Plasma Membrane | Biology | PDF

So now we're going to focus on the plasma membrane for a little bit here. And plasma membrane, we're going to talk about what it's made of. We'll also want to just briefly mention the fluid mosaic model. So this is not a rigid system. So it's going to be made of largely phospholipids, as we'll see i...

So now we're going to focus on the plasma membrane for a little bit here. And plasma membrane, we're going to talk about what it's made of. We'll also want to just briefly mention the fluid mosaic model. So this is not a rigid system. So it's going to be made of largely phospholipids, as we'll see it, or glycolipids, as the case maybe. And these are free to laterally move around throughout the membrane and stuff like this. And then also some of the other components, different membrane proteins and stuff like that, also often free to move around throughout the membrane, as long as they're not like rooted to the cytoskeleton in some way, shape, or form. So that's the fluid mosaic model, something you should definitely file away in your head. And three major components in talking about the plasma membrane. Phospholipids or glycolipids. And then we're going to have cholesterol. And then we're going to talk about the different types of proteins that might be associated with the membrane as well. All right. So phospholipids, we already studied their structure, typically glycerol, back bone, two fatty acids, and then some sort of phospholipids. This is like phosphotylcholine, phosphotylserine, phosphotyl enolamine. Notice the names phospholipids, whatever, and stuff. And we've got that polar head group. And then the two long-chain fatty acids are the two non-polar tails facing the interior. You can also glycolipids involve, because oftentimes some of your lipids are going to get glycosyl. They're going to have polysaccharides added and stuff like this. And oftentimes these are involved in recognition and things of that sort. One of the things, big keyword here, you're supposed to know that the membrane is asymmetrical. So what that means is that the outer layer of the plasma membrane and the inner layer of the plasma membrane don't have to have the same composition. So the phospholipids might be different. The fatty acids on those phospholipids might be different. Things about sort, even the distribution of the different proteins and stuff like that. And carbohydrates probably going to be different on both sides of the plasma membrane. So definitely a note to say that the plasma membrane is asymmetrical. So again, we'll talk about cholesterol very briefly in this context, but also things we mentioned earlier in this chapter in talking about cholesterol. So it's going to modulate membrane fluidity. And most of the time we'd like to associate that with that low temperatures, it increases membrane fluidity. But the opposite is true at high temperatures. High temperatures cholesterol actually decreases membrane fluidity. So it's probably most accurate to say that it modulates membrane fluidity. So however, most of the time the context it's usually brought up in is kind of assumed to be at low temperatures. And in that specific case, it definitely is increasing membrane fluidity. So the last component of membranes we'll talk about are the different membrane proteins. And they follow into a few different classes of proteins and you should understand those sanctions between these classes. So we've got peripheral membrane proteins which are just kind of loosely associated with the membrane. Just via some of the interlecular forces and different electrostatic interactions that are possible. But not anchored to any way or shape or form. So integral membrane proteins are going to be anchored to the membrane. But outside of the anchor, the rest of the protein is really just going to be on one side of the membrane, the extracellular side or the intracellular side. And then finally, transmembrane proteins are going to completely span that membrane. So it's worth noting we talked about this with the transmembrane proteins at the transmembrane region, part that are actually spanning the membrane, typically as composed of non-polar amino acid residues and stuff like that. So it's interacting with those fatty acid tails of the phospholipids. So this includes channel proteins, carrier proteins and porens. Things involved in transporting something across the membrane. In other sense, that you need a protein through which these are going to travel. That spans the entire membrane. So in the final talk about membrane receptors, and these are involved in recognition. They typically involve glycoproteins, can be involved in interacting with hormones or other different chemical messengers, and relay that signal into the cell. We call that signal transduction. Well, now we've got to talk really briefly about intercellular junctions. There's three major types you need to know. Gap junctions, type junctions, desmosomes. So gap junctions are junctions between cells that allow the passage of nutrients and things of this sort between the cells. So they're kind of holes in the membranes if you will between the cells. You might see these in cardiac muscle cells, not the only place. So in cardiac muscle cells, when you've got depolarization during a heartbeat and stuff like