in this video we're going to talk about the plasma membrane the plasma membrane of cells is made up of something called phospholipids and here is a typical structure of a phospholipid the phospholipid is made up of three subunits it has a phosphate group a glycerol molecule and two fatty acid tails the head of the phospholipid is the phosphate group and it's polar which means that it's hydrophilic hydro means water phil means love so it loves water the tails are nonpolar and so they are hydrophobic they don't like water they tend to stay away from water so what we have here is known as the phospholipid bilayer which makes up the cellular membrane on the inside of the cell is an aqueous environment an environment that contains water and the same is true on the outside of the tail i mean on the outside of the cell rather notice that the heads of the phospholipids are oriented in such a way that they're facing water it's because that they're polar they're attracted to it whereas the interior are the tails of the phospholipids and they're directed away from water because that region is hydrophobic it's nonpolar so that's the structure of the phospholipid bilayer it's made up of phospholipids and that comprises the cellular membrane or the plasma membrane now let's talk about the structure of the phospholipid so the phospholipid as we said before it's made up of a glycerol molecule and it contains two fatty acid non-polar tails in this example the first fatty acid which is this one here is a saturated fatty acid because it doesn't have any double bonds the second one is an unsaturated fatty acid because it has a double bond now the third part is the phosphate group the phosphate group is attached to two carbons which is also attached to a nitrogen atom with three other methyl groups on it the nitrogen atom has a positive formal charge and this oxygen has a negative formal charge so that's the typical structure of a phosphate i mean a phospholipid so this part here is the head of the phospholipid that is the polar part as you can see by the formal charges it's going to be attracted to water so it has a phosphate group two carbons and a trimethyl amine group well this is technically an ammonium group since there's four carbons attached to that nitrogen atom and these two fatty acid chains that are part of the phospholipid that is the hydrophobic part of the molecule so those are the two tails that's the part that is oriented away from water and in the middle we have a glycerol molecule i'm not sure what just happened there so keep that in mind the phospholipid is composed of a phosphate group a glycerol molecule and two fatty acid chains the plasma membrane of a cell is described by the fluid mosaic model the cell membrane is fluid in that the phospholipids and the proteins that are embedded in this membrane they're free to move they can move sideways forwards backwards they're in motion among a sea of particles now what we have here is called an integral protein because it's embedded in the cellular membrane this here is a peripheral protein because it extends on the exterior of the membrane and you can call this one a surface protein because it lies on the surface of the membrane now on the cell membrane you'll find carbohydrates attached to proteins when you see that this is called a glycoprotein glycol for carbohydrate and then protein you know that is so that's a glycoprotein now when a carbohydrate is attached to a lipid in this case a phospholipid this is going to be called a glycolipid now on the cell membrane cholesterol can also be found in it cholesterol has a hydroxyl group represented in blue and it has 4 fused rings so that's a typical representation of cholesterol the purpose of cholesterol on the cell membrane is to maintain the fluidity of the membrane when the temperature goes down the membrane will become rigid the phospholipids will move closer to each other as the temperature decreases so the fluidity of the membrane decreases cholesterol acts to increase the fluidity so as these phospholipids move closer to each other under colder conditions cholesterol acts as a spacer keeping them pushed apart now when the temperature rises the membrane can become too fluid so cholesterol acts in such a way to decrease the fluidity of the membrane so it keeps the phospholipids together when the temperature rises so the purpose of cholesterol is to maintain the fluidity of the membrane it prevents it from being too rigid and it keeps it from being too fluid so that's the basics of the plasma membrane of the cell there's more to talk about it but i just want to give you a brief introduction into it