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Video transcript

let's explore the fluid mosaic model of cell membranes now why is it called the fluid mosaic model well if we were to look at a cell membrane and just to be clear what we're looking at if this is a cell right over here this is a cell and this is its membrane this is the membrane it's kind of what keeps the cell the inside of the cells separated from whatever is outside the cell we're looking at a cross-section of its surface we're looking at a cross-section of its surface we're down here down here this is inside the cell if we look at it relative to this diagram this is inside the cell and this is outside this is outside and when you zoom in when you zoom in and this this little part right over here this is actually a phospholipid bilayer that forms it and so when you hear that you might say well what is a phospholipid and it's a good question because when you understand what a phospholipid is it starts to make sense why it would form a bilayer like this and why is the basis for so many membranes and biological systems so this is indicative of a phospholipid and as its name implies and let me write that down this is a phospho phospho lipid it's a lipid that involves a phosphate group and in general the word lipid and we have a whole video on lipids means something that that doesn't dissolve what so well in water and that's true is the case of this phospholipid you have these hydrocarbon tails that are coming from fatty acids and so these hydrocarbon tails they have no obvious charge or no obvious polarity we know that water is a polar molecule that's what gives us its hydrogen bonds and it's attracted to itself but these don't have those and so they're they're not going to be attracted the water the water is not going to be attracted to it to them and so that these tails are hydrophobic so you have hydrophobic hydrophobic tails and these are really kind of the lipid part of the phospholipid and then you have the phosphate head you have the phosphate head right over here and as you can clearly see this has some charge charged molecules do well in polar substances like water they're going to dissolve well and so this part right over here is going to be hydrophilic hydro hydrophilic and actually molecules that have a hydrophilic part and a hydrophobic part there's a special word for them amphipathic a word that i sometimes have trouble saying so phospholipids are amphipathic and fair Pathak which means that they have both a hydrophilic end a part that is attracted to water and a hydrophobic end that is not attracted to water and hopefully that starts to explain why they organize themselves in this way because you can imagine the hydrophilic heads are going to want to be where the water is which is going to be either outside the cell or inside the cells and the tails are hydrophobic the water is going to go away from them or they're going to go away from the water and so they're just going to face each other and they're going to be on the inside of the membrane but the really cool thing is a structure like this having this amphipathic molecule allows things like these by leopard these these lipid bilayers I should say to form it's actually fascinating if we think that if you go far back enough even before life in cellular form formed that you might have had phospholipid spontaneously forming these these tears of that where you have a bilayer a lipid bilayer so you could imagine something let me see if I drew a cross-section let me see if I could draw it relatively neatly so I think I'll draw half of it just because you get well I'll draw the whole thing hopefully you get the idea so that would be one layer of the phosphate heads facing the outside this is the inner layer and I'm doing a cross section right over here and then you have your hydrophobic tails so your hydrophobic tails let me do that in a different color so your hydrophobic your hydrophobic tails I think you get the picture we have a bunch of hydrophobic tails on either end and you could spontaneously form a structure like this which starts to feel like a well maybe there's a protocell forming and obviously to actually have real life you have to have some form of information that can be passed on and you have to have some type of metabolism and the cell is living in all of the definitions of life but at least this basic structure of the cellular membrane you can imagine how it forms in kind of a in a pre life state even by by virtue of amphipathic molecules like a phospholipid so fair enough we're able to form this phospholipid bilayer but what are all these other things that I have drawn here well these are proteins and these are examples of so this is a protein right over here this is a protein this is a protein and I just drew some kind of blobs just be indicative of the variety of proteins but the important thing to realize is is when you think of cells there's all of this diversity there's all of this complexity that is on or embedded inside of its membrane so instead of just thinking of it just kind of as a uniform phospholipid bilayer there's all sorts of stuff there's all sorts of stuff maybe this review this is a cross-section there's all sorts of stuff embedded in it and we see it right over here in this diagram you could say there's a mosaic of things embedded in a mosaic is a picture made up of a bunch of different components of all different colors and you can see that you have all different different components here different types of proteins you have proteins like this that go across the membrane we call these transmembrane proteins they're a special class of integral proteins you have integral proteins like this that might only interact with one part of the the bilayer while these kind of go across it you have things like glycol lipids so this right over here this is a glycol lipid glyco glyco lipid which is fascinating it lodges itself in the membrane because it has this lipid end so that's going to be hydrophobic it's going to get along with all of the other hydrophobic things but then it has an end that's really a that's a chain of sugars and that part is going to be hydrophilic it's going to sit outside of the cell and these chains of sugars these are actually key for cell-cell recognition your immune system uses these to differentiate between which cells are the ones that are actually from my body the ones that I don't want to mess with the ones I want to protect and which cells are the ones that are foreign the ones that I might want to attack when people talk about blood type they're talking about well what type of specific glycolipids do you have on certain on cells and there's all sorts of and that's that's not all we're talking about when we talk about glycolipids as kind of a way for cells to to be recognized or to be tagged in different ways so it's a it's a fascinating thing that these chains of sugars are can lead to such complex behavior and frankly such useful behavior from our point of view but you don't just have you don't just have sugar chains on lipids you also have sugar chains on proteins this right over here is an example of a glycoprotein glyco glycoprotein and as you can see when you put all this stuff together you get a mosaic and I'm actually not even done there you have things like cholesterol embedded cholesterol is a lipid so it's going to sit in the hydrophobic part of the membrane and that actually helps with the fluidity of the membrane making sure it's not too fluid or not too or not too stiff so this is cholesterol cholesterol right over there so you see this mosaic of stuff but what about the fluid part and I just talked about cholesterol value and making sure that it's just the right amount of fluidity what's neat about this is this isn't a rigid structure if if this thing were to be jostled around a little bit or maybe it would be plucked out somehow the phospholipids would just spontaneously rearrange to fill in the gap you can imagine these things are all flowing around that this membrane actually has a consistency of kind of of oil or salad dressing so it isn't this dis you know it isn't like a rubbery texture like you might imagine a membrane like a balloon it's actually fluid if these things can move around but it has but even though it's fluid it's good enough to separate the two environments the the environment inside the cell from the environment outside of the cell and that's where the named fluid mosaic model comes from
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