this. So it is these gap junctions that allow the passage of ions and stuff through to depolarize from cell to cell during muscle contraction. Tight junctions. These are going to seal the space between cells. This most notably happens like in your epithelial cells in your small intestine. So when you want to absorb nutrients from inside the limit of your small intestine into the bloodstream on the other side of the walls of that small intestine, so you want to make sure that those nutrients have to pass through the epithelial cells and therefore going to get absorbed into the bloodstream. If they were able to squeeze through the cracks in between the cells, that would be a problem. And so tight junctions are going to seal the space in between those cells to prevent such leakage from happening. So, and then finally talk about desmosomes. And desmosomes like to think of them as... They're probably the strongest of these inner cellular junctions. So, and they're there to kind of resist mechanical stress. So anywhere inside the limit of your small intestine into the bloodstream on the other side of the walls of that small intestine, so you want to make sure that those nutrients have to pass through the epithelial cells and therefore going to get absorbed into the bloodstream. If they were able to squeeze through the cracks in between the cells, that would be a problem. And so tight junctions are going to seal the space in between those cells to prevent such leakage from happening. So, and then finally talk about desmosomes. And desmosomes like to think of them as spot welds. These are probably the strongest of these inner cellular junctions. So, and they're there to kind of resist mechanical stress. Anywhere you might find mechanical stress, typically you're going to find these desmosomes. And so it turns out that in heart muscle tissue, which is expanding and contracting during muscle contract... Take a look at the Three muscle contractions. A lot of mechanical stress there. And so definitely a good place to find desmosomes. You're going to find it in the mucosal lining of your GI tract as well, where again depending on if you've just eaten or have started yourself, you're definitely going to have some expansion of contraction going on there. your skin, so lots of mechanical stress possible at your skin, and desmosomes present there as well. You should know that these guys are anchored to the cytoskeleton on the inside, which is definitely different from what you see both with gap junctions and tight junctions. Next, we're going to briefly talk about the cytoskeleton. We've got three different classes of proteins involved in the cytoskeleton. We've got microfilaments, intermediate filaments, and microtubules. Microfilaments being the smallest, microtubules being the largest, and you definitely know the distinction here. So, your microfilaments are made of the protein actin. So, in this case, they're involved in cellular motility. So, it can lead to helping change the shape of the cell to provide locomotion stuff like this, also involved in muscle contraction, also involved in cytokinesis, and also involved in a little bit of cellular transport, although we usually kind of describe microtubules as playing a larger role in that. There are ATP-dependent myosin proteins that can walk along the actin and carry things with them, or in the case that's also the source of contraction of the muscles where you've got myosin motors walking along the actin. Intermediate filaments, these are going to support the overall shape of the cell and give it some some some rigidity and things of a sort. And then finally, microtubules, a little bigger class. They're made of the protein tubulin. And this kind of, again, functions as a network of railroads, if you will, for intracellular transport. We can carry vesicles in different even organelles that are made in different parts of the cell as needed. Also involved in mitosis. This is, again, your microtubule organizing centers, the centrials. So, and the spindle apparatus that is created is also made of these microtubules. Finally, you're also going to find them in both flagella and cilia in eukaryotes. And again, we mentioned that the flagella in prokaryotes were made of the protein flagellin, whereas in eukaryotes that are made of microtubules, and that's a distinction worth noting. So, also associated with those eukaryotes flagella and cilia is this nine plus two arrangement of microtubules where you've got nine pairs of microtubules arranged around two central microtubules. So, you might hear that associated with it as well, and again worth filing away a little piece of trivia. So, I want to have a brief discussion about membrane transport. So, yourselves are isolated from surrounding environment by this plasma membrane, but we still need nutrients to make their way in and waste products to make their way out and things of this sort. So, we have to have transport across that membrane. We've got some vocabulary we need to learn associated with that. So, first, talk about diffusion. And diffusion is just the travel of a solute from areas of high concentration to areas of low concentrations. Why, if I open up a bottle of cologne in the corner of room, but eventually everybody in the room might have a chance of smelling it, because that cologne