Membrane Structure 1

even though we tend to think of lipids as being the main components of membranes, Proteins actually tend to make up the majority of membranes, and that's because proteins allow membranes to have functions. Proteins air the doers and biology. Now, in addition to proteins and Fosse Philip, it's membranes also contain sterols. And, um, chief among these is cholesterol, which you can see in this molecule right there. And basically, uh, cholesterol tends to fill in the gaps left by the fatty acids of the phosphor lipids, so it actually decreases the fluidity of the membrane. As a result, the outer membrane the plasma membrane, tends to have a large amount of cholesterol, because this membrane is in part responsible for the structural integrity of the cell, whereas internal membranes tend tohave mawr phosphate title cooling because they're not as responsible for a structure of the cell, and they tend to be more fluid Now when we say the membrane is fluid, we literally mean the membrane is a fluid. It is a fluid substance. There's no Covalin bonds between the membrane components and actually fost Philip IDs tend to move around a lot, in fact, and when you actually track the motion of a fost Philip in. What you find is that Foss Philip it's tend to move around a lot laterally so they'll go like this way or that way. But it's rare for them to flip flop. Between the outer and inner membrane are mono layer of the membrane, I should say, uh, however, there are enzymes whose job it is is to flip Foss Philip. It's between the mono layers and their names are literally a kid. You not flip pace, flop pace and scramble ace. And in this image we have I'm just gonna jump out so you can see it better. We have the This is the outer mano layer, and this is the inner mano layer, and you can see that the job of flip aces to take a phosphor lipid from the outer layer and flip it to the inner layer. Flop based does the opposite. Moving a fost Philip it from the inner layer to the outer layer and scramble is actually a does sort of a combined job. It takes ah Foss Philip it from the outer layer and moves it to the inner layer and also takes a possible lipid from the inner layer and moves it thio the outer layer. Now it's important to note that, um, there is actually an asymmetric distribution of phosphate lipids in the membrane and phosphate title Ethanol, Amine Foster title searing and Foster title and knows it'll tend to be found on the inner mono lair, whereas phosphate title, Colin and Sprenger Mylan tend to be found on the Outer Montel Air. Uh, that's not to say that there isn't any foster title calling on the inner model air, for example, just a much smaller amount compared to how much is on the Outer Montel air. That's what I mean by asymmetric distribution. And additionally, the distribution of phosphor lipids is different in internal membranes, meaning the total composition. And, um, the mono lair in which they're found in tends to be different. And it's actually it's kind of like the opposite of the distribution on the outer mono layer, and the reason for this is because of how inner membranes interact with outer membranes. So if you think of the let's call this the outer membrane so our outer mano lair is gonna be made up of blue in our intermodal layer is going to be read right? And I'm just using these colors to symbolize the difference in composition of lipids. So this is what our outer membrane are Plasma membrane, I should say. Looks like plasma membrane, a new internal membrane in the cell. Oh, and just to be clear, this is outer and this is in er on internal membrane basically has the opposite appearance. Its outer mono layer is going to be like the red one, whereas the intermodal air is going to be more like the blue one. So this is more like an internal membrane. And again the red is going to be, in this case, the outer facing portion and the blue, the interfacing portion. So this is the inside here and again. The reason for this is because of how the internal and plasma membrane interact right. The internal membrane will often form vesicles and those vesicles, as we can see in this image rate here, those vesicles will actually merge with the plasma membrane. And you could actually draw this process in reverse because it does occur in reverse, where you have the plasma membrane pinching inward and forming a vesicles that goes into the cell. So if you think about it, if the plasma membrane pinches in ordered right, it's going to look like this. And when that vesicles totally pinches off, I'm just gonna jump out of the image. Here, you see this better, it's gonna look like this. Whereas if we had a vesicles join the membrane, the opposite process would occur, right? So that's a nice way to think of why there's a difference in the composition between internal membranes and and external plasma membrane. If you think about it in terms of how they interact, it makes sense that they're kind of they kind of have, like an opposite distribution of possible lipids. Now the composition of membrane lipids is related to the temperature of the organisms environment. And basically, uh, the reason for this is because organisms want to maintain a plasma membrane with a certain level of rigidity. And if you live at much higher temperatures, you're gonna want mawr saturated fatty acids because if you live at higher temperatures, it your membrane is naturally going thio, where the components of your membrane are going to be subjected. Thio more energy, right? So you don't want them to become so fluid and move around so quickly that they fall apart right in the membrane falls apart. So you wanna have mawr saturated fatty acids, right? Remember, saturated fatty acids are gonna have stronger vander balls. Interactions and saturated fatty acids tend to have higher melting points. So if you live at a higher temperature, you're gonna have more saturated fatty acids so that your membrane doesn't become too fluid and fall apart. Conversely, if you live at colder temperatures, you're gonna want more unsaturated fatty acids because remember, unsaturated fatty acids have those CIS double bonds that they're going to result in kinks. And because of that, those unsaturated fatty acids aren't going to be ableto lineup is tightly, so there's going to be weaker Vander Waals interactions between them, and as a result, they're going to be able to stay mawr fluid at lower temperatures instead of hardening up into that crystal like structure. So, just to reiterate, um, you tend to find saturated fatty acids in the membranes of organisms that live at higher temperatures, and you tend to find mawr unsaturated fatty acids in the membranes of organisms that live in colder temperatures. So, uh, you know, this is all because temperature effects the lateral diffusion of the memory and possible lipids, and you want to maintain a certain level of rigidity. It's also worth noting that, um, even within the membrane, the distribution of possible lipids isn't even even within one mono layer. You actually tend to find these things called rafts, and they're basically just concentrations of single lipids and cholesterol molecules. So basically, those single lipids and cholesterol molecules form these high density pockets there mawr dense than the other foster lipids. So when you look at a new image of the plasma membrane, they appear like rafts. In a notion, that's where the name comes from. But they're basically just these high density pockets within the membrane. All right, let's flip the page.