So the plasma membrane, that's the lipid bilayer that has phospholipids, right? Well, do you know about lipid rafts, cholesterol, glycoproteins, transport proteins, the fluid mosaic model? There's a lot that goes on with this seemingly boring organelle. So what all do you actually need to know for the MCAT? We're about to tell you. Let's get right into it. All righty, so welcome to another application session where we are looking at reviewing the plasma membrane. It's a really boring organelle, but it's huge on the MCAT. It's actually really involved in the way that your immune system works, the way that cancer evades our immune system, and things like that. Not all that's tested on the MCAT, but we're going to jump into a lot of the stuff that is. If you're part of our program, one of the purposes of these sessions that we do is to tell you what's high yield from a specific chapter of content that you're going through. So we gave you a little bit of an overview of high yield material and also a little bit of mid yield material, stuff that you don't have to study quite as well, but that we still want you to know a good bit on and that's tested fairly frequently. So high yield stuff is definitely going to be the plasma membrane and active and passive transport, which is the stuff that we're going to actually kind of teach you and go through today. But mid yield things would be all the organelles in the cell, especially those membrane bound organelles and then oxidative phosphorylation. Yeah, so as far as the mid-yield stuff, because we're not really going to expand on it, I'll kind of tell you what you need to know really briefly, and then, Meg, you jump in and tell, you know, disagree with me or correct me. So as far as the celery nails go, you really just need to know the general basics for them. You need to know that a ribosome cranks out proteins. You need to know the difference between the rough ER and the smooth ER, which is that the rough ER... is actually the source for making proteins. It's where the ribosomes are going to be attached to. You need to know the smooth ER is involved in lipid synthesis and some detoxification reactions. You need to know that the lysosomes are responsible for breakdown of macromolecules and things like that. What organelles am I missing? Mitochondria, the powerhouse of the cell. You need to know that one, of course. within each of those, there's like separate subjects, right? So there's, there's like, there's translation, which you got to know the whole process of translation. And that occurs at the site of the ribosome, but you don't have to know, um, you don't have to like really associate the two that much, uh, at least on the MCAT and the same thing with the mitochondria and the majority of respiration. So even though things like glycolysis, um, happen in the cytoplasm, a lot of the things that you're going to be looking at for respiration and metabolism, you know, the big terrifying word, are going to occur inside of the mitochondria, like oxidative phosphorylation is a good one, the citric acid cycle, all those things are going to happen inside of the mitochondria, but we'll cover those in later subjects, you know, like don't Whenever you learn the mitochondria, you don't learn the electron transport chain at the same time, right? You learn the mitochondria creates ATP. It has its own genetic code, which is kind of weird. And then mitochondrial inheritance is not on the MCAT, is it? I don't think so. You get your mitochondria from your mom. That's going to be the extent. Yeah. Yeah. I can't remember if that's on the MCAT or not. I know we did it in med school, but yeah, mitochondria, you inherit them from your mother. And so if there's a mitochondrial disease, if your dad has it, none of the offspring will have it. If the mom has it, 100% of the offspring will have it. That's it. That's all they could test you on. But you will learn organelles as like their main function. and then go learn the separate processes in a later chapter. I agree. I think that organelles are going to be tested on the MCAT with how they relate to the biochemistry that's on the MCAT. So, you know, I think of like a lot of questions that I've seen on the Golgi body relating to how they exocytose. proteins and stuff like that. But yeah, I definitely like the functions and how they relate to a couple of things within the cell that are highly tested on the MCAT. Yeah, the Golgi, that's a good one. It's the post office, right? So it packs up your proteins and chips them off. But that's about the extent of it. All right. Yep. You just need, you need more than one, more than just phospholipids. So that's a good segue into kind of like what you want to know on the plasma membrane. And part of it's the composition. So you need to know what the plasma membrane is made of. And mostly it's kittens. Now, mostly it is phospholipids. You're going to see a lot of lipids. In the plasma membrane, you're also going to see a lot of proteins, because proteins are going to make up your ion channels and things like that. And you're also going to see some carbohydrates. So it's a good idea for us to kind of like talk about what the role of a lipid, a protein, and a carbohydrate would be in the balance of a plasma membrane. So let's kind of break it up