this is the video for the higher level section of b2.1 on membranes and transport now when we talk about the fluid mosaic model we said fluidity means how easily something can move around that is not something that is a fixed property of cells okay so the components of the cell membrane can actually determine how fluid it is so if something is less fluid and things are less able to move around that's going to in turn make the cell membrane less permeable so how do we control that well saturated fatty acids are less fluid than unsaturated fatty acids so phospholipids that include more saturated fatty acids going to result in less permeability different temperatures can also require different amounts of fluidity so colder habitats so I think of this like cold I get stiff and things are hard hard to move around well I want to make sure that the cell membrane doesn't become stiff and you know not movable so in colder habitats cell membranes are going to be cont comprised of more unsaturated fatty acids again to control that fluidity so whether or not something has more fatty acids that are saturated or unsaturated can be dictated by its environment and how fluid it needs to be in addition to the composition of those fatty acid Tails animal cells can add an additional feature to help regulate fluidity and that is something called cholesterol so this is again only in animal cells and cholesterol is a molecule that looks like this it is hydrophobic just like the rest of the lipids so it's going to hang out here in the hydrophobic tail region okay so I'll try to kind of like Circle this here's cholesterol and it's non-polar so I I would find it in the non-polar um hydrophobic region of my lipid by layer and this again is something that can regulate fluidity and if you're regulating fluidity you're also influencing permeability what's cool about cholesterol is that it can stabilize membranes at higher temperature and also prevent them from becoming really stiff at low temperatures so we say that it helps regulate the fluidity of a membrane and even if the temperatur is veryy like really warm or really cold it can prevent that membrane from becoming too fluid or too stiff now membrane fluidity is a very very important concept when we think about the formation of vesicles and the impact on moving things into or out of the cell so vesicles look like this um they are little pouches right made up of a lipid by layer and they can compartmentalize um different regions of the cell or surround different things and they're really important for this process called endocytosis so let's work on our root words here cyto means cell Indo means into okay so I'm going to be moving things into the cell or on the inside of the cell and this is a process of bringing materials into the inside of the cell by engulfing it this is so cool so what a cell can do is it can actually make an indentation in its cell membrane and then a small piece of that membrane pinches off so you can imagine that indentation kind of like pinching off and then surrounding that material that was once on the outside of the cell and then once that pinching process finishes up that now becomes a vesicle so what's so cool is that the lipid bilayer components of this vesicle literally used to be part of the cell membran right here they've just been pinched off and again we need the cell membrane to be fluid to be able to move in order to make that happen okay now regardless of how fluid the membrane is this is an active process so it's going to require ATP now vesicles aren't just important for endocytosis they're also important for moving things around within a cell we don't want components to get lost or spread out in the cytoplasm so we use vesicles to transport things within a cell let's take a look at this example let's say that on the rough ER okay we've freshly synthesized a protein see these little dots those ribosomes on the rough ER they are from manufacturing proteins that will eventually be exported from the cell that rough e r will then take that protein and wrap it in a vesicle which will then travel over here to our GGI okay so the GGI is going to accept that vesicle okay and when it does that vesicle fuses with the GGI and releases the contents to the inside of the GGI the GGI can modify that in a lot of different ways that's in another topic and then it will repackage it into another another vesicle okay and this vesicle will then fuse with the membrane to expel or excrete that material that protein to the outside of the cell so these vesicles for transport are very important that process of a vesicle fusing with the cell membrane and releasing its contents to the outside that is something called exocytosis so again cyto meanings cell EXO meaning I'm going to the outside so process of moving materials to the outside of the cell the material to be exported via exocytosis must be wrapped in a vesicle and that's because this vesicle will eventually fuse with the membrane so this could be like a waste product or it could be some kind of product for secretion like a protein it's an active process right to get this to happen that means I'm going going to need to supply energy in the form of ATP but it's the vesicle part here that I want to pay close attention to this vesicle is eventually going to become part of the cell membrane so it needs to be made of the same components so vesicles made up of a lipid by layer it fuses with the membrane so what's so cool about this is that now this cell membrane contains um parts that used to be the vesicle which the Gogi manufactur Ed and wrapped around that protein fun stuff it just so happens to be that cells are going to use a very similar process when they want to grow so let's say I have a cell and this cell wants to grow bigger cute well little vesicles are going to pinch off of the Ruff so let's say here's my vesicle um that is coming off of the ruffy r this vesicle is going to fuse with the cell membrane so what that will look like is this vesicle will fuse with the cell membrane and just become integrated into the cell membrane and now my cell has grown just a little bit I've only shown you one vesicle you can imagine if I have 20 vesicles doing that my cell is going to expand quite a bit so in the first part of this video we've dug a little deeper into that like lipid by layer and how things like vesicles really really help the cell perform its functions now we're going to switch gears and talk a little bit more about the proteins that are embedded within that lipid by layer and how exactly they help cells um regulate what comes in and out channel proteins were something we covered in the standard level portion from this topic and channel proteins are there um to help things that are polar or that have a charge move in and out of the cell cells can regulate their permeability by either having those channels or not having them but they can also regulate the permeability by opening or closing them and one type of protein that can open or close is something called a voltage gated ION channel it's exactly what it sounds like it is a channel protein that is meant for moving ions either into or out of the cell that is going to open or close just like a gate based on changes in voltage okay so that's kind of the definition of what it does this there's a lot more detail here about how voltage gated ion channels work in a different topic um but for now we'll just say that this gate is a mechanism um and it's