The Lipid Bilayer

Hello everyone in this lesson. We are going to be talking about the lipid bi layer and the different types of molecules that are going to come together to form the outside of ourselves. Okay, so the lipid bi layer obviously is going to be the outer membrane of ourselves that allows ourselves to take in certain materials or get rid of certain materials and it protects ourselves from the outside world. Now membrane lipids are going to be composed of a combination of a whole bunch of different things and they're going to be composed of a combination of non polar tails and polar head groups. In general, this is going to be how the molecules of the lipid bi layer are going to be create they're generally going to have non polar or hydrophobic tails and they're going to have polar or hydra filic head groups and this is going to allow the lipid bi layer to have an internal hydrophobic layer and external hydra filic layers which gives it its unique properties. Now many molecules that build or make up the lipid bi layer are going to be an thep a thick Now these are going to be things like certain types of lipids inside of the membrane. Also, membrane proteins, especially trans membrane proteins are going to be empathic and what this means is is that they are going to have a hydrophone phobic portion and a hydra filic portion. Like we just talked about in the sentence above, they're going to have two different characteristics inside of the same molecule to give the lipid bi layer its unique characteristics. So now let's talk about some of the different types of lipids that you're going to find in the lipid bi layer because, well, we obviously have to have lipids in there because it is the lipid bi layer and the first one is probably going to be the most famous, the most recognizable of all the lipids. The phosphor, a lipid, I'm pretty sure you guys have heard of this before, but just remember that the phosphor lipid is the most common lipid found in the membranes of the cell, including the lipid bi layer and all those little vesicles and different types of membranes inside of the cell as well. And they are going to have a hydrophobic tail in a hydra filic head like we talked about above and they are really going to make up the bulk of this membrane, They are going to be the foundation of this membrane. Everything else is pretty much going to build off of these. These are just the building blocks for the rest of the membrane. Now, there can be different types of phosphor lipids. Generally, when we talk about phosphate lipids, we don't always talk about the diff types, we just talk about them in general but there are going to be different types and this is going to be based on the composition of their head group. Remember they have a hydro filic head group where you can have different types of molecules in that head group which is going to give them unique properties, which is going to give them their unique names. Some examples are going to be right here, but there's a whole bunch of different types of phosphor lipids that we're not really going to get into. I'll show you a picture of some down below, but we're not going to get into all those specific types. Just know that they exist. Okay, so the next type is going to be the stuffing go lipids which have a pretty cool name. And they are going to have a sting go sign. Let me highlight this. They're going to have a single sign which is going to have an amino alcohol with a long hydrocarbon tail linked to a fatty acid. But basically they're just another specialized type of lipid that exists inside of the lipid bi layer. And again, similar to the phosphor lipids. The composition of their attached groups gives them their unique names. Some examples are going to be right here. So what do these guys actually do? Well, these are believed researchers believe these things actually protect ourselves, protect the inside of ourselves by maintaining stability of the lipid bi layer and protecting the cell from foreign objects or environmental factors, not allowing those things into the cell. So basically they're scattered around the lipid bi layer and they believe they function as protectors of the lipid bi layer and maintain that stability and some of them because of their unique properties can be classified as phosphor lipids but not all of them. Okay, alright, so now let's move on to glycol lipids. Glycol lipids are going to be pretty interesting. These are going to be lipids that are going to be attached to a carbohydrate or sugar group. Right? So there are many different types of like oh lipids as we can see here. But again, their uniqueness, their unique names, Their unique properties are generally going to come off of the sugar that is attached to the lipids. Now, what do glycol lipids do? They're scattered around the cell membrane And what do they do? They are generally used for cell to cell recognition. So how do cells recognize each other? Generally, glycol lipids are placed in the cell membrane of the cell and it kind of acts like a name tag or a banner saying I am a red blood cell. That's type a glycol lipids are going to be utilized to tell which types of blood cells you actually have. So your blood cell type is going to be written down via your glycol lipids, whatever type of like a lipid you have. So usually these are going to be utilized to force l recognition and cell type identification. Okay, so we also have sterols sterols are going to be steroids that have a hydroxyl group attached to them and a short hydro carbon chain And generally they're going to be quite rigid. So one of the most famous examples of a store all is going to be cholesterol. We've all heard of cholesterol before. Usually when you think of cholesterol, you think it's not a good thing, there's bad