into that real quick. Maggie, what do you see the role of lipids in the plasma membrane being? Okay, so there's different types of lipids. You see glycol lipids, you also see cholesterol lipid rafts, and the most common one is the phospholipids. And so the purpose of them is to maintain that membrane fluidity. That's what cholesterol's purpose is. When the temperatures are very low, obviously the plasma membrane is going to want to become more frozen. And so cholesterol actually I don't know the process and you don't have to know the process either, but somehow it, um, it helps to increase the fluidity so that the plasma membrane doesn't freeze in low temperatures. Um, whereas in high temperatures, um, the phospholipids are going to want to become all loosey goosey and want to go everywhere, but cholesterol kind of holds the fort down and holds it all together and actually decreases fluidity. um as far as funny meme or not meme but there's a super funny like cartoon drawing hopefully i can like uh find it and put it on the screen um i thought you were going to i saw it in the powerpoint earlier yeah but i was like that's stupid your humor's stupid john but um i'll find it maybe All right. And then for phospholipids, you hear about the phospholipid bilayer, which is just the idea that the phosphate heads of the phospholipids are on either side of the plasma membrane and that the lipid tails kind of point towards each other on the inside. What do you like a highly tested thing? I feel like of the plasma membrane is. hydrophobicity and the fact that hydrophobic molecules can pass straight through the plasma membrane. That's because the majority of the plasma membrane that inside like the peanut butter of the sandwich is those lipid tails. So, um, I always think of plasma membrane as being hydrophobic. The transport proteins is when you have to have something hydrophilic come through, then you got to have a transport protein or something like that. And what's kind of weird about that, what was really weird, and it was hard for me to learn because it seems backwards, because you're right. hydrophobic things can go in and out. Even like steroid hormones, they're big, and they're super hydrophobic, and they can just go in and out the plasma membrane as they want. They buy nuclear receptors to elicit their gene expression, which means they can just straight up, just say deuces to the doorman, and go straight through the plasma membrane and get to the nucleus, which has always been weird to me because... the head of a phospholipid. A phospholipid is actually made up of a phosphate head, yeah, phosphate head, and then some tail groups, and it's hydrophilic. So the very first thing that the extracellular materials are going to come into contact with is hydrophilic. which means it likes water, right? And so it was always really hard for me to remember this idea like, okay, I know that steroid hormones can go through, but it doesn't make sense. Why the freak can that happen if the first thing they're going to come into contact with is hydrophilic? Well, Maggie explained it really well earlier in that the majority of the cell membrane is going to be hydrophobic. And not only that, there's something called the fluid mosaic model that describes the plasma membrane, which is the idea that this whole thing is just a bunch of small molecules that are just kind of like rotating constantly. And so like even the heads, and this is a little bit deeper than you got to get, but even the heads are like... it's called like flipping. They literally flip back and forth. And so I say all that to say like you can, you could easily imagine these small polar head groups getting kind of displaced a little bit and revealing this giant sea of hydrophobicity that you're going to go into. and the way that I memorized that, the way that I, and I still think about it today, like I literally took my first biochem test in med school, had this subject on it, and I literally thought about this as I was taking it. I remember that the head is hydrophilic, because I always think, what do you lick with? It's your tongue. Is your tongue on your head, or is your tongue in your head? or is your tongue in your tail which i guess that's a different subject but um it's in your head right so i just feel lick you lick with your head ah no residencies are gonna fight me oh yeah but i like i like how you mentioned the fluid mosaic model i always think of it as just kind of like stuff's floating around like think of um a pool that has like floaties in it and toys in it. And then some kids in it, like it's mostly water, but you've got these kids swimming around the kids at the transport proteins and the floaties are the, or maybe the floaties would be the transport protein because those have like a hole in the middle for us solutes to go through. And then the kids are the cholesterol. cause they will, um, they're bad for your heart health. And then, um, yeah. And then they got other stuff like glycoproteins and sterols and stuff, um, just kind of swimming around and things can kind of move. And the lipid rafts, if you learn a little bit about them, although you don't really have to know much about them for the MCAT, they'll move throughout the membrane as well. So, you know, we kind of learned in, in grade