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Passive transport and selective permeability
- [Voiceover] - What I wanna start thinking about in these videos are ways for molecules to go across a cellular membrane, either from the outside to the inside, or from the inside to the outside. And the first type of transport of molecule across membranes that I'm gonna talk about is transport that does not require energy. It's all about molecules moving down their concentration gradient. And that type of transport we call passive transport. Passive transport. So it does not require energy. It's really just about things moving down their concentration gradient. Let me write, "Move down concentration gradient." Move down concentration, concentration gradient. Now, if you have this cellular membrane, a lot of things might want to move down their concentration gradient, but this membrane is selectively permeable. It's going to be more or less permeable to different types of molecules. So let's think about these different types of molecules and think about how they might diffuse passively across the membrane. So if we have really small molecules, we can say, "OK, they might be able to fit in the gaps "between the hydrophilic heads, "and they might be able to fit between the gaps "of the hydrophobic tails and get through." So being small is good. If you're small, that aids transport, passive transport, aids diffusion across the membrane. And in particular, it really helps to be small and non-charged. Small and no charge. So examples of that could be things like carbon dioxide. So carbon dioxide, it's a small molecule, it doesn't have a charge. So carbon dioxide molecules, if I have a higher concentration on the outside, on the outside, actually let me do the other way around. Let's say I have a higher concentration on the inside than I have on the outside. Well, just as we learned in the diffusion video, in a given amount of time, you're gonna have more carbon dioxide molecules interacting with the bottom, going from inside of the cell and interacting with the membrane, than from the outside of the cell. And sure, they don't have any charge, and so they're not going to be particularly attracted to the hydrophilic head of our phospholipids, but they're also not going to be repelled by them. And you're gonna have more on the inside interacting with the membrane than the outside, and so, since they're small, some of them are gonna able to pass through, and they're also not going to be bothered by the hydrophobic tails, and you're gonna have things going both ways, but you're gonna have more going from the inside to the outside than from the outside to the inside. So they're gonna move along with their concentration gradient. So carbon dioxide can actually diffuse quite well across cellular membranes. Another molecule that can is molecular oxygen. Molecular oxygen can also diffuse quite well across cellular membranes. So if I have a higher concentration of oxygen on the outside than I have on the inside, because it's small and it's non-charged, it's not gonna have problems. It's not gonna be particularly attracted to the hydrophilic heads, but they're small and they're gonna be able to pass right between them, it's going to be indifferent to them, and then it's gonna be able to pass through all of these hydrophobic tails, and since you have a higher concentration on the outside than the inside, you're just going to have more in a given amount of time, more random interactions of the ones going in that direction than the ones going in that direction, so you would have a net inflow into the cells. So these things are going to be able to diffuse fairly... whoops, these things are going to be able to diffuse fairly naturally. And of course, they are going to be obstructed by just the structure, by all of these molecules here that make up the actual cellular membrane, but they're going to be able to get through. Now, what about things that would have a lot of trouble getting through? So things that would have a lot of trouble getting through, would be things like a sodium ion. A sodium ion. Or a potassium ion. Why would they have trouble getting through? Well, let's just imagine. Let's say I have a higher concentration of sodium on the outside... than I have on the inside. Well, they might be attracted to the hydrophilic heads here that have some charge, but there's no reason why they would then want to go any further. They're going to be attracted to the hydrophilic heads that have charge... and the hydrophobic tails have nothing interesting for them. They're gonna wanna maybe clump around the... phosphate heads, but not be able to migrate all the way through. So things that have outright charge are gonna have trouble just passively diffusing. We'll see in future videos that there's other ways for them to get through. You have things like channel proteins, which essentially give them tunnels, and we'll talk more about that. But just naturally, natural diffusion is going to be hard for things like this. Now, what about things that are in between? What about things like water molecules? And water is incredibly important, because cells are living in an aqueous environment. They're surrounded by water on the inside of the cell and the outside of the cell. And water is in between, because it doesn't have an outright charge, but it has a partially. Water molecules, oxygen, two hydrogens. Oxygen likes to hog the electrons. It has a partial negative charge on that end. The hydrogens have their electrons charged, have a partial positive charge end... a partial positive charge on that end. And we call these phosphate heads hydrophilic, because they're attracted to water, and water is attracted to it. So the water molecules, for sure, are going to be attracted, are going to be attracted to the hydrophilic heads. but their charge isn't so strong that they can't, if you have enough interactions, a lot of them will be attracted, but some of them will actually make it through. The water molecule is small enough, and its charge is not strong enough, I guess you could say, It has some polarity, but it's going to be able to make it through, not as easily as carbon dioxide or the molecular oxygen, but it will be able to slowly diffuse through. And as we'll see, there's other ways that this can be facilitated, where the water can go through once again. We'll see in future videos things like aquaporins, tunnels through the membranes, so it doesn't have to deal with all of this business right over here. And of course, if you have really large molecules, if you had a big, honking protein right over here, this would have trouble. This would have trouble. It would have trouble even physically getting through the gaps, not to mention whether parts of it are hydrophobic or hydrophilic. So hopefully this gets you a sense of the types of things that can diffuse through a cellular membrane. This is a form of passive transport, and in the next video, we'll talk about facilitated passive transport.
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