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Plasma Membrane Transport
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Today we're going to be talking about transport. No, not transportation - I'm not talking about subways and cabs and Lyft drivers. I'm talking about transport. How we get substances across plasma membranes. So stay tuned!
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Hi and thanks for tuning in today we're talking about transport, which is a discussion about how we get things across plasma membranes and into and out of cells. So if you remember, all cells are surrounded by a plasma membrane. This selectively permeable barrier prevents things from getting into the cell as well as things from getting out. But this is a double-edged sword, right, because that means not only do we prevent bad things from getting in and good things from
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leaving, but we also trap bad things like wastes in our cell and we prevent good things like nutrients from being able to make it in. So what's the solution to that problem? Well the solution to that problem is transport. So we're going to talk about two major types of transport, but in order to talk more about that we have to talk a little bit about these things called gradients.
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So because plasma membranes act as a semi-permeable barrier between things quite often we end up with what we call a gradient of a particular substance. What that means is on one side of that selectively permeable barrier we have a lot of something and on the other side of that selective permeable barrier we have not a lot of something or less of that. Now according to the way physics in the universe works things will always naturally flow from where there
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is a lot of it to where there is a little of it. We call that difference between the area of high concentration an area of low concentration, we call that a gradient. And we call that movement of particles to disperse evenly from areas of high concentration to low, we call that diffusion. And diffusion is something that happens naturally. Things like to be approximately evenly spaced out with respect to each other. So as long as things are happening where they're going
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from areas of high concentration to low, that is going to fall into the category of passive transport because it's not going to require energy to occur. So how does that happen when you have that selectively permeable barrier that is the plasma membrane between substances? Well, let's talk about that. So first we'll start by talking about passive transport. The easiest form of passive transport to understand is what we call simple diffusion. There are some things
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that are able to pass freely across the plasma membrane. These are small hydrophobic substances. These are things like carbon dioxide and oxygen and nitrogen. They're small enough, they're hydrophobic enough where they're allowed to squeeze between the hydrophobic tails of the phospholipids and make it into and out of the cell freely. Now that's a good thing in one way because that means we don't have to dedicate any sort of energetic transport to bringing oxygen in and out
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and carbon dioxide in and out of our cells. The downside is because they go through free diffusion we're not able to regulate that behavior at all. So in other words, we're not allowed to control how much oxygen and carbon dioxide are going into and out of our cells. So there's a bit of a good and bad there. But what if something is too big or too hydrophilic and isn't able to make it across
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a plasma membrane, but is still going down its concentration gradient, it's going in the correct direction energetically, can that still come in through passive transport? And the answer is yes. A lot of people seem to think that passive transport can't involve proteins. Of course it does! In fact the majority of things that make it into your cell through passive transport do so through a process known as facilitated diffusion. Facilitated diffusion is simply passive transport
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that utilizes dedicated transport proteins. So if you go back to our previous video on the plasma membrane, you'll remember that there's a whole group of proteins that exist within plasma membranes called transporters, and that's a large part what these guys do. So facilitated diffusion can occur as long as a substance such as a sugar or a protein or something like that, which is too big or too hydrophilic to squeeze between the phospholipids and make it into our cell,
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can still go down or work with its concentration gradient. It can still diffuse. In this case it's going to go through a dedicated transport protein. Now the cool part about that is this:
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because it requires a protein to do so in many cases we can regulate that behavior. We can turn off a transporter or we can turn a transporter on if we need it to function. So while it can still go without the indirect input of energy through passive transport, we can at least turn the spigot on or turn the spigot off. A specific example of facilitated diffusion involves the diffusion of water, and that is what we call osmosis. So osmosis is pretty
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much the facilitated diffusion specifically of water and they give it its own category because of the central importance that water molecules have to life itself. So water is the solvent of life and almost everything that we're bringing into and out of a cell is what we call a solute. So anything that's dissolved in the cytoplasm or dissolved in a fluid is referred to as the solute.