school, the plasma membrane was a lipid bilayer. And that was basically the end of it. Um, but now the, the model has kind of changed to be the fluid mosaic model to actually incorporate all of the different things that are in the plasma membrane. Um, even though John talked crap about it, um, saying it was a boring organelle. I like the plasma membrane. I'm glad. I'm happy. I'm happy for you too. So yeah, that's kind of, that's kind of the basics of like the lipids in the plasma membrane as far as proteins go. And let's keep, can you think of anything else that lipids do in the plasma membrane? fluidity. They separate the exterior from the interior. They allow the selective permeability. That's pretty much it, right? Yeah, I think so. Okay. What do proteins do then? Typically, they're like channels for large molecules to get through or even charged molecules to get through. Yeah, good. So we think about channels. We think about, if you look at other membranes, because the conversation of plasma membranes generally extends to the membranes of other organelles as well. and so you're really looking at a bunch of different things. Antiporters, symporters, these pumps that we're going to get into a little bit later for active and passive transport, that's pretty much what you see. Proteins and carbohydrates also are really big identifiers in the cell. And so there's one, I think it's CTLA-4. You'll have to check me on this, but CTLA-4, I'm pretty sure, is a protein that we express on our cell's plasma membrane that tells cytotoxic T cells, please don't F me up. Like, please don't kill me. and that's another function that proteins can have. It's also a very similar function that carbohydrates have. Something really interesting that cancer does is it will actually produce some of those. It's kind of snarky, but it will produce some of those proteins that will say... you know, go ahead and pass over me and don't kill me. And then something else that's kind of interesting is that's actually a target for chemotherapy now, is we'll actually target some of those because we know that cancer cells are snarky and they like to. create some of those. So that's lipids, proteins, and then the carbohydrates are all about signaling and identification. You don't really even have to know that for the MCAT. But in med school, it's huge, the idea that carbohydrates allow us to identify molecules. But on the MCAT, it's not even on there. You just need to know that, yeah, there technically can be carbs in the plasma membrane, and there will not be nucleic acids in the plasma membrane. They are not present. So I know we've talked a lot and we've said a couple of times, like, but you don't have to know that for the MCAT. So to, to wrap up what you do need to know about the plasma membrane, the fluid mosaic model, and how there's just a lot of things in the plasma membrane that are just floating around. Um, you do need to know that you need to know that hydrophobic things pass straight through the plasma membrane, whereas hydrophilic things need something like a transport protein or a pump of some sort. Um, you need to know that there are, um, glycoproteins and glycolipids in the plasma membrane. And that's about the extent of what you need to know about them. You just need to know that they're there. Um, you need to know that cholesterol increases membrane fluidity at low temperatures and decreases it at higher temperatures, even though that's not highly tested. I would go ahead and say, can't hurt to learn that. Um, I have a question for you, John, because they will ask you some application questions and they love to talk about amino acids. What kind of amino acids would be in like this portion of the protein? Oh, okay. So let me walk you through my thought process. So the region that she's highlighted, colored in black, is... generally, she's not talking, well, she's pointing at the protein because that would make up the amino acids, but she's referencing the hydrophobic portion of the plasma membrane. And so she's saying what amino acids do I expect to be on the exterior of a protein that runs in the hydrophobic portion? it's probably going to be hydrophobic. So I would imagine that my answer choices would be like, you know, three hydrophilic ones like glutamate, spartic acid, and then they would do something to try to be like, try to trip you up or be a little sneaky and give you one that's not charged like serine. And then they would have one that's super obviously hydrophobic like valine and you would pick that one. I've had a question like that. Yeah. I mean, I think so. I mean, I just made that up, but I did have a question like that. That was like, what would be the peptide sequence for a transmembrane protein? And the answer was one that started out hydrophilic, like had one or two amino acids that was hydrophilic, then a big long chain of hydrophobic. And then when that was hydrophilic. And so I guess it was referencing something that would like, you know, go all the way through. and would hit the hydrophilic heads as well as the hydrophobic tails? That was kind of a crappy question, I thought. Not my MCAT, but I've seen a practice do that. That's pretty intense. There's something called signal sequences or nuclear localization sequences. That's what it's