 So remind yourselves, we're talking about channels now. And remember that channels are an example of facilitated diffusion. So a channel is basically a tunnel in the cell membrane. I drew one here. This is an example of a channel. And now I'm going to talk about two molecules that are super important to us that have channels in the cell membrane. And that's potassium and sodium. And if you think about it, like, let's go ahead and just draw... Oh, look, I already started drawing you the potassium channel. Think about potassium. Potassium is... Oh, gosh, let's just make sure I have enough room here. Potassium is an atom that quickly loses an electron. So it's pretty easy to steal potassium's electron, which is how it ends up with a positive charge. But it's kind of big for a life-oriented ion. And super important. So I'm going to draw a picture of it. What just happened? Seriously? That's not cool. That cannot be what my potassium looks like. Okay, so potassium is relatively large. Because of its positive charge, it also ends up having water molecules attached to it, like water molecules kind of, I don't know, hug it, stick to it, stick to it is better. Because water molecules are polar, and you know that already. Compare that to sodium. We also have sodium channels. I'm going to draw my sodium channel over here. And my sodium channel, I'm going to draw it this way on purpose. I'm going to draw my sodium channel significantly narrower. And I'm going to let you think about why that is. Go ahead. Is sodium or potassium a bigger molecule? Potassium is bigger. Potassium has a molar mass of like 39, and sodium has a molar mass of, I think, 22. So the, I mean, sodium, I don't know what I just said, but sodium has a molar mass of 22. Sodium is smaller than potassium. Okay, right now. If you look at this, are you like, dude, I know why potassium can't get through a sodium channel. Do you know why? Look at it. Potassium's huge. Is potassium going to be able to fit through the sodium channel? Not a chance. So that alone is going to make it like a deal breaker for potassium, and there's no way that potassium is going to get through the sodium channel. Sodium is also attached to water molecules. And now watch what happens here. In fact, I need to make my water molecules more realistically sized. Okay, something like that. All right. You might look at this and be like, okay, sodium is smaller than potassium. So why can't sodium go through the potassium channel? They're both positively charged. Like, if potassium can go through, why can't sodium? Watch this. Potassium channels have these like brushes on them. And it's like a little entry gate. And watch what happens. The entry gate, those little things on the potassium channel touch the water molecules on the potassium and like scrape the water molecules off the potassium. Once the water molecules are scraped off the potassium, guess what potassium can do? Can totally go through the channel. Like, no problem whatsoever because the water got scraped off. Take a look at if I were to draw in this view, if I were to draw this sodium atom with its water molecules attached. Still, I mean, same thing. It's an ion. It's positively charged. But check out, I can't brush off the water molecules off of the sodium ion, which means by the time this guy gets to the potassium channel, it's like too big to go through. Uh-uh, nice try, doggy. You did not get your water molecules brushed off of you. You can't come through this opening. Seriously? Are you ready for the potassium or the sodium channel? Because look at the sodium ion with its water molecules. How in the world is it going to get through? Are you worried? Well, you should be worried because if you can't scrape off the water off of your sodium, you're not getting the sodium through. Guess what it does? The sodium channel is covered with negative charges inside that are like, we love you, sodium. Please come into this awesome negative charged tunnel and we'll have a tea party on the other side. And the sodium ion is like, I'm feeling the pull. I can't, it's like the tractor beam. Is that from Star Wars? The tractor beam is pulling the sodium in. And sure enough, that negative charge is so strong that the negative pulls that sodium and the sodium water molecules just get sloughed off and the sodium gets water sloughed off and passes right on through. Potassium is like, I want to go through the tractor beam too, but can the potassium molecule even fit? Look at that thing, it's huge. I'm sorry, potassium, you're going to have to drop some pounds before you can go through this tractor beam. That's like good luck because if potassium drops some pounds, then it's not potassium anymore, right? And then it's going to be sodium and then it can totally go through. How unbelievable is that? Channels can be gated. So both of these channels can actually have doors on them. And you can imagine that if I had a big old gate on my channel, I don't care how big the tractor beam of negative charged pull is. I don't care how great the water molecules are being scraped off. If there's a lid on the tunnel, you're not going through. Now, not only can you have gated channels, you can have gated channels that open depending on certain stimuli. So if you flick the channel, like apply a mechanical stimulus to the channel, like my small people do to each other's heads, you will actually open the channel. And then you can imagine that you can't get in. You can get in. And we can actually send messages that way. Okay, channels are so cool. If you have a ton of channels in your cell membrane and you open them all up, you can imagine that diffusion can actually happen super fast through a channel. I love that so much. Now, I mean carrier proteins are equally as cool as channels. So let's go talk about carrier proteins next.