cholesterol and there's good cholesterol and cholesterol is very, very important in animal cells. What does it do? It is actually going to allow the fluid nous, the fluid let the fluid capacity of the cell membrane. Now you may be thinking, wait a minute. You just told me that these sterols are very rigid. They are very rigid. How does this actually lead into the cell being more fluid? Like well, these cholesterol molecules are going to be placed at different areas inside of the cell membrane and basically it ensures that the cell membrane stays fluid. Even when it's really, really, really cold or it stays strong and solid, even when it's really, really hot. And maybe that fluid membrane wants to melt away. Those cholesterol molecules are going to ensure that it doesn't do that. So basically these cholesterol molecules are utilized to keep the fluidity of the cell membrane at a normal equilibrium point and you can actually add cholesterol to your cell membrane and you can take it away depending on the temperature that the cell is XP experiencing and I believe that on a normal temperature around 30% of your cell membranes are going to be cholesterol but that can change depending on the temperature. Okay, so now let's look at our examples. These are just some examples of the different types of fossil lipids, because usually when we talk about possible lipids, we don't talk about all the different types, but here are some interesting types and they're all going to be unique in one location and that's going to be the head or the hydro filic head portion of the phosphor lipids. And this is where they're going to get their unique names based on the characteristics of the molecules and the atoms that make up the hydra filic head of the phosphor lipids. And if you guys were wondering this what cholesterol looks like. It's a pretty rigid molecule that your cell puts into your membranes and takes out of your membranes depending on what it needs. Okay, everyone, I hope that was a great overview of the different types of lipids that you're going to find in your lipid bi layer. Let's move on to our next topic.

Okay so now we're going to talk about by layer formation and fluidity. So the important thing you need to know is that lipid bi layers are fluid. I feel like a lot of times when people are studying the cell they kind of think of it how you see it in the picture in your textbook just sort of this two D. Structure that's just sitting there and stuck in place. But that's not the case at all. Everything in the cell is moving and especially the lipid bi layer is moving. And so um lipids form a super highly flexible lipid bi layer when they're put into water. So if we just have a bunch of individual lipids and we just throw them into water it's actually the most energetically energetically favorable confirmation. Meaning that if we have these lipids these individual lipids and we just throw them all in a bathtub then they're going to form a lipid bi layer. We don't need enzymes for it. We don't need anything else sort of guiding the process. If you just throw them in together they'll automatically form a lipid bi layer because it requires no energy to do so right they just form it and its they like being that way it takes energy to actually separate the bi layer not form it so because it doesn't require any energy and they want to be in that confirmation when you throw lipids together they form a lipid bi layer and so this is actually super super important because if you just have all these lipids and you throw them in water they form this by layer. What you can do is you can actually form these self sealing compartment. So what do I mean by that? Well like a cell for instance a cell exist in lots of different environments. There are bacteria that live in water and it's just surrounded by water. But the bacteria itself is not affected by that water because the lipids have sealed all of its bacterial inside inside the bi layer. And so because lipid self seal, they automatically form that. And if something disrupts it, say that bacteria gets poked with something and it disrupts that by layer the bi layer just goes and fixes it automatically. It just comes back together. And so one of the examples that you'll often hear when discussing the lipid bi layer is using golf balls in a bathtub. Right? And so you can imagine that if you have a bathtub it's half filled with water and you throw a bunch of like a ton of golf balls in it, they're going to evenly distribute out across the bathtub. So what we mean by self sealing is if I were to just jump into the bathtub, right, I would create this huge um break in between the golf balls, right? Because my body is separating the golf balls apart. But if I were to remove my from the bathtub, those golf balls will come back together because they, because there's so many of them, they self seal lipids are the same way you have a lipid on a bi layer of a cell and it gets poked or wounded or damaged in some way it'll come back together self seals so they spontaneously form and they can reform if something tears them apart. And this is really important, mainly because it forms a boundary, it separates what's inside the cell versus what's outside the cell and that is what allows us to live like. That is really the most basic thing we need to live. We have to be able to separate ourselves in the outside world. And even the smallest, most simplest bacteria can do that because they have a lipid bi layer that separates themselves forms that boundary and does that without any energy and self seals if it were to be damaged. So this is super, super important for understanding biology, but also understanding evolution because this is one of the very first things that had to evolve to create life is the ability to