hi and welcome to chapter five in this first video we're looking at the composition and function of cell membranes so what are the functions of our plasma membrane or cell membrane the first includes being an outer border of cells and of membranous organelles because it is the outer border of the cell it controls what can enter and exit the cell the membrane will also be involved in receiving signals from the external environment and sometimes these signals will result in initiation of certain signals or cellular responses inside the cell finally the cell membrane can also allow the cell to stick to or adhere to neighboring cells and we saw this in a previous lecture when we talked about tight junctions desmosomes and GAP Junctions although we previously described the membrane as being primarily composed of phospholipids in a phospholipid bilayer where the nonpolar Tails face inwards and the polar heads face the outside and inside of the cell we often also call the plasma membrane a fluid mosaic model because there are so many other components that make up the cell membrane including cholesterol proteins and carbohydrates the the cell membrane is considered fluid because these components are not stuck in place there is some lateral movement left and right for all of these molecules the structure of the cell membrane and that phospholip bilayer was really discovered around the 1950s when transmission electron microscopy became more popular and prevalent this allowed researchers to see that the cell membrane was actually made of a bilayer of phospholipids rather than a single layer that they originally thought it was composed of and for a typical cell we'll see that plasma membrane is about 5 to 10 nanometers in thickness when we're looking at that bilayer and about 50 percent of the membrane will be made up of proteins the lipids will be about 40 percent of the membrane and then the carbohydrates will be about 10 percent but that's for a general cell depending on the type of cell these percentages can vary for the carbohydrates on the surface of the cell membrane we can see they can either be attached to proteins in the form of glycoprotein a carbohydrate protein complex or glycolipids a carbohydrate lipid complex but as I mentioned previously the main component of the membrane is really the phospholipid and the phospholipid is known as an amphipipilic or amphipathic molecule because it has both water loving and water fearing parts so in our phospholipid if we remember from our previous chapter the hydrophobic Tails which are down here there are two of them these are made of two fatty acid chains these are nonpolar not water loving they are in fact water fearing in contrast the hydrophilic head the top part over here is composed of a glycerol molecule and a phosphate group and sometimes there are additional attachments on that phosphate group looking at the fatty acid tail portion down here the fatty acid Tails can be saturated which means that the tail region is only comprised of single bonds or it can be unsaturated like the one shown towards the right side remember that when they the fatty acid tails are unsaturated and these are CIS these are a CIS type of double bond then we have kinks or bending in the tail which makes the tail or that fatty acid in particular more fluid so these are more fluid whereas the saturated tails are more solid if you have a bunch of phospholipids they'll often self-arrange themselves based on their amphipathic nature where the water loving portions will face the outside and the water fearing portions will face the inside in some kind of shape so here we see down over here a bilayer sheet very similar to the cell membrane but our phospholipids can also arrange themselves into a structure known as a liposome and that also looks almost like a miniature cell where I have a bilayer and a spherical shape the polar heads are facing the inside of the liposome but also facing the outside of the liposome and then finally we also have the micelle at the top right where it's a single layer and I can see the fatty acid tails the nonpolar or water fearing region faces the middle and the water loving or hydrophilic regions those polar heads face the outside proteins are also an important component of membranes they will have a variety of functions including Transporters where they move things across the membrane into or out of the cell we have protein receptors that bind to for example hormones that signal the cell to perform some kind of function we have enzymes that speed up reactions or we even have proteins involved in adhesion either to surfaces or to other cells of these proteins there are two types we have peripheral proteins and integral proteins peripheral proteins are only found on the surface of the membrane either the inside or the outside for example here we have a peripheral protein it's only on the inside surface of the cell integral proteins are integrated completely in the membrane such as this one and I can see it passes through both sides of the phospholipid bilayer for these proteins that are embedded in the phospholipid bilayer they must themselves have both hydrophilic and hydrophobic regions to do so so parts of the integral proteins will be hydrophilic the parts that face the inside and outside of the cell but the part of the proteins that are embedded within the fatty acid tail region or the water