Cell membrane introduction
So when you go swimming or showering, have you ever wondered why don't your cells in your body fill up with water or why don't the substances in your cells leak into the pool? Well, the reason is because we actually have a very important structure that prevents this from happening. This is what we call the cell membrane. The cell membrane is what's on the outside of a cell. So if we have a very basic picture of a cell here with a little nucleus on the inside, this pink outside layer is what we call the cell membrane. The cell membrane can protect our cell from the outside environment, and it can determine what can enter and leave our cell. This is a property that we call semi-permeability. It is somewhat permeable. Somethings can enter, while other things cannot. So since this is such an important part of our cell-- in fact, it's one of the reasons why we can actually survive in the world. So what actually makes up this structure? Well, the main building block of a cell membrane are what we call phospholipids. There are other substances that make up our cell membrane, but the most important building block are phospholipids. And so phospholipids have three major components. The first is a phosphate head group. The second is a glycerol backbone, and the third are two fatty acid tails. So the way we draw this is we give the phosphate head group kind of like a head. It's a circle. And two fatty acid tails hang down from it, kind of like strings on a balloon. So the way I kind of remember this is a phospholipid looks like a balloon, but with two strings. Now, where's our glycerol backbone? Well, our glycerol backbone is actually what it sounds like. It's what holds the fatty acid tails to our phosphate head. It's the backbone of this molecule. So it's usually not drawn in the picture. But just remember that it's there, and it holds our two fatty acid tails to our phosphate head group. So this structure actually has a very interesting property. Up here, this head group is actually hydrophilic or polar. So hydrophilic means that it's water loving. This phosphate head group will do whatever it can to get to water. It loves water. But these fatty acid tails, because they're very, very long carbon chains, this is hydrophobic. I remember hydrophobic because a phobic or phobia is fearing. So hydro is water, so it's water fearing. These two fatty acids will do whatever it can to get away from water. A molecule that has both of these things together is what we call and amphipathic molecule. It means that the molecule has a hydrophobic section and a hydrophilic section. So in water, what would this do? So let's say we put a ton of these molecules in water. Once in water, the hydrophobic heads want to be as close to water as possible. But the tails don't. So what will happen is these phosphate groups are going to cluster together while the tails try to shield themselves away from water. But since this is a substance that's in water, water's going to be down here, too. So this will actually form a really unique structure, because the fatty acid tails are going to start grouping like this. And the phospholipids are going to be kind of upside down so that the phosphate head groups can be close to water, while this inside section can be hydrophobic and away from water. This is what we call a phospholipid bilayer. This is the basic structure of a cell membrane. And like we mentioned, this inside section is going to be hydrophobic. So now we have the structure that looks kind of like this. We call this our phospholipid bilayer, or lipid bilayer for short. But doesn't this section here also interact with water? How can this structure be like this if this section here still touches water? And we know that the fatty acid tails don't want to touch water. Well, in a cell in real life. What actually happens is we end up with the structure that forms a circle like this. Now, this is a fairly crudely drawn picture. In a cell, this wall is actually pretty thin compared to the entire body. So you'll notice that this water here doesn't become a problem anymore, because in our actual cells, water can be on the outside and on the inside. And no matter where this cell membrane touches water, it's always going to be the phosphate head groups that are hydrophilic that are seeking out water. And inside the cell membrane, we actually have a hydrophobic section. So moving on to new picture, we mentioned before that the cell membrane is semi-permeable, and we're going to explore that a little bit more. So I've taken the liberty of pre drawing a very long picture of a cell membrane. So as we mentioned, the cell membrane is actually a sphere that surrounds our cell. For the sake of this lesson, we're going to draw it out in a straight line. And we're going to say that this can be the outside environment or the extracellular, and this can be the inside or the intracellular. So you'll notice that the cell membrane has these phospholipids packed really closely together, so usually small molecules are what can pass through the cell. Another property of the cell membrane that we've discussed is that this inside section right here is really hydrophobic. So generally small, nonpolar molecules can pass through our cell membrane. This is what we call passive diffusion. So what is a good example of a small nonpolar molecule? Well, the most common type of small nonpolar molecule tend to be gasses, things like, O2, for example, or CO2. These are things that surround us every single day. And our cell, in a sense, breathes these molecules in and out of our cell. So gases can very easily pass through our cell membrane, and it's very fast. They are small and they are nonpolar. So what else does our cell interact with every single day? Well, the most common one is water. So water's actually a pretty small molecule, and it's polar. So something else that's similar to water is ethanol. This is like alcohol that we can drink. So how do these interact with our cell membrane? Well, we said that the cell membrane likes small molecules, so these can actually pass through our cell membrane. But our cell membrane prefers nonpolar molecules. So these are actually going to pass through really slowly, and they can pass through because they're so tiny that they kind of sneak by, but pretty slowly, because this very hydrophobic region is still not going to like having the water in there. So if we have small polar molecules, what about something that is a large but nonpolar, like benzene? Benzene can actually pass through our cell membrane. Even though it's large, it's nonpolar. So it's going to get along really well with that hydrophobic region in our cell membrane, but it's going to pass very slowly. Now, as a little bit of a fun fact, benzene used to be used in labs for students and researchers to wash their hands. Scientists actually found out that benzene can pass through our cell membrane and cause harm to ourselves. What about something that is large and polar? Well, a molecule like this would be sugar or glucose. Glucose actually cannot pass through our cell. It's large and it's polar. It's the complete opposite of what the cell membrane allows to pass through the cell. So glucose will have to be absorbed by ourselves through other means, but it cannot pass through the cell membrane. What about charged molecules? These are also all over the place. What's an example of a charged molecule? Well, something like a chloride ion, a sodium ion, or any sort of ion. Another pretty common charged molecule are actually amino acids. And since these are charged, they're so incredibly polar or charged that they also cannot pass through. So, in summary, our cell membrane protects our cells and determines what enters and leaves, a property that we call semi-permeability. And this cell membrane is made up of a whole bunch of phospholipids put together. Since our cell membrane has a very large hydrophobic region, it prefers nonpolar molecules. And since these phospholipids are packed so closely together, our cell membrane also prefer small molecules to pass through. So our cell membrane is semi-permeable, allowing, generally, small and nonpolar molecules to pass through the cell membrane.
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