separate what could potentially become living in the future to the external environment. So lipid bi layer is super, super, super important. Now we're gonna go back to that bathtub model with the golf balls. And so this model is actually called the fluid mosaic model, not with the golf balls, but with the lipids. And so the fluid mosaic model describes the nature of membranes. And what that means is that it just describes the fact that the lipid bi layer is moving around, it's fluid, it's not stuck in place. The lipids don't just bind to each other and just sit there rigid. Said they're moving around like golf balls would in a bathtub, if you fill them up, they potentially could spin, they would switch places with one another, um and they would just move all around the bathtub. And lipids do the same on the cell membrane. So there's three types of movement that you need to know about. We have lateral movement, rotational movement and traverse movement. Or you can call this diffusion uh Typically in your book, it's called diffusion. Um But another term for it, what the simplified version of it is how the lipids are moving. So in lateral diffusion. What it means is the movement of individual lipids and a single by layer um sort of switch places. So what you would need to know about this is if you have two golf balls next to each other in the bathtub and then all of a sudden they're moving, they're moving and then they switch places. So it's just moving across the surface of the of the bathtub for the golf ball. So it's sort of moving laterally or moving across the surface across the surface rotational diffusion is a single lipid, An individual lipid and it's when it's rotating. So you can imagine that golf ball is just spinning in place, right? So we'll think rotational is spinning in place And it actually in lip. I mean golf balls that are going to move this fast. But lipids in a violator can actually move super fast, up to 500 spins a second. So golf balls in a bathtub are not going to do that, but lipids can super fast and then the final one is traversed diffusion and this is the most rare. And what would happen is if that golf ball decided to traverse the bathtub and go to the bottom of the tub and just sit there at the bottom. Now, you can imagine that's going to be a super rare movement. Um and it also is for lipids, right, lipids usually don't just decide to flip to the other side of the membrane, but they can so far to describe this would really flip to other side. And so, like I said, here is extremely rare, but they can do it. Now, studying this movement has become very difficult for scientists, but if you are a scientist, you are interested in studying this. One of the things you would need to know is that liposomes are made in a laboratory. So synthetically made, they're made by scientists and they're like lipid balls essentially. And they're used to study how lipids move in a membrane. And so that liposomes are how scientists have figured out these three different types of movements. So here's an example of a by layer, we have our lateral diffusion where if this red lipid here is moving across the surface to this other place, we have rotational diffusion where the this individual single lipid is rotating in place. And then we have the most rare type, most rare type, which traverse diffusion, where this blue lipid here, just for some reason, decided to go to the other side of the membrane and ended up over here. So lipids move, they form boundaries, they self seal, and these are the three types of movements that lipids can do in a membrane and they move a lot and they're always moving. So important to know lipids move their fluid, so with that let's move on.

Okay, so in this video we're gonna talk about bi layer composition and asymmetry. So what that means is what we're going to talk about is what the by layer is made of. So its composition. And the fact that it's not symmetrical, meaning that each side of the bi layer is different, has different things in it. So first let's talk about composition. So the composition. So what the membranes made up of the type of lipids in the membrane are really important because they affect how fluid that membrane is, so how much it can move around and then what it can do its function. And so the fluidity of the membrane, so it's moving around is controlled by not only the amount of lipids, so how many lipids are in it, but also by the type. So certain type of lipids make it more fluid and other types of lipids make it less fluid. So some examples of things that make it more fluid are shorter chains. So the shorter hydrophobic change. So the average is around 18, 20 carbon chains and a lipid. Um But if you're shorter than that, say uh 14 for instance, which it can go down to 14, then that's going to make the lipid more fluid. It's going to be moving around more. If you want something, if you want a membrane that's less fluid, you can make the hydrocarbon chains longer. Right? Exactly. But there's a second thing that you can do and that is saturated hydrocarbon chain. So what does that mean to be saturated? You go over this in chemistry, Well saturation has to do with the number of double bonds and saturated hydrophobic chains have no double bonds. And so those without any double bonds, that means they're all single bonds and that means they're saturated and if they're saturated that they're more rigid. And so that's going to make the membrane less fluid. Whereas unsaturated, which have double bonds are more fluid. So what are some things that make it more fluid? It's unsaturated bonds, it's shorter chains. Those types of things make the lipids in the membrane more fluid. And so um another way, so that's sort of the composition, different things that lipids can look like. But remember