is traveling from an area of high concentration in the bottle out to areas of lower concentration across the room. So, that's diffusion. It is spontaneous, it's governed by entropy considerations, but does not take the input of energy for diffusion. Now, as most assist the diffusion of water, this is a little bit tricky, so we can see a little bit backwards in some cases, but it's the diffusion of water. And it's where water is going from a high water concentration to a low water concentration. So, if you look, when we talk about diffusion, we usually talk about spittily the diffusion of solutes, and the solutes are going to travel from high-solute concentrations to low-solute concentrations. But with osmosis, we're talking about the diffusion of water now. And if we look at that in terms of water, no problems. We're waters going from a high concentration of water to a low concentration of water. But if you look at it in terms of solute concentrations, it's going to seem a little bit backwards. And so, water going from a high concentration of water, or why would you have a high concentration of water because you have a low concentration of solutes, and then water is going to go from a high concentration of water to a low concentration of water, or why would you have a low concentration of water? Well, because you have a high concentration of solutes. And so, if you look at osmosis in terms of water, no problem, same definition as the fusion, look exactly the same. High concentration of water to low concentration of water. But if you look at it in terms of solutes, it's going now from low-solute concentrations, and water is traveling to high-solute concentrations. So, keep in mind that different students often get confused just a little bit. Some new words in terms of relative solute concentrations, hypotonic, hypotonic, isotonic. So, oftentimes, we look at these words in the context that's putting a cell in a surrounding solution. So if we take a cell here, and we'll just put it in some sort of extra cellular environment, we'll keep it real generic here. So if the environment has a higher relative solute concentration, we'll call that environment hypertonic. Hyper means above or beyond. So a hypertonic solution, just one with a higher relative solute concentration. And in this case, higher than the cell that would place it. So hypotonic, on the other hand, hypo means low or below, just like hypoglycemic means you have a low blood sugar. So lower below here means you have a lower relative solute concentration. So in the case of maybe I take a red blood cell, not in a hypertonic solution now, but maybe in pure water. Well, that pure water definitely has less solutes than what's inside a red blood cell. And so we'd be placing that red blood cell now in a hypotonic solution. So then finally, isotonic iso means the same or equal to. And so an isotonic solution is just one where the two regions you're comparing are equal in solute concentration. So you can definitely stick a cell inside of some sort of salt solution where the inside and outside have exactly the same solute concentrations. All right, so what's the result of sticking a cell inside a hypertonic environment? Well, if all of a sudden there's a higher salt concentration out here, then what's water going to want to do? Well, water's going to want to travel outside the cell. So, and again, that's just pure osmosis. Water wants to go from where there's a lower, I'm sorry, a higher concentration of water to lower concentration of water, but that means a lower concentration of solutes out to a higher concentration of solutes, in this case, out to that hypertonic solution. So the result is either creation or plasmolysis. So creation, you're going to get the shriveling up of a cell. So whereas plasmolysis is something we're reserving talking about plant cells. So I don't really shrivel up because of the cell wall, so but there is some shrinking that goes on in the process. And then finally, the opposite, what if you stick, in this case, your cell inside of a hypotonic solution, like again, that red blood cell in pure water. Well, in that case, where's the water going to want to go? Well, if I've got pure water outside now, now the water wants to rush inside the cell. In the case of red blood cell, it's often going to lead to it rupturing, and we call that cytolysis in this case. So that we've identified some of the vocab we need. We've got to talk about two major classes of transport, passive and active. So it really comes down to require the input of energy, yes or no. If it does not require the input of energy, that is passive transport. If it does require the input of energy, usually in the form of ATP hydrolysis, well then we're going to call that active transport instead. Now with passive transport, we've got two options, either simple diffusion or facilitated diffusion. So, and simple diffusion just means they can diffuse right across the cell membrane. And so oftentimes small solutes or non-polar solutes can diffuse right across. So small non-polar molecules like carbon dioxide, so it can diffuse right across the membrane. Some of the non-polar things like steroid hormones can often diffuse right across the membrane, and they're just going to undergo simple diffusion. So, but you can also have facilitated