say resting membrane potential, but the concentration gradients it establishes are also very important. The sodium having a high, it establishes, it pumps sodium ions out of the cell against its concentration gradient. So we say that sodium ions already have a higher concentration outside, but it keeps pumping them out. And to do that, it needs to use ATP. It breaks up ATP into ADP and a phosphate group. It hydrolizes it. And so that's why it's sometimes called an ATPase. It's an enzyme that helps break up ATP. But it uses that, and it uses that energy to pump sodium out of the cell and potassium into the cell. And then, as we'll see, that sodium that's pumped out of it, that kind of forms a potential energy because it starts to build a chemo-electrochemical gradient which can later be used to power secondary active transport. We'll talk about that in a few seconds. Now this is in animal cells, the analog in plant cells, fungi, protus prokaryotes is the proton ATPase, or the proton pump, which does the same thing, but it does it, instead of doing it in two directions, it does it for protons, it pumps the protons out of the cell against their concentration gradient. So even though you have a higher concentration outside than inside, it'll continue to pump them out, but to do, to power it, it uses ATP to change its conformation in the right way. And so that's why it's of, this is often called the proton ATPase, this is called sodium potassium ATPase, that's our friend the sodium potassium pump, this is called proton ATPase, and you wouldn't see these in the same cell. So maybe I'll draw a little line over here, this would be in plants, fungi, protus, things like that. This would be in animal cells. But both of them are actively using energy. They are directly using ATP to transport things against their concentration gradient, which is why we call it active transport. Now, because you have these concentration gradients, or these electrochemical gradients are established, those can be used to do other forms of active transport. And that's what we call secondary active transport. So this right over here, this is my little depiction of a symporter, and this is a sodium glucose symporter. And what it does, is it leverages the sodium flowing down its concentration gradient. And once again, that was established with the sodium potassium pump, so it's flowing down its concentration gradient, but it's leveraging that energy, you can imagine putting a little wheel under a waterfall to make it spin, to also make glucose, to transport glucose against its concentration gradient. So the glucose already has a, will have a high concentration gradient here, low over here, but it's transporting glucose against its concentration gradient. We talk about it in other videos. And then another example of secondary active transport is an antiporter, or an exchanger. In the symporter, a cotransporter, they're both going in the same direction even though one is going with its concentration gradient, that's essentially powering it, and the other one is going against its concentration gradient, that's why it's active transport. With an exchanger, they're going in opposite directions, so you have the sodium calcium ion exchanger, and here the sodium is going down its concentration gradient, and that fuel's taking the calcium ions outside of the cell against its concentration gradient. So once again, anytime something is going against its concentration gradient, and once again, in this case, it's calcium, it's going to be active transport. But since the sodium ions, the sodium ions are going in a different direction than the calcium ions, we call this an antiporter, while this right over here is a cotransporter, a symporter. Now you might say hey, isn't the sodium potassium pump, isn't this an antiporter? Things are going in different directions? And the difference is, both of these, this is primary active transport and the sodium potassium pump, both of these things are going against their concentration gradient. And a true antiporter, it's really secondary active transport. One of them is going with their concentration gradient, going down it, which is providing the energy, to take the other thing against its concentration gradient. So anyway, hopefully this gives you a high level overview of the various forms of transport, and it gives you more appreciation for how beautiful and intricate and mesmerizing a cellular membranes and all the different things that cells have to do actually are.