this is the video for the standard level portion of B 2.1 on membranes and transport now all cells have a plasma membrane or you may hear that referred to as cell membrane um those two terms are synonymous and it is composed of two layers of phospholipids and that is called the lipid by layer by meaning two it controls what enters and leaves the cell so in order for a cell to function it has to be compartmentalized it has to have some kind of control over what comes in and what does not and we call that ability um semi permeable or selectively permeable again those two words mean the same thing now an ability of a molecule to get through that membrane or its permeability is in general based on the size and the charge of that molecule large molecules are not as permeable so they're going to require different way waves of getting into or out of the cell and the membrane is not permeable to polar or charged molecules and the reason being these little hydrophobic tails of the phospholipids they really don't like them okay so they're going to repel anything with a charge like an ion or something that's polar like let's say glucose so what is able to get through that membrane either in or out well things that are small and things that are non polar so something like an oxygen molecule those can easily pass through the membrane everything else is going to need some kind of alternative method for either getting into or out of the cell now it's important to remember that molecules are always in motion okay and that motion is going to result in random collisions okay so the closer molecules are to each other the more frequently they are going to collide with each other and what happens when molecules Collide is that they bounce off of each other and move off into random directions and so when that happens we're going to get these molecules bumping into each other and colliding with each other and moving off into random directions and that is going to result in these molecules spreading out going from where they are really crowded to where they are less crowded and then less likely to bump into each other that is diffusion diffusion is the passive movement of particles from an area of high concentration where they're really crowded to low concentration where they are less crowded and when we say passive what we mean is that there's no energy required okay so we don't need any energy to be added to this system this all just happens based on the movement of those molecules now this this can happen with or without a membrane so I've kind of shown you this process without a membrane remember they were all really crowded in here and they just diffused outward so that can happen right if you think about spraying perfume in the air those molecules just diffuse when it comes to going across the membrane molecules are going to move from a high concentration to a low concentration again without the input of energy okay so some things that can diffuse across the membrane these are going to be our small non-polar things like oxygen and that movement is going to continue until we reach equilibrium so equilibrium is when the concentrations are the same on either side of the membrane so that movement from high to low will continue until that equilibrium point is reached at which time defusion or the net diffusion of molecules will stop now everything else requires help getting either into or out of the membrane only those small non-polar things can just diffuse and in order for those molecules to either get in or out of the cell it's probably going to require a protein so remember embedded within the cell membrane we're going to have different proteins some of them are going to be peripheral proteins just sitting out here facing either outside or inside but a lot of them are going to be integral protein prot so integral proteins span all the way across that cell now I like this little pneumonic jet rat to remind me what these different proteins embedded in the cell membrane can do so they can help cells join together they can act as enzymes they can help with transport more on that in a minute they can help with cell recognition or attachment to other cells or they can either be like transduction like receptor points for for hormone signaling what we're going to really focus in on is this one here so this t for transport and these proteins that we'll talk about can either act as channel proteins or as protein pumps both channel proteins and protein pumps are what we call integral proteins and that means they are transmembrane proteins so this prefix trans meaning across and that means it goes all the way across the membrane all the way from outside the cell to inside the cell and that means it's going to be coming into contact with both the polar heads and the non-polar Tails so that means it's going to need polar regions and nonpolar regions all right in addition to that it's got to make sure that its polarity matches whatever is moving through here so if I have something like glucose which is polar or an ion which is charged that means the amino acids lining this inner Channel also need to be polar so we really got to think about this Form and Function if a protein is going to function as a transmembrane protein it's form those amino acids need to be aligned with the polarity of the different parts of the membrane so some examples are things like ATP synthes which is an enzyme in cell respiration channel proteins which we'll talk more about in a bit and protein pumps now peripheral proteins are proteins that are only attached to the surface or periphery of the cell membrane they're not going to go all the way through like one of those channel proteins so here's a great example of one okay so it is only um facing like the outside of the cell it doesn't go all the way through some of these proteins can function as glycoproteins for cell recognition cytochrome C is an electron transport chain protein and there's lots of different examples here but again their different functions are going to require them to have different forms again they need to be their amino acids in different places need to align with the polarity of those phospholipids in a minute we'll take a look at a special channel protein called an aquaporin and an aqua por and I love this it's exactly what it sounds like it's a pore to help aqua water move into or out of a cell and that process water moving into or out of a cell or across a membrane is called osmosis okay the net movement of water molecules across a semi-permeable membrane it is passive that means no energy is required and so let's say I have a scenario like this down here and these little red dots are going to represent some kind of solute okay so I don't know let's say glucose for fun and you can see that I have a high concentration of glucose and a low concentration of glucose and they are separated by a semi-permeable membrane so I'll show you that a little bit down here that's this semi-permeable membrane now let's say this membrane is not permeable to this solute oh no it's stuck but equilibrium still needs to be reached so if the solute cannot cross the membrane then water will cross the membrane and it will keep moving until the solute concentrations are equal and so water we'll find will always flow towards the areas of high solute concentration okay so some of this extra water in here maybe I should label that more clearly so some of this extra water is going to move towards the areas of high solute concentration and that will continue until not maybe not the amounts but the concentration is equal now even though water is polar and even though these Tails really don't care for water because they're