Hello students. In this lecture, we're going to talk about the plasma membrane structure. The plasma membrane structure is really important to understand because it's going to dictate what gets into the cell and what and and what goes out of the cell. And the permeability of that membrane determines how separate the environment is inside the cell from the outside. And so very very important the plasma membrane isolates the internal structures. It keeps it separate. It allows the internal the inside of the cell to maintain homeostasis despite a changing environment outside of the cell. And so this is this is very important. It provides support for the cell. it also there's going to be proteins and glyco glyop proteins and glyolippids that can provide identification and communication. So this is a way to communicate with other cells. The structure of this membrane is is fluid in that it changes depending on what's going on. So so it's it's a dynamic process where the membrane itself can change. And so we'll talk about that. It defines the intracellular fluid and the extracellular fluid. So intracellular ICF, we're going to use this abbreviation. So be familiar with that. ICF is intracellular fluid or cytool. ECF is extracellular fluid. So that's outside the cell. Another term for that is interstatial fluid. So that's the fluid in between cells outside. um will use those terms for this class. So be familiar with that and I will use those abbreviations ECF and ICF. Okay. And the importance of keeping those separate is that we can maintain that homeostasis inside and we so that cell can carry out its specific functions. And remember the plasma membrane is a phospholipid billayer. So that means those phospholipids we talked about in previous chapters, they have a phosphate head and a and a and two fatty acid tails. And the fatty acids are hydrophobic and the phospholipids um head is the phosphate head is hydrophilic and that is going to have a charge. And so this molecule itself is what we call amphipilic. And then the way that they're arranged is the the two layers, the tails come together, the fatty tails come together. So the inner portion, the the inner portion of the sandwich, so to speak, or an Oreo cookie, let's say, the filling is hydrophobic and the outside the cookie, the surface areas, the cookie of the Oreo is hydrophilic. And those are the parts that interact with the cytool or the intracellular fluid. and the extracellular fluids which are watery which makes sense. So the hydrophilic portions the heads are interacting with the watery solutions and the hydrophobic portions are tailtotail inside sandwiched in in between. So um again the phosphate head is hydrophilic and the fatty acid tails are hydrophobic. And so when you look at that, here's the head. Here are the two tails. And if we and when you put these phospholipids together in a watery solution, they're literally going to arrange in this ma in this manner together. And so they will orient on their own in this in this way. So the the fats are together and the um phosphate heads are a apart up towards the watery solution and this creates that membrane. And so you can see here, here are the heads, here are the tails, here are the heads. So if something were to want to come through this membrane, it would have to go through this fatty region, this this sandwiched region in here and it must be hydrophobic to get through that. Okay. So if I want to cross a plasma membrane, this is what you have to know and have to understand is that if I go across a plasma membrane, I have to be hydrophobic. Okay? Or very small. Very small things can get through. Like water is hydrophilic. It's it's polar. It's very tiny. It can kind of get pushed and squeezed through because it's very tiny. It doesn't go through well though on its own. It's it's very hard to get through. It actually does better if it has protein channels, but it can get through. Gases are really small and um they move through on their own and of course lipid soluble stuff or hydrophobic stuff goes right through because it's soluble within these tails. So it just floats on through. So th those things will diffuse down their gradient. We'll talk about transport later in the next couple of lectures. those things will be able to get across. Things that are not hydrophobic, so things that are hydrophilic or polar can't get through these tails on their own. They will need help. They will need some sort of doorway to help them through or protein carrier or channel or something to help them get through that. And this is so important when it comes to um hormones and how they interact with with a cell, when it comes to drugs and how they um are administered or how they get into a cell or interact. All of these things are going to be determined by this solubility, this ability to cross the membrane or not. And so we'll talk about this all the time in this class. So please pay attention to this. Please understand what can cross and what can't. So what else is in the membrane? So we have this phosphoipid blayer. We've got proteins. There are proteins that are just kind of on the surface and then there are ones that go all the way through. The ones that are on the surface are usually for some sort of like communication or some sort of attachment. They're peripheral proteins. The integral proteins are the ones that go all the way through. They're usually um acting as receptors or transport proteins um to they're they're they're allowing things to go through or communicating on both sides so to speak. They can also be structural proteins as well. So you can see here we've got an example of a membrane and we've got our phospholipid billayer here and you've got a channel protein here which is kind of an open doorway. You've got a peripheral protein here. You've got a structural protein winding through here which is probably um some sort of attachment. Um this could be a structural protein. It could also be a signaling protein. you've got some um you've got some glyolippids here