actually referencing to that will allow it, will allow like once cells get synthesized, if they, their first few proteins that they create are hydrophobic, it will immediately go and it will pause translation and it will insert it into the endoplasmic reticulum. and so that's what they were actually getting at, but you didn't even know that, and you rocked it, so that's freaking great. But yeah, I mean, I'm sure that that's why the body works like that. Like, I'm sure that the body didn't make the endoplasmic reticulum to be like, you know what, I freaking love these four amino acids. I'm going to make these my best friend. I'm sure that it was like just a factor of, hey, the inside of the... membrane is hydrophobic. So I bet the ones that are, if I want to send something there, I'm going to make something hydrophobic. That's really cool. Yeah. Um, I also, you know, the last thing I, I think I have on the plasma membrane, I actually have two things. Uh, the plasma membrane for prokaryotes is different from eukaryotes and I wouldn't have thought that this was a high yield thing, but I actually have a couple of questions on that, that I found on AAMC material. So, um, um, And just remember, I guess, that eukaryotes have like sterols and glycolipids, whereas bacteria or prokaryotes are going to have peptidoglycan, that peptidoglycan cell wall. They're going to have that as well incorporated with their plasma membrane. So I wouldn't have thought that was a high yield thing, but. it was on a question. So I just want to mention it. Um, I also want to mention, I think about the plasma membrane sometimes with, um, exocytosing and endocytosing. So if you've ever like wondered how that works or wondered how the plasma membrane does that. Um, so, you know, if a protein, okay, let's take it from, let's take it from what, where John was, um, going off of if a ribosome is, is transcribing. some mRNA and it comes across a signal sequence, then it's going to go to the endoplasmic reticulum, make the rest of its protein, feed it into the ER. The ER is going to, it's like the highway of the cell, right? So that protein is going to travel along the ER to the Golgi body. Then the Golgi body is going to, it's kind of like a lava lamp. So it's going to like break off a little piece with a vesicle that has that protein in it, and then exocytose with the plasma membrane. And so, you know, you have like a ball and then it hits this, and then it kind of like just feeds out and creates part of that wall as well. and then endocytose would be like, if this is our plasma membrane and something came along and it needed to get inside the cell, then it would kind of like wrap a little hole right there and then kind of go back in. Plasma membrane is, it's dope, but it's also kind of like a lava lamp. Like it's, it's constantly, um, having to. have this homeostasis between exocytosis and endocytosis so that the plasma membrane doesn't get too big or small. Um, and it can maintain a certain cell size. I thought that was important to throw in there because our students asked for a little bit more teaching in these things. So I thought I would, uh, bring around town. My cat is bringing everything around. She is all up in it. She needs attention right now. Anything else, John? No. What's next? Some questions. Let's tear them up. Okay. So I normally take a little bit of excerpt from the passage, but really the only thing you need from the passage to answer this question is to know that we're in a prokaryotic cell. Okay, John, you want to take it or me? Sure, I'll take it. Which other cellular components are likely to be located near the lacY underscore or sub, anyways, transcript in the cell membrane? So yeah, the passage tells you this is prokaryotic, but also the lac operon, which you do need to know for the MCAT, is only in prokaryotes. So this could have technically been a stand-alone question. It just would have been a doozy of one. So I'm gonna rephrase this as which of these components is only in. prokaryotes. Which of these is definitely in prokaryotes? A says, or which of these is in prokaryotes cell membranes? A, proteins and glycolipids. Well, that's in both of them, so that won't tell me. So I'll say maybe to A. B says glycolipids and sterols. I believe you just said that sterols were only in... eukaryotes, right? Okay. As well as glycolipids. Oh. So I'll say maybe not to A then. So B, glycolipids and sterols are both only in eukaryotes. So no. C, sterols and phospholipids. Sterols are... definitely only in eukaryotes. So I'll say maybe not. And then B says phospholipids and proteins. So yeah, those are both components of the plasma membrane. I really thought this was going to be going for peptidoglycan. Otherwise, I would not have volunteered for it. But I would go with D as the correct answer. And I forgot to highlight this second thing. D is the right answer. Okay. Correct. Great job, John. Thank you. I had to use the phone a friend. And then I'll take this one and it says, which type of molecule is least likely to be found in a eukaryotic cell membrane? um that's pretty straightforward we're looking for something that's not in eukaryotes so um phospholipids those are definitely in eukaryotes those are definitely in like every plasma membrane um cholesterol we talked about