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Video transcript

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 cells 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 ourselves 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 some things 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 hanging 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 there are 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 an 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 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 is 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 these 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 this 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 acids Held's 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 a new picture we mentioned before that the cell membrane is semipermeable 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 or we're going to say that this can be the outside environment or the extracellular and this can be the insulin 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 gases things like o 2 for example or CO 2 these are things that surround us every single day and our cell in a sense breeze 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 is 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 a 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 water in there so if we have small polar molecules what about something that is 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 and our cell membrane but it's going to pass very slowly now it's 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 our cells 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 our cells through other means but it cannot pass through the cell membrane what about charged molecules these are also all over the place once 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 ourselves 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 in nonpolar molecules and since these phospholipids are packed so closely together our cell membrane also prefers small molecules to pass through so our cell membrane is semipermeable allowing generally small and nonpolar molecules to pass through the cell membrane
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