hi folks welcome back to the next video this video is all about the cell membrane and so the cell membrane is comprised of four different components uh you may have only learned three but we're going to add a fourth one here and so hopefully you remember things like a phospholipid bilayer proteins we'll call it membrane protein to be specific and carbohydrates here uh phospholipid bilayer this is the bulk of our cell membrane this is pretty much what most the cell membrane is made out of and because of that the properties of the phospholipid bilayer heavily dictates kind of how the cell membrane kind of operates and functions then we have proteins and you can see the proteins are kind of studded within the phospholipid bilayer sometimes you'll have some that kind of face just inside sometimes you'll have proteins that just face it outside sometimes you'll have protein to kind of span the whole kind of thickness of the cell membrane or the phospholipid bilayer here either way these proteins here provide transport functionality here and we'll really get into that in the future videos here but they kind of help transport various different chemicals and other materials or at least allow for that and other functions like self signaling and other things like that that we'll get into later in the semester then we have carbohydrates this we won't really focus on too much in this video series or really in the semester but these carbohydrates kind of line the outside of the cell membrane here and they kind of are used for cell to cell recognition so other cells can kind of sample these carbohydrates here and then determine kind of that identity and type and origin of the cell the last part here are cholesterol so cholesterol here these are um these things here right here here these are molecules these are lipid molecules right that are embedded within the phospholipid bilayer and the function of them is to provide fluid fluidity and stability to cell membrane this one's a little bit hard to explain but one way to think about this is if you think about water actually so if you think about water water boils at 100 degrees Celsius and it freezes at zero degrees Celsius right and if you have pure water that's the case but if you start throwing things into the water you throw salts or electrolytes then the boiling temperature and the freezing temperature change the boiling temperature actually increases and the freezing temperature decreases and now to get too technical about it but the reason that's the case is for water to evaporate or for water to freeze they have to take on kind of new chemical forms and for water if it's pure water that's easy to do if there's only water molecules around but if there's other molecules that get in the way like salts and whatnot it makes evaporation or freezing basically chemically harder to do so it takes a colder temperature to freeze or a hotter temperature to boil all right think about that for the cell membrane here the phospholipid bilayer can also freeze it can freeze up quite literally and it can also break apart especially due to hot temperatures and if the cell membrane was purely made of phospholipid bilayers you know the phospholipid Biola could freeze that kind of you know cool uh kind of Colder temperatures and bursts at hotter temperatures but if you add cholesterol something that's not a phospholipid to this mix here it will that kind of increase the resilience to kind of rupture or to kind of freeze up kind of like the same way if you add salt to water it kind of increases the barrier to freeze and boil hopefully that makes sense if not don't stress about it too much it's not super critical that you understand the exact kind of mechanisms of why cholesterol is embedded in the cell membrane but just know that's what cholesterol does for cell membranes hence that's why also cholesterol is really important for our body because we need it to kind of keep the stability and fluidity of our cell membrane so let's focus on that uh phospholipid bilayer because remember cell membrane is mostly made up of this fossil lipid bilayer so if you just kind of take a small chunk and zoom into it this is what you see right so we just have a small chunk of the bilayer here here's the extra cellular fluid here the intracellular fluid here and then each one of these kind of yellow things the two tails here that's a fossil lipid here and in fact if you just get a bunch of these phospholipid molecules throw them in water they will naturally make spherical membranes kind of like what you see here they just naturally do that and the reason they do that is because these fossil lipid molecules they have a polar and a non-polar portion of their molecule now that's really weird because in the last video we talked about how you can have molecules that are covalently bonded they can be polar or nonpolar and that's true right you can have water which is polar and then you can have like oxygen molecule or like a fatty acid which is nonpolar right but you can also have molecules that have regions that are polar and regions that are nonpolar and those molecules here are called amphipathic molecules so in this phospholipid the head portion this kind of spherical portion this part of the molecule is polar actually right it has an uneven distribution of charge but these long tails they are reminiscent of those fatty acid chains and so these are hydrophobic so again if you throw fossil lipids it's just a molecule into water they'll form the spherical oh these kind of uh spherical kind of membranes on their own because what happens is the heads will try to line up to be where the water is at it's extra cellular food has mostly water so all the heads will point this way and then there's fluid here right and just so fluid there's water here right so all the heads here will point this way because the polar parts are attracted to water right there the hydrophilic heads and the hydrophobic tails and all polar parts will stay inside to stay as far away as possible from the water and so that's how you get your phospholipid bilayer just by the fact that these molecules are ampipathic here now this kind of this chemistry here basically is really important because it determines or helps determine what kind of molecules can pass through the phospholipid by there we're not considering you know the membrane proteins or anything other special parts of the cell membrane if you just look at the cell membrane there just a phospholipid bilayer portions here again we have the similar diagram where here is the outside of the cell this is the extracellular fluid here this is the intracellular fluid in here this is our phospholipid bilayer we're going to ask a simple question what molecules are able to pass through this membrane right and so if you go back to the previous picture you can imagine like you know these are these there are these tiny gaps in between these molecules you can Envision that you know there are some molecules that can pass right through so what types of molecules can do that right so first of all proteins that typically can do that one reason and it's pretty obvious for the most part proteins are pretty big as a molecule they're pretty chunky right they're big molecules they can't squeeze in between the fossil lipids to get by so we have this kind of bludgeoned Arrow showing they can pass through so proteins on the inside proteins outside on their own they're not passing through the phospholipid bilayer but this lipid here is a little bit can do so because lipids are nonpolar and the bulk of the volume of the phospholipid is also non-boy yeah we have the I just told you we had the polar heads here and here right but most of this volume here is nonpolar those hydrophobic tails and so because of that remember like dissolves like these lipids can easily intermingle within these kind of hydrophobic tails and kind of pass through so lipids typically can pass through cell membranes or anything nonpolar really can pass through these membranes but not these proteins so already we have a situation where this fossil a little bit by there lets sum things in but not other things and because of that you can say that the files a little bit by there is a select has selective permeability meaning it can things like lipids can pass through it it's in in other words you can say lipids kind of permeate can permeate through this bilayer or but protein scan so it's select in that sense it's elected about what can permeate or what can pass through this phospholipid bilayer all right so proteins can go through but lipids can what else ions ions cannot why is that because fossil lipid lipids are electrically neutral and ions that hopefully if you remember from chemistry they are not electrically neutral right and so because of that they kind of repulse each other right if there's a positive ion the even though this is electrical electrically neutral here uh the ions just going to bounce off it right and vice versa if this ion was an anion it would bounce off this electrically neutral um uh phospholipid bilayer water for the most part doesn't pass through kind of for the opposite reasons of lipids water is a polar molecule so for the most part water can pass through because it is a polar molecule then carbohydrates carbohydrates they're polar so again they can pass through and compare the water they're actually pretty big and so they have a size and a polar reason why they can't pass through right so to summarize here really what molecules can pass through the phospholipid bilayer it's really chemicals that are small nonpolar and electroneutral right so these kind of molecules typically do that a good example this steroids steroids are small they're nonpolar and electric and neutral so they pass through easy right now you may think about this a little bit more like wait a minute water is really important to the cells carbohydrates are really important to the cells if these don't get through how does the cell do its business how can it survive and that's where we get other modes of Transport that are not purely based off diffusion so in the next video we're going to talk about diffusion because that's a really important mode of transportation but then we're going to start talking about the fancier forms of transportation that allows things like water and carbohydrates to pass through because again they can't pass through by the phospholipid bilayer they're going to find other ways to do so okay folks that's it for this video I'll see you for the next one