that and with membrane fluidity and so nope those are definitely going to be in eukaryotes glycoproteins are in eukaryotes so i'll say maybe not to a b and c but D says peptidoglycan, which I, I know peptidoglycan cell wall is like the bacteria, um, cell wall. And then I also know that it's incorporated in the cell membrane. So I'm going to say D and I know that these are not in, you care in prokaryotes. I didn't know that. I guess I'm sure about it. well I mean this one was just asking what's not in a eukaryote so technically they could be in prokaryotes I don't really know yeah but you said sterols aren't in proks and in cholesterol oh is that a is that a yeah they're all interesting I guess not I don't somebody look it up let us know yeah all righty um so I like those two questions transport you Yeah, I do too. Actually, I lied. I don't. Go ahead. Sometimes they're easy and sometimes they're not. I was going to say those two questions about the plasma membrane were what prompted me to talk about prokaryote versus eukaryote plasma membranes because I wouldn't have thought, like I said, that it was high yield to know the specifics, but I would have gotten both of those questions wrong probably, or at least the first one. so all right let's get into active and passive transport okay john you want to start or yeah i'll teach it so when when we look at active versus passive what we mean is if it's active we have to invest energy you in order to move a molecule across a membrane. And if it's passive, we do not have to invest energy to move that molecule across a membrane. And so they both break down into two different kind of like subcategories. Active transport, if you look on the left, it breaks down into primary and secondary. Primary means we're going to snap ATP, right, because that's like the energy from the cell. That's the body's energy currency. That's where we store energy as. And so if we're in active transport, we need to use energy to pass something. Then we're going to break ATP, and that's going to directly lead to the solute, or I guess we'll just call it a molecule. passing from one side of the membrane to the other. That's primary active transport. Secondary active transport is a little bit different. And so we're still using something like ATP or like a GTP, some kind of high-energy molecule. And we're going to break that bond, use the energy from the bonds that are reforming after it, and we're going to move a molecule across the membrane. like a sodium. Chlorine is a really common one. But the chlorine or the sodium that we're moving across is not what we technically really wanted to move across. That's just what gets moved across by investing the original ATP. And so the molecule that we're interested in moving actually goes by a product of the original molecule moving. Meg, can you explain that a little bit better than I did? I feel like I got in the weeds a bit on the secondary. No, I think you did a good job. You know, looking at this... this figure right here, I think is a good way to think about it. So you can see when it says low and high, that means like the solute concentration of whatever you're trying to move. So you can see in red every time, if it's going from a low concentration to a high concentration, that's not energetically favorable. And so we're going to have to have some kind of um, you know, as some kind of energy input. That's why all of the active transports have some sort of energy input and they have some sort of low to high concentration movement. So, you know, you can see with the antiport. that you have two, you can see what the secondary active transport all the time, you're moving two molecules kind of at the same time. And so with the antiport one would be low to high, and then one would be high to low and they would kind of go, um, in opposite directions, like one's coming in and one's going out. Um, but with the SIM port, um, you could, I could kind of think of it like you're sneaking your friend by or whatever at a party. It's like, okay, I have a good example. So, you know, at like frat parties or whatever, um, you know, all girls are invited, like girls get in free or whatever. But if I was going to like sneak my guy in for like, this is, um, let's see. Sexist. this is this is girls at a frat party so there's a high concentration of outside and they can easily get inside to where there's a low concentration of them but this red one there's a high concentration of guys in the party already because it's a frat so you already got like 20 guys there automatically and so i'm i'm gonna have to like sneak him in with me so you see both arrows are going um in this direction and maybe my iPad will like load. Yeah, they're both going in that direction. But one of them, you know, has to kind of go with the other. Yeah, the key words you're looking for here, if they give you a word problem instead of like a diagram, would be against the concentration gradient. So if you're moving something against the concentration gradient, which just means you're going from an area of low concentration to high concentration, it's active transport. Path of transport is from high concentration to low concentration. So Meg, what's the difference between facilitated diffusion and simple diffusion? So, facilitator, you can