it's also the type of lipids in the membrane can also affect fluidity. And a big example of this is cholesterol and cholesterol can really affect membrane fluidity. And what it does is cholesterol is actually the short rigid molecule and whatever it gets into the membrane that also makes the membrane more rigid. So it's short, it's rigid and it sort of sinks into the the lipid membrane and that makes it less fluid. So membranes less fluid less permeable and cholesterol is actually a really frequent way membranes adjust their fluidity so they add more cholesterol if they want to be less fluid or they remove it from the membranes that they want to be more fluid. And it makes up about 20% of the weight of lipids and animal cells. So that includes ourselves as well. So here we have saturation of lipid hydrophobic chains. Remember saturation equals no double bonds and unsaturated equals a double bond. So this one is a mono unsaturated meaning it has one double bond in it right here. And you can see that when you add a double bond into this lipid, it creates a little kink, right? Right here, where the lipid lipid tails are, those hydrophobic tails are no longer perfectly aligned with each other. They sort of separate out from each other. And when they do that means that when they insert into the membrane you have this extra space here, right, there's all this extra space. And what this means is this extra space makes it more fluid. Whereas when you're tightly packed as in a saturated lipids, it's going to be less fluid because everything is tightly packed together. So that is sort of the composition. But now let's talk about asymmetry because there are all these different lipids, right? There's different lipids of length, lipids of saturation versus saturation, different types of lipids cholesterol and things like that. And the position of these membranes in the membrane are not equally distributed on each side. So what that means is that this side of the membrane by layer and this side of the membrane by layer can be different. So one side can contain more cholesterol. The other one can contain more double bonds more unsaturated um hydrophobic tails. It just means that they're not equal the two sides of the membrane do not look alike. They're not symmetrical. We call them asymmetric. And so because they're not alike, we have to be able to distinguish them. It's not that we can just say, oh that's a bi layer. All the lipids are the same etcetera etcetera etcetera. No, they're different. And so we have to give them different names. So we call them either the side of solid face or the extra cellular face. So the side of solid face is going to face what it's going to face the side. It's all right. That makes sense. The extra cellular face is going to face the extra cellular environment. But sometimes we call this the aluminum face because it can either face the extra cellular environment. So outside the cell or it can face inside aluminum. So for instance, if we have a membrane here, this would be the extra cellular face. And this would be the side of solid face. But if we had a testicle, right? So here's a circle. This would be the Luminal face of the membrane facing inside the vesicles facing inside the lumen of the vesicles. And this would be the side of solid face because now we're in a vesicles were inside the cell so that outer membrane is still facing the side of saul. So those are the two ways that we name lipids. Now, there are some enzymes that we need to know about. It's not just lipids in the bi layer, there's also enzymes. So the first one is flip aces. Now what did flip aces do? Well they flip lipids to the other side. And so flip aces are important because they're very specific. They don't just move random lipids. They're not just these enzymes aren't just coming up to buy layers and flipping whatever they want willy nilly. No they are taking specific lipids and they flip them to the other side. And generally these enzymes are the er and the golgi which is where lipid synthesis really takes place and forms in the south. And so flip aces are really responsible for making that asymmetry right? Because if it was just random it's probably likely that something would be on one side. Some things would be on the other. But flip cases they say no this is going to be asymmetrical and it's going to be exactly how I want it to be. So they target out specific lipids and move them to the correct side of the bi layer that they're supposed to be in. Another enzyme you need to know about is possible light basis and what these are their enzymes, they break bonds and um they are only found on this side of Solich side. So that means that lipids on the side of solid side are being exposed to these possible light base these possible life bases are coming in. They're breaking bonds between lipid molecules and what that does is it can change the structure, It can change the fluidity. And it it just causes that one side the side of solid side of the membrane to be different from the other side of the membrane. And then finally we'll talk about lipid rafts. And and will you might mention these again when we talk more about membrane proteins. But lipid rafts are sort of sections of the membrane that they're called functional domains, meaning that they're kind of different from the rest of the surrounding membrane. They have certain proteins, they have certain lipids in them. And use actually those lipid rafts have some kind of function. So they collect a bunch of lipids that maybe do the same thing. And so that little section of the membrane is a lipid raft and it has a particular function and that function can be a lot of different things. So let's look an example of a lipid raft. So we have our bi layer here. Here's a lipid raft where these um