hydrophobic water's small enough that it can still um in some small amounts make it through the phosph lepid bilayer however especially for cells um that have a lot of water moving in and out like in our kidneys we need that process to be much more efficient and that is going to require a special type of transmembrane protein called an aquaporin so these are special channels that make it easier for water to move via osmosis so what these aquaporins do is you can see they kind of like form a barrier okay a hole in the membrane where we don't have to worry about water coming into contact with those hydrophobic Tails so it makes osmosis happen a lot faster and easier cells can actually do really cool things like controlling the amount of aquaporin um to make things permeable or not permeable to water so cool um it's just important for right now to understand that aquaporin are special channel proteins that allow for the movement of water and that movement of water towards an area of high solute concentration is called osmosis now you've heard me mention channel proteins a couple of times channel proteins are associated with our third type of passive transport so we've talked about osmosis the passive movement of water diffusion the movement of molecules from areas of high concentration to low concentration and now we'll talk about the third type which is facilitated diffusion in my experience it's really easy to get confused here and think that facilitated diffusion requires energy because this word facilitated kind of sounds like helping but it is not facilitated diffusion is passive so the passive movement of particles from areas of high concentration to low concentration through a channel protein it is the same thing as diffusion right so molecules moving from areas of high concentration to low concentration it's just that in diffusion those molecules are moving in between the phospholipids and in facilitated F diffusion they are moving through a channel protein but since it is going from high concentration to low still passive no energy required why do we need channel proteins then well remember these hydrophobic Tails don't like charged or polar molecules so things like glucose or other polar molecules or ions are going to require these channel proteins okay to kind of like make a hole in those phospholipids what's so cool about this is that channel proteins are specific to different molecules so I need a glucose Channel and I need a sodium ion Channel and I need a potassium ion Channel I need different channels for different molecules and cells when we say that their membranes are selectively permeable well one of the ways that they control which molecules come in or out is by either manufacturing certain channel proteins or not so if you're a cell that wants to let glucose in you better be manufacturing a glucose Channel if you're a cell that doesn't want to let glucose in you don't manufacture that some channels can even be manufactured but open or close at different times and so all of this relates back to this idea of Form and Function the function of the cell is going to dictate the form right so which channels either need to be produced or opened in order to let those molecules into and out of the cell now let's talk about a different type of molecular transport which is active transport I love this it's exactly what it sounds like it's active it requires energy and the reason that it requires energy is because unlike diffusion and facilitated diffusion going from high concentrations to low concentrations in active transport things are being moved from areas of low concentration to areas of high concentration and this is what we call against the concentration gradient so if I was going to give molecules feelings I would say these molecules um like being spread out and having plenty of space and you're going to need to put energy into them if you were going to force them into a crowded area in reality it's about those collisions that if you're going to force them into a place with lots more of collisions and you're going to keep them there and keep them from bouncing away you're going to need to add energy into that and so how do we get these molecules to be going into really crowded spaces well we're going to need something called a protein pump it's exactly what it sounds like it's a pump made out of proteins and they are specific to different molecules so I might have a glucose pump or I might have a sodium pump again very specific and when you add in this energy molecule called ATP then it's going to change the shape of this protein ever so slightly to move that particle towards that area of high concentration and so that is really why we need energy in the form of ATP here just so to sum this up a little bit membranes have to be selectively permeable now there are some things like oxygen and like water that are always going to be at least somewhat permeable but the cell needs to control what goes in and out and again to do that it can either manufacture or not manufacture certain channel proteins or it can open or close them it can shoot to manufacture um certain kinds of protein pumps um and just in general the structure of the lipid by layer and these hydrophobic tails is going to naturally um cause some um molecules not to be able to pass through the membrane so there are a lot of different structural components that all relate to this function of semi-permeable um semi-permeability of that membrane now the membrane isn't just there to control what goes in and out of of cells there are some other really cool features of the membrane that also Aid in its functionality um and a lot of them have to do with recognition right so glycoproteins are proteins that are embedded in the membrane and they have a carbohydrate chain always sticking to the outside so this is going to be really important both for recognition and for joining cells together glycol lipids are exactly what they sound like they are a lipid they're going to be embedded within those hydrophobic tails with also a carbohydrate sticking on the outside these are going to be particular to UK carotic cells procaryotes don't have these and again they're used for recognition we're going to talk a lot more about these um in the immune system um one of my tips here is that if you get a little bit confused use your knowledge of prefixes here so glyco means sugar this is a complex of sugar and protein okay and again this is a complex of sugar and lipids both embedded in the membrane both with some recognition features here um so if we get confused you can just remember those and we'll end this video by just talking about the model of the membrane in general now this is something that you should know how to draw so you may want to take some time later on to practice drawing this um and when you're asked to draw this you may be asked to draw What's called the fluid mosaic model now Mosaic means that it's made up of many parts fluid means that those parts can move around so what's really cool is that if this protein isn't needed here but it's needed elsewhere in the membrane it can just whoop move over to another part now this is the current model of the cell membrane it hasn't always been the model that has been widely accepted but it's the current model with the most evidence to support that this in fact is how cell membranes or plasma membranes are structured so what do we need to remember about this the lipid by layer of those phospholipids and the different protein components that can be moved around um and definitely something to keep an eye on um both from a functional point of view and from a form point of view and make sure we can draw that