fearing region of the membrane these will have to be hydrophobic so if I were to take an integral membrane protein and guess what kind of amino acids comprise this protein in the areas that face the inside and the outside of the cell where the water is found these hydrophilic regions then even though I don't have to memorize this chart I told you guys before if I gave you an amino acid and I asked you to guess where it was found in the integral membrane protein I would say oh that inner and outer region that faces the water would probably be comprised if this was my if these were my two choices the outer and inner portion would be more likely to have lysine and amino acids like lysine that is water loving whereas the part of the protein that is facing the hydrophobic regions that would be more likely to have something like leucine in that region the third major component of the membrane are carbohydrates and specifically oligosaccharide carbohydrates remember oligo refers to few a few sugars in length a few monosaccharides in length and these are always on the exterior surface of the membrane they're always bound to either proteins which creates glycoproteins or bound to lipids forming glycolipids so I see a few here there's one that's a glycoprotein and I see another one over here glycolipid on the right these are important in cell cell recognition and attachment and in fact if you know your blood type you may have heard of the blood types before and there is for example type A blood type B type A B type O then that actually is defined by the type of glycoprotein you have on the surface of your red blood cells if you're type A you have type A glycoprotein on your red blood cells if you're type B you have type b glycoprotein a b that means you have both and if you're type O you do not have either a nor B on the surface of your red blood cells although most of our glycoproteins are advantageous to the cell's function sometimes they're not for example and our immune system we have two major types of cells called B cells and T cells and T cells that are part of our immune system have a CD4 receptor on them a glycoprotein that unfortunately recognizes HIV human immunodeficiency virus the virus that causes AIDS as self it sees it as something that should not be destroyed and HIV can bind to these CD4 receptors and these T cells respond by allowing HIV into the cell and we'll talk more about this when we get to the chapter on viruses membrane fluidity is important to make sure the membrane is not too loose doesn't fall apart when it gets too hot like in the summertime but also not too rigid that it freezes and cracks in the winter time so how do we moderate or maintain a certain level of fluidity in the membrane these are the factors that can influence membrane fluidity the first is phospholipid type so earlier I mentioned if you have saturated fatty acids in that tail region of the fossil lipid these are more rigid they're more solid so SFA stands for saturated fatty acids these would make the cell membrane more rigid whereas unsaturated fatty acids would allow the membrane to become more fluid because they don't pack as tightly the second factor is temperature warmer temperatures increase the fluidity of the membrane whereas colder temperatures make molecules more compressed and more rigid and then finally cholesterol is embedded throughout the membrane and it's a nice buffer it actually can increase or decrease the fluidity of the membrane in the winter time cholesterol prevents the membrane from freezing and maintains fluidity but in the summertime or when it's too hot cholesterol can also increase rigidity the plasma membrane is also asymmetric where the inside portion the portion facing the interior or cytoplasm of the cell is not the same as the part of the plasma membrane that faces the outside environment for example proteins on the interior portion of the cell membrane will be anchored or attached to the cytoskeleton proteins we talked about before such as microtubules microfilaments and intermediate filaments exterior proteins on the other hand can bind to the proteins and sugars found in The extracellular Matrix finally glycoproteins on the surface of the cell can bind to substances the cell needs for example hormones or different types of cell signals because of the composition of the plasma membrane mainly due to the phospholipids but also due in part two the proteins the plasma membrane is selectively permeable most things cannot pass but some molecules can so this allows the cytosol the inside of the cell to differ from the extracellular environment and the fluids in the extracellular environment for example we're going to say later that the concentration of sodium and potassium ions are different inside and outside of the cell if we're looking at transport and what kinds of things can move into and out of the cell and how they can move we're going to look at two methods one type of transport across the membrane is known as passive this means no energy is required for the molecule or molecules to cross the second type is active requiring energy for the molecules to pass through the membrane so the second video we're going to look at passive transport and in our third video we'll look at active transport that takes us to the end of the first video for chapter five in our second video we're going to talk about passive transport