Membrane Structure 1 - Video Tutorials & Practice Problems
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Membrane Structure 1
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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 are the doers in biology. Now, in addition to proteins and phospholipids, membranes also contain sterols and chief among these is cholesterol which you can see in this molecule right there. And basically, cholesterol tends to fill in the gaps, left by the fatty acids of the phospholipids. 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 to have more because they're not as responsible for 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 fluid substance. There's no covalent bonds between the membrane components and actually phospholipids tend to move around a lot in fact When you actually track the motion of a phospholipid, what you find is that phospholipids 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 or or monolayer of the membrane I should say. However, there are enzymes whose job it is is to flip phospholipids between the monolayers and their names are literally, I kid you not, flippace, flopace, and scramble ace. And in image, we have I'm just going to jump out so you can see it better. We have the this is the outer monolayer and this is the inner monolayer. And you can see that the job of flippase is to take a phospholipid from the inner layer to the outer layer and from the inner layer to the outer layer and scramblase actually, does sort of a combined job. It takes phospholipid from the outer layer and moves it to the inner layer and also takes a phospholipid from the inner layer and moves it to the outer layer. Now, it's important to note that, there's actually an asymmetric distribution of phospholipids in the membrane. And phosphatidylethanolamine, phosphatidylserine, and phosphatidylinositol tend to be found on the inner monolayer whereas phosphatidylcholine and sphingomyelin tend to be found on the outer monolayer. That's not to say that there isn't any phosphatidylcholine on the inner monolayer for example, just a much smaller amount compared to how much is on the outer monolayer. That's what I mean by a symmetric distribution. And additionally, the distribution of phospholipids is different in internal membranes, meaning the total composition and the monolayer 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 monolayer 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 monolayer is going to be made up of blue and our inner monolayer is going to be red, right? And I'm just using these colors to symbolize the difference in composition of, of lipids. So this is what our outer membrane, or plasma membrane I should say, looks like. An internal membrane in the cell. Oh and just to be clear, this is outer and this is inner. An internal membrane basically has the opposite appearance. Its outer monolayer is going to be like the red one. Whereas the inner monolayer 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 inner facing 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 right 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 vesicle that goes into the cell. So if you think about it, if the plasma membrane pinches inward, right, it's going to look like this and when that vesicle totally pinches off and I'm just going to jump out of the image here so you can see this better. It's going to look like this Whereas if we had a vesicle join the membrane, the opposite process would occur. So, that's a nice way to think of why there's a difference in the composition between internal membranes and the 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 phospholipids. Now, the composition of membrane lipids is related to the temperature of the organism's environment and basically, 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 going to want more saturated fatty acids because if you live at higher temperatures, your membrane is naturally going to or the components of your membrane are going to be subjected to more energy, right? So you don't want them to become so fluid and move around so quickly that they fall apart, right? And the membrane falls apart. So you want to have more saturated fatty acids. Remember, saturated fatty acids are going to have stronger Van Der Waals interactions since saturated fatty acids tend to have higher melting points. So if you live at a higher temperature, you're going to have more saturated fatty acids so that your membrane doesn't become chewed fluid and fall apart. Conversely, if you live at colder temperatures, you're going to want more unsaturated fatty acids because remember, unsaturated fatty acids have those cis double bonds that are going to result in kinks. And because of that, those between them and as a result, they're going to be able to stay more fluid at lower temperatures instead of hardening up into that crystal like structure. So, just to reiterate, you tend to find saturated fatty acids in the membranes of organisms that live at higher temperatures. And you tend to find more unsaturated fatty acids in the membranes of organisms that live at colder temperatures. So, you know, and this is all because temperature affects the lateral diffusion of the membrane phospholipids and you want to maintain certain level of rigidity. It's also worth noting that even within the membrane, the distribution of phospholipids isn't even, even within one monolayer. You actually tend to find these things called rafts and they're basically just concentrations of, sphingolipids and cholesterol molecules. So basically, those sphingolipids and cholesterol molecules form these high density pockets. They're more dense than the other phospholipids. So when you look at an image of the plasma membrane, they appear like rafts in an ocean. Where the name comes from but they're basically just these high density pockets within the membrane. All right. Let's flip the