not really actually a gate it's a piece of the protein that kind of like opens and closes when the protein changes shape why is the protein change shape while when you have changes in voltage changes in charge on either side of the cell it causes the protein to change shape and it can open or close so again we'll talk a little bit more about how this functions in neural signaling in a different topic but for now we want to keep our eye focused on Form and Function so the form is we have this voltage gate voltage gated ION channel and the function is that it can help cells move things in or out or not depending on the voltage which is is going to change at different points in a neuron transmission so in thinking about how this plays out in transmission between two nerves um I want to take a closer look at something called a nicotinic acetal choline receptor that's a big name this receptor is a protein okay now again this is something that's covered more in depth in our chapter our topic on neuron signaling um but this nicotinic acetal choline receptor is an integral protein and it is on the neuron that is going to be receiving that message now the message comes in the form of a neurotransmitter called acetal cine so acetal Coline is going to defuse out of the neuron that is sending the message and it is going to attach to and bind to that AAL choline receptor okay and when that happens it causes a change in the shape of this acetylcholine receptor and it opens up the protein so again we've been talking a little bit about this gate mechanism now when that happens it allows these ions that are outside of the cell to move into the cell and that is going to kick off a whole sequence of events that causes an action potential or a nerve signal or a nerve transmission to now happen in this cell Okay so again all of that mechanism about how neural signaling works is covered in another topic but it's important for now that we understand the form and the function of these proteins it is both a receptor and a channel protein for ions now what's really cool is that this is all reversible so if acetal choline detaches the shape changes and whoop the gate is going to close and so this is one of the ways in which cells use their proteins to regulate what's coming in and and out of the cell now when I mentioned that ions are going to move into the cell they're only going to do that if a concentration gradient has been established so remember ions are going to move from areas of high concentration to areas of low concentration so if we want that to happen we first need to establish an area of high concentration and to do that we're going to need active transport specifically a protein pump now in a nerve transmission we need to keep an eye on two ions sodium and potassium so there's a really cool protein pump called the sodium potassium pump remember they are specific to the ions that they pump um and it's going to use ATP to establish a concentration gradient so what it does is when ATP binds to this protein it causes is a change in shape and it's going to force sodium to be pumped to the outside of the cell okay so we're going to have three sodium ions pumped outside of the cell that's what extracellular means and two potassium ions pumped into the cell so this awesome protein pump is forcing both of these to move against the concentration gradients okay and it's doing both of them at the same time super cool so it actually has a double function here okay so it's establishing a high concentration of sodium out of the cell and a a high concentration of pottassium inside the cell um via protein pumps which require ATP and because it's doing both of them at the same time um we call it an exchange transporter so exchange it's doing double duty it's doing both of those ions at the same time um just in opposite directions and again this uh topic is about Form and Function um you don't need to necessarily know how they function in nerve uh Transmission in this topic but overall it's important to be able to relate those ideas um of how nerve Transmissions work that's in um theme C but also how we establish those concentration gradients to begin with and that's in this topic on form and function um of the membranes and how that works with transport next we'll talk about an example of something called indirect active transport and we'll be looking at this example of sodium dependent glucose co-transporters so this is exactly what it sounds like Co meaning with it's a transport protein that is going to move sodium and glucose at the same time both of these molecules are going to end up moving into into a cell so this is going to start with a sodium pottassium pump and I've drawn that here in Black okay this sodium potassium pump is going to use ATP as an energy source to actively pump sodium ions outside of the cell now this accomplishes something really useful which is a high concentration of sodium ions so at this time there's a high High concentration of sodium ions outside of the cell and a low concentration of sodium ions inside of the cell we had to use ATP to do that okay there is also a low concentration of glucose outside of the cell and a higher concentration inside of the cell but the whole reason for doing this is we want to be able to get that glucose into the cell okay the problem here is is that we're moving it against the concentration gradient so ordinarily this would also require active transport but remember we've just pumped this sodium outside of the cell so this sodium is going to move into the cell through the co- transporter okay and it's moving from high concentration to low but when it does that it's also going to bring the glucose with it so because this sodium is moving from high concentration to low it's moving passively through this co-transporter and even though we're taking glucose from an area of low concentration to high concentration it's still going to be passive because it's coupled with the passive movement of this sodium ion from a high concentration to low so this is exactly how glucose is reabsorbed in the kidneys if you've learned about that already and energy is required but it's indirect because it's not the energy that we're not using the energy for this part for moving the glucose the energy is used to establish the high concentration of sodium ions on the outside of the cell using the sodium potassium pump so again this is an example of what we call indirect active transport and we're using a protein called a co- transporter now the last specialized protein function we'll talk about is something called a cam or cell adhesion molecule and these are very important for tissues tissues are many cells working together to perform a common function so what I've drawn here are like some cells let's say these are cells in your intestines and their goal is to get maybe like food molecules okay to go from inside the intestine into like your bloodstream like a capillary okay so we'll have that that somewhere down here well it's really important that things only go in This One Direction what we don't want is any food molecules kind of like escaping in between cells so to prevent that cells use these things called cell adhesion molecules they are proteins that basically help these cells form really really tight junctions okay so they're going to help cells join together and form a tight Junction so that molecules are going where they belong and not kind of like getting lost in between these cells