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Now, when we talk about osmosis we're talking about the facilitated diffusion of water. Osmosis goes through a dedicated set of transporters that are known as aquaporins that specifically allow water molecules to cross across the plasma membrane. And because again this is diffusion, this is passive transport, we actually don't regulate the movement of water. If water is in abundance inside the cell, it will leave. If water is in abundance outside of the cell,
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it'll come in. Now I know what you're thinking and this is kind of weird right? Because if we're talking about concentrations, if we're talking about concentration gradients, what we're talking about is the amount of dissolved solute in a sample. So how can there be less water inside of a cell as outside? Water is the solvent, right? Well kind of...and this gets us to a conversation about
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something called tonicity. Tonicity refers to the amount of dissolved solutes within a solution. In biological systems we're almost exclusively talking about aqueous solutions involving water since water is the solvent of life. We measure tonicity typically in the concept of osmolarity, referring to the amount of dissolved solutes that are in that sample.
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Now what happens when a cell is placed in a solution where there are fewer dissolved solutes? Now we have to think about this from two different angles. If there are more dissolved solutes inside of the cell than outside of the cell, one way to sort of rectify that gradient, one way to resolve that gradient, would be to allow all of those solutes to cross the plasma membrane and balance out the solute concentration. Right? But that can't happen
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and one of the main reasons why is most of those things can't freely diffuse across the plasma membrane. They go through some sort of facilitated diffusion mechanism, or, even though there might be a grand total of more solutes on one side of the membrane, individually those gradients may not be outwardly rectifying, as we say. So the other way to balance that gradient out, the other way to rectify that gradient, would be instead to allow water to come into the cell, right,
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and just balance out that equation. And that is how life typically has to do it. That's how cells have to regulate that. So while we tend to think of it as there are more solutes inside of the cell than outside, another way to interpret that is there's just not enough water in the cell to make sure that the solute concentrations are equivalent. That solution that's outside of the cell that has fewer dissolved solutes is what we call a hypotonic solution, and if
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you place an animal cell in hypotonic solution, in order to sort of balance things out, what's going to happen is water is going to rush through those aquaporins into the cell and that's going to cause the cell to swell. In an animal cell that's likely going to cause the cell to burst.
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That is why, for example, if you have a patient who is dehydrated you do not give them an IV bag full of pure water. You would be giving them a hypotonic solution. Water would rush into their red blood cells and their red blood cells would explode and now you've created a whole other problem for that patient. Instead we like to use what we call an isotonic solution, which we'll cover in a minute. But you may be thinking, "Wait a minute, we don't give plants
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Gatorade". We don't give plants energy drinks because, you know, they need the electrolytes. No, we give them pure water. Why can plants and fungi tolerate pure water - tolerate a hypotonic solution? Well the answer is simple, they have a cell wall. It's really that simple. That cell wall gives them the rigid structural support needed to keep them from exploding when they're exposed to a hypertonic solution. In fact, they prefer hypertonic solutions because what ends
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up happening over time is as those plants begin to lose water, if you don't water them enough? What happens to the plant? It wilts. Why does it wilt? It wilts because the the cytoplasm is drying out and the rigid structural support of the cell wall isn't enough to keep the plant upright and it begins to shrivel and droop because it ran out of water. Give it water again, (whoosh), a hypotonic solution, swells back up, and the plant stops wilting and goes back to its normal form.
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Unless you're me. I don't have a green thumb. I kill pretty much anything I try to grow, so typically the plant just dies at that point. But what about the opposite? What happens in the opposite case? What happens if we have what we call a hypertonic solution? What if there are more dissolved solutes on the outside of the plasma membrane than there are on the inside? Well in that case, water is going to try to leave the cell to balance out that equation.
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Water leaving the cell is going to cause the cell to shrivel and that will happen in both plants and animal cells. The cell will begin to shrivel. So a hypertonic solution is also quite harmful to cells because in order to try to balance things out water is going to leave.