proteins have accumulated these similar proteins, but also different types of lipids. So this bright green thing here is cholesterol in this section has a lot more cholesterol than other sections of the bi layer. And so this lipid raft is going to have some kind of function. It looks this case. It's a trans membrane protein. So it's probably gonna be transporting things across the membrane. And the cholesterol is gonna add that rigidity to that section of the lipid raft so that those membranes so that those proteins stay in place. And um so this lipid raft has a particular function that the rest of the membrane that this part of the membrane here and this one here doesn't have. And so there can be different lipid wraps on different sides of the membrane. So what we talked about is lipid bi layer, its composition. It's made up of different types of lipids that have different um characteristics whether they're short, long saturated, unsaturated, different types of lipids, including cholesterol. And we've talked about the fact that the two different sides of the membrane don't look the same. They're different. They have different lipids, they have different proteins, they can have different enzymes that affect the lipids on either side. And they have different lipid rafts. And so this creates this by layer that's not only just acting as a barrier but has all these different domains that are unique to that particular portion of the membrane. And that's going to be super important when we start talking about sort of signaling networks and interacting with other cells. And so the bi layer is really about asymmetrical part of the bi layer is super important for allowing different things to happen on either side of the membrane. So super crucial for cell biology. So with that let's move on

Okay so now let's talk about lipid assembly. So these lipids have to get made somewhere and then they travel to the places in the cell. They're gonna be remember a lot of organelles are encased in lipids including the Golgi that has a bunch of lipids in it. The plasma membrane has lipids different vesicles in the cell. So these lipids have to come from somewhere and they are assembled in the er so that's where they come from. So lipid synthesis only happens on a particular portion of the er though it's not just happening everywhere it's happening on the side of solid surface. So that means that where the er faces the side of salt. Um that's where lipid synthesis is taking place. And what happens is lipid synthesis doesn't synthesize a bi layer. Right. All it does is it creates one mono layer. So it just creates one lipid on the outside of the side of solid face of that E. R. And then there will be enzymes that come in to create those by layers. So the enzymes that come in and create the by layers are called scramble aces. And what they do is they take one um they take a lipid from that one model layer. So from the model layer where the er is being created and and scramble it to the other side. Now a lot of times they get questions well what about flip bases are these similar to flip bases. And while their function is kind of similar there are two separate proteins And the reason is that flip this is our specific. They target specific lipids and they flip it to the other side to maintain that asymmetrical membrane scramble aces are not specific. They're just coming in and they say oh you're creating all these things we need to create a membrane. So I'm going to flip you flip. Beautiful beautiful view and it ends up scrambling all the lipids up and they scramble them to the other side. Just sort of mixing those lipids up to create the bi layer but they're not specific at all. So not specific. So these newly formed membranes once they're formed on the er they pinch off the er and they form small vesicles and these small vesicles then travel to wherever they need to go. If they're going to the plasma membrane they'll travel to the plasma membrane they're going to the Golgi, they'll travel to the Golgi wherever they are meant to go the nucleus. They travel there so they all the lipids are synthesized in the er then scramble aces sort of helped make those by layers. And then once they've gotten enough lipids assembled where they can actually pinch off the er they pinch off the er and travel to where they're going now some of course remain in the er because the er needs lipids as well but most of them get transported out to other organelles. Um So let's talk about a term that you're gonna see in your book called lipid droplets And what these are is that these are vesicles and they have excess lipids in them. And so the body the er has been making all these lipids but it doesn't really have anywhere to put them. So lipid droplets are actually just storage of those lipids and you can kind of think in a human body we eat a bunch of fat and that gets stored as fat cells And in a cell we don't necessarily have fat cells. We can't store fat and fat cells in a cell. But what we can do is create lipids and store them in lipid droplets. And so interestingly fat cells just talking about have a lot of lipid droplets which makes sense. So here we have lipid violators synthesis. So here would be the er this would be the er side and we are creating a bunch of lipids and you can see they're all right here they're being created on this one side of the membrane and there's a ton of them. So what we need is we need scramble aces to come in and flip some of them over to this side. So scramble ace comes in and flips them over to the side. So we get a sort of more equally distributed um lipid bi layer and when this occurs this will pinch off the er and then travel to wherever it needs to go to replace the lipids in that organ l so that's lipid assembly with that let's move on