see they got these big green blobs here. Those are transport proteins, right? So they have to have some sort of channel to go through. It's probably because they're hydrophilic molecules. This is going great with the plasma membrane lesson. Whereas simple diffusion, if it's like a hydrophobic molecule, then it can go straight through. Can water go straight through? Yes. So water can go straight through one of the channels. So water is actually facilitated diffusion. Yes, exactly. So the name of the channel that water goes through is aquaporins. So osmosis or whatever is facilitated diffusion of water through an aquaporin. um so you got anything else to say about passive transport i was going to give an example for active transport no that's good okay um so i was thinking about the sodium potassium pump i think i thought at one point you were gonna mention it but i john correct me if i'm wrong would that be an example of co-transport see let's think it through because i think they're both going against their concentration gradient they they would both be going against their concentration gradient. So you snap an ATP. Yeah. So it'd be co-transport. Yeah. So if you, if you think about the sodium potassium pump, um, which is common in like nerve cells, like axons and stuff like that, right, John? Yeah. It's, it's, it's going to be in like every cell pretty much. Cause you're still going to have to maintain a, um, membrane potential across every cell. gotcha um so that'd be a good example of co-transport and then john was talking about chlorine and how it has to go through like a channel and so i'm I'm unsure of which transport chlorine goes through. That's beyond the scope of the MCAT, but I have seen a couple passages on the MCAT that talk about when those chlorine pumps can go wrong and it's mediated by a single mutation in the gene. And if you don't have a good chlorine movement across your membranes. then you can get cystic fibrosis. That's right. That's what I'm thinking of is cystic fibrosis. Look at you go. yeah i always thought that was interesting i've seen i've seen like one or two like i'm pretty sure at least two uh passages on cystic fibrosis and it'll it'll go into like the specifics of the gene and everything during the passage but i always thought it was cool how like one little thing could go wrong keep pouring out of the cell and then water's obviously going to follow those charged molecules and so that's why you get all that like thick accumulation of mucus that's normally would have more water in it would be looser Yeah, you'll notice a lot of that happens with the MCAT. Like the stuff that they test you on, like anytime you read a disease on the MCAT, you're probably going to see it again in med school pretty soon, pretty early on, because they use like, like they have characteristic diseases that they use to teach certain subjects, even if they're like not common at all. Like you're not going to see it whenever you're practicing medicine and still use it to. teach because it's it's it follows the rules it plays nice whereas most diseases don't plays nice with science this sick fibrosis is ugly okay well just a couple questions i think just one question so i i noticed when i looked I know, right? I noticed when I was going through my test, this is taken from the AAMC, or I think it's one of the AAMC exams, maybe the sample test or something. But I got this one wrong. And it was like one of the only ones I got wrong in the section. Slight flex. I was feeling good that day. But I got this one wrong. And so I thought it was interesting. But you want me to do it john are you i mean let's see you redeem yourself okay you're so dumb so um so let's see i'm a little nervous to be honest um so it says consider the structure it gives a structure of this molecule called stn in the passage and it says considering the structure of stn what is the most likely mechanism for its entry into the cell Okay, so basically, how does this molecule get into the cell? We just talked a lot about the plasma membrane and the ways that you can transport through the plasma membrane. So let's see if we actually, you know, can put our money where our mouth is. A says active transport. So what is that? That's requiring ATP. That is, you know, going against your concentration gradient. I have no prior knowledge to tell me the concentration of STN. outside or inside of the membrane. So I'm not really comfortable saying active transport at this moment. I'll say maybe not, but I'm not going to cross it off completely. B says receptor mediated endocytosis. So I was talking about endocytosis a little bit earlier. That would be for like really large molecules, probably larger than this STN right here. Um, but you know, maybe, maybe this STN could come on and latch to a receptor. And then that receptor could trigger the plasma membrane to endocytose it. But, you know, usually I think about a little bit bigger molecules, endocytosing like proteins. um, C says diffusion directly through the membrane. So, uh, that, that makes a little more sense. You know, I see a little bit of these polar groups, these, um, you know, I think I see some amides at the top and these carbonos hanging off and then some nitrogens throughout like these, I think they're called heterocycles. um