diffusion. And so oftentimes larger molecules or highly polar or charged molecules or ions, they can't just diffuse across the membrane, so they're going to need some help. They're going to need some sort of carrier or channel protein to facilitate their fusion. Now they're still moving down their concentration rate. They're still going from high concentration to low concentration. So no energy is required. They just need help in getting across that non-polar membrane. So now active transport is going to require ATP hydrolysis. It's going to require the input of energy. And why are we going to require energy? Because now we're going against the concentration gradient. We're trying to pump something against its concentration gradient from areas of low concentration to high concentration. The opposite of what diffusion wants to do spontaneously. And this is going to require energy. And again, this can happen in one of two ways. It can even happen in primary active transport or secondary active transport. In primary active transport, we simply are using ATP to pump that solute across against its gradient. Done. In secondary active transport, we're going to involve at least a couple solutes here. And we're going to pump one solute across the membrane and again against its concentration gradient. And then we're going to let it diffuse back across the membrane in the spontaneous direction and carry some other solute with it. In that case, that's what makes it secondary active transport. So we'll look at a special example of this here on the next slide. All right. So it turns out the sodium potassium pump, which plays a huge role in maintaining resting membrane potential, is an example of primary active transport. So if you look inside the cell, there's a high concentration of potassium ions. I like to think of circle K. It's a convenient store in gas station in many parts of the country. And if you look at its symbol, it's circle with a K inside. And it's convenient to remember that potassium has a relatively high concentration inside the cell and a very low concentration outside the cell. Well, for sodium, it's the exact opposite, relatively high concentration of sodium outside the cell and a low concentration of sodium inside the cell. And the sodium potassium pump is involved in maintaining that. So the sodium potassium pump, it's driven by ATP hydrolysis. And it pumps two potassium ions into the cell against its gradient. Notice, potassium would rather diffuse outside the cell by diffusion passively, if it could. So but the sodium potassium pump is pumping it into the cell even more. And then three sodiums are exchanged and pumped outside the cell also against its gradient. So both ions become against their gradients. That's what makes it active transport. And we're directly tied there transport to ATP hydrolysis directly. That's what makes it primary active transport. Now secondary active transport, again, you're going to use ATP hydrolysis to establish a concentration gradient in one solute and then use the diffusion back across that gradient to use the transport something else. So in the case of the sodium glucose co-transport. So in this case, we use the sodium potassium pump to establish the sodium gradient, where we have a high concentration of sodium outside the cell. And then it turns out there's a protein that involves the co-transport of sodium ions and glucose into the cell. And so we've established this gradient using ATP hydrolysis to cause energy to have all the sodium outside the cell. But we can let it diffuse back into the cell and take glucose with it. And so in this sense, we are using energy to get glucose in the cell, but kind of in an indirect way. And that's what makes it secondary active transport. And then finally, we have one other mode of membrane transport that's going to involve either endocytosis or exocytosis, in which case we're having parts of the membrane either or vesicles fused into the membrane or we're pinching off vesicles from the membrane one way shape or form. So we take a look at endocytosis. Endo means inside. And so endocytosis, we're bringing things into the cell. And so those things are going to come in. We're going to form a structure around them and pinch off a vesicle to bring them inside the cell. And so when they come inside the cell, they're now inside of some membrane bound vesicle. Now, exocytosis is the exact opposite. We're going to have some things enclosed in an intracellular membrane bound vesicle that vesicle is going to fuse with the cell membrane and its contents can get emptied out. So that would be exocytosis. Now, in endocytosis, we've got three different types we can talk about. We can talk about phagocytosis, often referred to as cellular eating. So in this case, we're doing endocytosis of solid material. Whereas, pinocytosis, cellular drinking, now we're doing endocytosis of solutes dissolved in water. And then finally, we can have receptor-mediated endocytosis. So it's not generic. So some receptors have to be bound that's going to result in the formation of a coated pit. And so it's endocytosis is going to be triggered by receptor-binding. And you get this lovely coated pit, and it's going to bring in a vesicle with those coated pits, if you will, on the interior. Now that vessel is brought to the cell.