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Current time:0:00Total duration:7:00

Passive transport and selective permeability

AP.BIO:
ENE‑2 (EU)
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ENE‑2.E (LO)
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ENE‑2.E.1 (EK)
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ENE‑2.E.2 (EK)

Video transcript

what I want to start thinking about in this 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 going to talk about are is transport that does not require energy it's it's all about 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 so down 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 well they might be able to fit in the gaps between the the hydrophilic heads and maybe they might not be and they might be able to fit between the gaps of the hydrophobic tails and get through so being small is good so small if you're small that ADEs transport passive transport i'll aids diffusion across the membrane and in particular it really helps to be small and non charged small and no 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 then 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 the going from inside of the cell and interacting with the membrane then from the side of the cell and ensure 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 they're going to have more on the inside interacting with the membrane on the outside and so since they're small some of them are going to be 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 out of oxygen on the outside then I have on the inside because it's small and it's non charged it's not gonna have problems you know it's it's not gonna be particularly attracted to the hydrophilic heads but they're small they're gonna be able to pass right between them it's gonna be indifferent to them and then it's gonna be able to pass through all 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 fairly whoops these things are going to be able to diffuse fairly fairly naturally now and of course they are going to be obstructed by 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 like a sodium ion a sodium ion or a potassium or a potassium ion why would they have trouble getting through well let's just imagine let's say I have a higher concentration on sodium on the outside on the outside then 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'll be they're going to be attracted to the hydrophilic heads that have charged and they're gonna they're not going to and the hydrophobic tails have nothing nothing interesting for them they're gonna want to they're gonna want to maybe clump around the the 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 the fusion 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's not doesn't have an outright charge but it has a partially water molecules oxygen two hydrogen's oxygen likes to hog the electrons has a partial negative charge on that end the hydrogen's have their electrons charged have a partial positive and a partial positive charge on that end and we've called these phosphate heads hydrophilic because they're attracted to water and water is attracted to it so the water molecules 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 it 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 it's 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 a carbon dioxide or the molecular oxygen but it can it will be able to slowly diffuse through and as we'll see there's other ways that this can be facilitated right where the water can go through once again these we'll see in future videos things like aquaporins a tunnels through through the membrane 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 like a big if you had a big honking protein right over here this would have trouble this would have trouble it would have probably 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 in the in the next video we'll talk about facilitated passive transport
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