but honestly the the carbon count kind of really outweighs that so i'm thinking that this looks pretty hydrophobic you know just a lot of aromatic aromatic rings aromatic aromatic um rings so he is looking good d says passage through an ion channel so this molecule is not an ion so wouldn't think that it would go through an ion channel um between b and c i just feel more comfortable saying that this molecule is pretty hydrophobic but it's not very big so i'm gonna go with c and that's how i would have figured it out if i was if my brain was working that day um what did you put the first time and I think I put active transport. I bet you were thinking. Because I wasn't. Yeah, I was. I was thinking that this was a polar molecule. And I wasn't thinking of active transport necessarily with concentration gradient. I was thinking like, what was it called? You were thinking polar versus non-polar. The carrier mediated. Yeah, I was thinking of like the carrier. The facilitated diffusion was what I was thinking of. Yeah, yeah, yeah. And you notice that they didn't put that. So polarity is like a, this is, I know you know this, Mags, but we can talk about it real quick. So polarity is kind of like a spectrum. It's not like there's a hard line. It's like after you cross this, you're no longer polar. Polarity is talking about relative to certain things. And so. Usually if the MCAT is going to test you on, is this nonpolar and it diffuses through the membrane or is it polar and it has to use a channel, they're going to make it really, really obvious that the molecule is polar and nonpolar. Here, yeah, there are a lot of rings, so I could definitely see the argument that it's polar or nonpolar. But man, there's a lot of like polar molecules. Like I see two carbonyls, I see a bunch of nitrogens, like I could argue pretty hardcore that this is polar. But you notice, well, back up, if it were polar, there would need to be an answer choice that said facilitated diffusion. Because that's how large molecules are going to diffuse. Large polar molecules are going to diffuse via. protein channels. There's not an answer choice that says protein channels. So the MCAT at least gave you that as kind of like a safety net. If you didn't know this was nonpolar, you could at least say, well, nonpolars are supposed to have protein channels. That would be facilitated diffusion. The only answer choice that says facilitated diffusion is answer choice D, like loosely says it, and this is not an ion. So even though ion channels are technically facilitated diffusion, it's a specific type, right? It's facilitated diffusion of an ion. And so you're not going to...this isn't going to work. Yeah, I didn't stick to my guns. I remember taking this question the first time and I really did just kind of, I guess, knowing that there's a transport protein in active transport and in facilitated diffusion, I kind of got tripped up and I kind of mushed them together when I really should have mushed active transport and against the concentration gradient together. Cause that's really what goes together more so than active transport and a transport protein. So, um, Both can have transport proteins, but only one goes against the concentration gradient. And receptor-mediated endocytosis is a specific type of endocytosis that I don't think is within the scope of the MCAT. Endocytosis is the idea of what it is, but receptor-mediated endocytosis is a specific type of endocytosis. I don't know that that's... within the scope of the MCAT. I've never seen a question on it, but the mechanism definitely is not, because it's uber complex, but the actual idea of it is fairly simple. There's a few, there's receptors that are going to accept certain molecules, and whenever the molecule binds to it, you literally just kind of form a pit, and then there's another protein that comes in and then closes that pit off. So... kind of, it's kind of in what it is, but don't worry. Don't go into the wormhole of trying to figure out that mechanism because you're going to Google it and you're going to see it and it's going to be terrifying. You don't have to learn it. You just need to know what it is. Yeah. Um, I had one more thing to say on these on, on transport and, um, now it's like slipped my mind. Um, what was I going to say? Oh, yeah. Um, so, you know, we talk about these carrier proteins, or these channels, or these transport proteins is really the catch all name for them. And I want to say that they are fairly specific. So like in that last, in that last question, we were doing answer choice D talked about ion channels, it's only going to let in certain types of ions. You know, there's a lot of transport proteins for a ton of different things around the cell in the plasma membrane, but they're going to be pretty specific. And so that's how we were able to mark off that an ion channel is not going to let in something that's not an ion. In fact, there's some like the sodium potassium pump that are just extremely specific to sodium, three sodiums and two potassiums. And that's what they do. And that's the only thing that they will push across the membrane. Alrighty. Is that it? Yeah, I think that's it. Alright, well I hope you all enjoyed our discussion of the plasma membrane. Let us know if you have any questions below.