and glyoproteins here. So, this is showing that you've got sugars attached to the proteins or fats um glyolippids. You've got sugars attached to the to the fats here. Um these are signaling molecules. You've got cholesterol. Cholesterol is very important. Cholesterol provides rigidity. So, um, cholesterol helps the the membrane stay more rigid. So, note all of these these phospholipids are kind of just floating around and they're floating next to each other. So, these proteins are moving within this membrane and everything's everything's pretty dynamic. It's moving throughout it. So, the cholesterol allows for a little bit of rigidity and the more cholesterol you have, the more rigid it is. This is important because when we have excessive cholesterol, cholesterol gets deposited into our membranes, especially our our blood vessels and it makes them rigid and then they can't um they can't expand and we lose elasticity and that's a problem. So that's that's how you can remember that cholesterol is there for rigidity. So you've got cholesterol in that membrane and I think we covered everything that's in there. So channels channels again are transmembrane proteins. They may have gates on them. They may not. Um they may be very selective or they may be open to lots of things. It just depends on what type of channel you have. They may be a pump. So they could be for active transport. So they may have an enzyme that like breaks down ATP and like pumps things against their gradient. It just depends on the type of protein they are. So that would be like a carrier protein. So you've got channels that are open. You've got carrier proteins that actually bind and move things. And these could use ATP or not. you have receptors, things that bind like chemicals or um well they would bind chemicals and then this could trigger a sequence of events within the cell or open a channel depending on what's going on. Um there the proteins could be enzymes that catalyze a reaction. We'll talk we talked about enzymes in in previous lectures and the proteins could pro provide structural support. So they could be connecting to other cells. They could be providing structural support to connecting to proteins within the cell or just supporting that membrane itself. And note that the the proteins that are there are not necessarily permanent. Um an example of this is with insulin. There are glucose is a polar molecule and it needs a transporter to cross the membrane. Insulin under the influence of the hormone insulin. We actually insert more glucose transporters into the membrane. And so that is a dynamic process. We're going to change the number of transporters we have depending on the presence of that hormone. And so this is dynamic. Again, proteins can link cells from one cell to another and they can provide communication. The cholesterol, we talked about this. This is the lipid. Remember that four ring structure that you had to draw in week one. Um, this stabilizes the membrane and prevents it from being too fluid and falling apart. glyolippids and glyoproteins usually there for communication and cell recognition. So knowing this and understanding the the complexity of the plasma membrane explain how the permeability of this membrane. So work with someone either at your house or in in around you and figure out how the permeabilia of the membrane may affect drug administration. Can you see how that this would this would allow you to or limit you on what how you can deliver certain drugs. So pause the pause the screen, write this down, and then we'll talk about it. Good. So hopefully you've come up with some ideas on the permeability of the membrane affecting your deliverability of drugs. So let's say I had a a steroid cream and a steroid is lipid soluble. And if I put that on my skin, do you think that that's going to go through the skin and into my blood? Good. It is because it can cross the membrane. It's lipid soluble. So, if it can cross the membrane, it can cross my skin. It can go across my cells and into the blood and get into the rest of the body. So, you can administer lipid soluble um medications. most not all lipid soluble medications but in general lipid soluble medications can be absorbed through the skin. So if it's hydrophobic there's the potential for that to be administered through the skin. This is why we can administer some medications with a patch or a cream that if we want to absorb them and maybe we don't want to absorb them but we should know that they are getting absorbed even if we're applying them on the skin. So like hydrocortisone cream, most people think, oh, I'm just applying it to my skin. No big deal. It's just for my skin is for a rash. But if I keep applying this every day, I'm actually absorbing that hydrocortisone and that's affecting me systemically. So that's important to understand. Whereas if I had a hydrophobic medication, let's say insulin or epinephrine or um growth hormone, all of these are water soluble. They are hydrophilic. Can I put them on my skin and get and absorb them? Correct. I cannot. So there's no way for me to administer them in that way. I would have to inject them. I would have to put them into into the bloodstream to get them into the body because there's not a way they're not going to be absorbed in that way. And so that's very important. It also is we're going to talk about this later when we get to hormones and transport, but this affects how they're transported um these medications. It affects their half-life. And so this will be very important moving forward in understanding how drugs interact in the body for your patients. All right. So, our next topic is going to be cell membrane transport. So, now that you understand the structure of the cell membrane, we'll focus on transport across the membrane. And this is going to be a foundational concept for the rest of physiology. And I know you've had a little bit of experience, but we'll refresh you guys on that and get everybody up to speed and then move forward with that. I'll see you there.