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So how do we prevent that from happening in our patients, for example, who might be dehydrated. Well we give them an isotonic solution. With an isotonic solution, the dissolved solute concentration is equivalent on both sides of that plasma membrane. Now does that mean water isn't going through aquaporins? Absolutely not! What it means is the net movement of molecules is even. An equal number of water molecules are going in as going
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out and the equation is nicely balanced. That's why, for example, if we need to rehydrate our patient we'll give them a bag of lactated ringers, for example. Something like that, which might prevent them - from their cells from exploding and dying. So that's what osmosis is. So if we're talking about free diffusion or facilitated diffusion or osmosis, we are talking about the movement of molecules in accordance with their gradients. In accordance with their chemical
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gradients. They're moving in the direction favored by the universe. But what happens if we're working against a gradient or we're trying to maintain or create a gradient? Iin other words, we're making things go in the direction they don't want to go. That's where active transport comes in.
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Active transport requires the input of energy because we're going to be making solutes move in directions that they don't want to move. We're going to be either moving them from concentrations of low concentration to high concentration or we're going to be trying to create a gradient by making a high concentration on one side of the plasma membrane or the other. And this, of course, is going to require energy because it's going against the natural flow of things.
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Now active transport can be broken down into two different varieties. Primary active transport, which requires the direct input of energy, and secondary active transport, which usually harnesses the power of an existing gradient to pay for that particular energy debt. We'll talk about that in just a little bit. So a great example of primary active transport comes from something called ion pumps. Ion pumps like the sodium potassium exchanger
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work at the plasma membrane and they basically hydrolyze ATP, release the energy by breaking down ATP, and converting into ADP to power the movement of ions against or to create a gradient. If we're talking about the sodium potassium pump, what happens here is using ATP as an energy source, the sodium potassium pump will pump three sodium ions out of the cell bringing in two potassium ions. An
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exchange of three for two. This is actually going to create a what we call an inwardly rectifying sodium gradient. In other words, there's more sodium on the outside that wants to come in. And an outwardly rectifying potassium gradient. In other words there's more potassium inside that tries to get out, outwardly rectifying, to get into the extracellular space. But the other thing to note is we're kicking three positively charged ions out and only bringing back two.
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There's also what we call an electrical gradient across your plasma membrane. This is what we call an electrochemical gradient. The gradient that exists is actually both electrical and chemical. What this means is in a very real sense all of your cells are electrified. There is actually a small voltage across your plasma membrane because of that electrical chemical gradient.
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Now we'll talk in just a minute why, but why would a cell want to spend a bunch of ATP doing this and creating this gradient? Well because that gradient can be harnessed for secondary active transport. But just hold that thought because I want to talk about another form of primary active transport, and this is what we call group translocation. So group translocation is found mainly in bacteria
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and plants and also archaea. So prokaryotes and plants. Group translocation quite often doesn't use ATP as an energy source. But in the case of, for example, the phosphotransferase system found in bacteria, it actually uses phosphoenolpyruvate as a source of energy. And this particular system is - the phosphotransferase system is actually used to bring sugar molecules, things like glucose
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and fructose into the cell again. It's a form of direct (active) transport because those things are actually moving from a low concentration out of the cell to a high concentration into the cell. So that requires energy to work against its chemical gradient. But it uses phosphenolpyruvate instead of ATP as its energy source. So those are two examples of primary active transport. What about secondary active transport? Well one of the things you have to realize is this
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chemical gradients or electrochemical gradients, no matter what the case may be, are a form of potential energy. So as far as the universe is concerned, as long as there is an equivalent, an energy equivalency, things can move in either direction. What I mean by that is this: quite often what happens in the case of transport is what we call coupled transport. The movement of one particular solute occurs, can come, at the same time as the movement of another solute.
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And as long as one is energetically favorable, it can power the movement of another one, another solute, in an energetically disfavorable direction. So let me give you a great example:
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if we look at the SGLT transporters on your intestinal cells, the job of those intestinal cells is to bring glucose in from your gut into those cells so it can be used as an energy source, and eventually get into the bloodstream. But the problem is this, technically speaking the glucose concentration inside of your cells is much higher than it is inside of your intestinal lumen. So in
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order to bring glucose in, that's going to cost energy. Glucose is moving from a low concentration to a high concentration. So how do we pay that debt? Well remember, we have a truckload of sodium trapped on the outside of our plasma membranes that desperately wants to come back in. So how do we pay for this? Simple - we bring glucose in through this transporter against its gradient. At the same time we bring a sodium ion in with its gradient. As far as the universe is
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concerned the energy debt is paid. The favorable movement of sodium in, pays for the disfavorable movement of glucose coming in. This form of coupled transport is what we call secondary active transport. It's active because it requires energy, but it's secondary because there's no ATP actually being spent. The ATP was actually spent by the sodium potassium exchanger or what's also known as
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the sodium potassium ATPase at the plasma membrane a little while ago. And this is a very common recurring theme when it comes to active transport. It turns out that the majority of transport across your plasma membrane that is active in nature utilizes an existing gradient as its energy source. Secondary active transport through coupled transport is an incredibly common thing. It's so common that about 1/3 or 50%, 1/3 to 50%, of all ATP in a given cell is used to power that
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sodium potassium ATPase or that sodium potassium exchanger. Why? Because maintaining that huge, inwardly rectifying sodium gradient or outwardly rectifying potassium gradient can be utilized by other transporters to bring things in or out kick things out of the cell
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against its chemical gradient. Why do this? Well the answer is it's a lot more energy efficient. You pay one particular protein with ATP to do its job and then you can utilize the results of that work to do lots of other forms of transport. If you think about your own home, for example, how many furnaces do you have? You have one and it's in your basement and it's used to heat every room in your house. Now, could you theoretically have a little heater in each one of your rooms
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and you and adjust them as you see fit? You could, but that's kind of a pain in the butt and it's less energy efficient. Instead think of the sodium-potassium ATPase as the furnace, as the furnace equivalent. In this example the sodium-potassium ATPase uses a ton of the cell's energy to maintain this sodium-potassium gradient, but this sodium-potassium gradient can be harnessed by hundreds of other transporters to perform active transport by coupling the inward
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movement of sodium or the outward movement of potassium with the disfavorable movement of things into and out of the cell against their particular gradients. It is an example of increased efficiency brought on by evolution. Now as you can imagine there are different types of transport proteins that move things across the plasma membrane. We tend to break them down according to how many solutes they move and in which direction those solutes do move. Now all of these different
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types of of transporters can be involved in either passive or active transport, so keep that in mind. The first type of transport is what we call a uniporter. Uniporters are simple. They move a single solute across the plasma membrane a great example of this from the world of passive transport are aquaporins. They move a single thing: water. They move water molecules back and forth across the plasma membrane. There are also active uniporters called calcium
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uniporters. So calcium needs to be brought into organelles or out of organelles, in particular the smooth endoplasmic reticulum. Well quite often they're moving against their gradient. So how do we do this? Well we use a uniporter that's coupled to the hydrolysis of ATP to bring those things, to load the smooth ER back up with calcium ions once they're no longer needed. There are what we call symporters. So symporters are kind of straightforward. Symporters move more than one
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solute in the same direction. So we talked just a minute ago about those glucose transporters
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in your intestinal cells. They are going to bring in one glucose molecule against its chemical gradient and they'll do that at the same time as they bring in one sodium ion with its gradient. That is what we call symport you're bringing two things in the same direction, two different things in the same direction. And then of course you have the opposite, which is antiport, and in the case of antiporters you are bringing two different, or two or more, different
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substances in opposite directions. The sodium potassium ATPase is a great example of this. You are going to be kicking three sodium ions out of the cell in exchange for bringing in two potassium ions into the cell. Antiport: multiple things moving in opposing directions. Okay. So like I said. those can be involved in both passive or active transport. The last type of transport that I want to talk about is what we call bulk transport. So bulk transport you have to
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remember will only be found really in eukaryotic organisms, in particular animal cells and some protists. Bulk transport is a form of active transport, but what it relies upon is the the
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bending of the plasma membrane or the creation of vesicles in order to move objects and move solutes across the plasma membrane. It is a form of active transport because this movement of the plasma membrane, the formation of vesicles, or the release of vesicles does require energy to do so. So there are two types of active transport, broadly; there are subcategories of each. The first one is endocytosis. So endocytosis, is the movement of large molecules, even whole cells,
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into the cell. So there are a couple subforms of this. One of them is what we call phagocytosis. So phagocytosis literally translates to "cell eating". So phagocytosis happens when a cell needs to eat large, bulky substances or even whole cells. For example, like in the case of macrophages or amoebas - they'll eat whole other cells through phagocytosis. What needs to happen is pseudopods are going to begin to extend from the - created by the plasma membrane. This will
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create a pit. This pit will be reinforced by a protein called clathrin that will reinforce that structure. Eventually what will happen is those pseudopods will extend around whatever is going to be consumed through phagocytosis. Those pseudopods will then sort of come in contact with each other. Remember the plasma membrane is fluid like that so they can kind off pinch almost like a bubble. Forming this will create the vesicle, which will then pinch off, and that vesicle is
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already on the inside of the cell. Now if this is a phagocytic vesicle, that vesicle is then going to fuse with the cellular stomach, the lysosome. That vesicle will then disappear into the lysosome releasing its cargo on the inside the lysosomes. Enzymes will then break that down and then that particular food can go on to be processed further through the metabolism of the cell. Another type
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of endocytosis is what's called pinocytosis. This literally translates to "cell drinking". It was originally discovered and thought that the cell was basically consuming strictly liquids, in particular water, to replenish its cytoplasm. That does happen, but quite often there are smaller dissolved particles that the cell's trying to ingest in bulk. Pinocytosis is basically akin to phagocytosis except you're not going to find large bulky solids in there. Same basic process
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with the pinching off and the forming of a vesicle and then the dissolving of that vesicle inside the cytoplasm. The third major type of endocytosis is what's called receptor-mediated endocytosis. Receptor-mediated endocytosis relies on substances to bind to specialized protein receptors on the surface before the endocytosis begins. This is in large part how things like for example, LDLs, so low density lipoproteins, this is how LDLs are consumed by cells to bring cholesterol in.
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It's also how, for example, neurotransmitters can be taken up by various cells, but it can also be used against us. So receptor-mediated endocytosis is also how viruses get into cells, for the most part. They bind to specialized receptors that trigger receptor mediated endocytosis. These viruses come in and lo and behold once they get into the vesicle that's formed, they are not food. Instead, they're viruses and they go on to hijack the cell and reproduce and so on and so forth.
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The opposing process of bulk transport is exocytosis and this is how things make it out of the cell through bulk transport. So those exocytic vesicles can form by budding off of lysosomes in the case of waste products. This is how uh lysosomes this is how your cell gets rid of bulk waste. For example if you have a macrophage that just went around and ate a bunch of of pathogenic bacteria, it's going to digest them in the lysosome then form an exocytic vesicle to kick
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the waste out of that cell so we can go on and eat more things. Another type of exocytic vesicle can form from either the the endoplasmic reticulum or the Golgi apparatus. So for example this is how neurotransmitters are released into a synapse by some of your neurons in order to send a signal. It also be how certain proteins like insulin, for example, are secreted by your pancreatic
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beta cells through the formation of these exocytic vesicles and exocytosis. Big thing to remember is this is only really going to happen in eukaryotic cells. Bacteria, plant cells, fungi, they can't do this. Why? Because they have big, bulky cell walls that are in the way. And these cell walls are going to prevent the formation of those pseudopods or the formation of the necessary vesicles to do
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bulk transport. So this is restricted mainly to animal cells and some group of protists, things like amoebas, for example. All right so that's our video on transport. Thank you so much for tuning in. I hope you learned a lot about how cells are able to import things they need, and export things they don't want, and prevent everything else from getting in. That's the story of the plasma membrane and that's the story of transport. I hope you guys learned a bunch. I hope
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you guys see you guys again real soon. Thank you so much for tuning in and I'll see you next time!
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