Hey everyone, Dr. D here, and in this video we are going to be covering Chapter 7 from our Biology 12th edition from Campbell. And this chapter covers membrane structure and function, so let's go ahead and get started. Dr. D, Dr. D, Dr. D, explain stuff. Alright, welcome back. Let's do it.
Chapter 7... membrane structure and function. So we're talking about all the ins and outs of the membrane today, what it looks like, you know, what it's comprised of, what are the components of the membranes, and how they work, you know, what's the function of the membrane as well. Remember that a membrane, when we're talking about the membranes of the cell, we are talking about a phospholipid bilayer. Remember that?
We talked about the lipids called phospholipids, which have a hydrophilic head as well as two hydrophobic tails. Remember that? And this is why lipids or phospholipids, I should say, are known as amphipathic molecules.
Amphipathic meaning they're part hydrophilic and part hydrophobic. They are amphipathic molecules. Now, remember that the membrane consists of a phospholipid bilayer. Remember that?
A phospholipid bilayer. So if we have phospholipids at the top layer, you have the hydrophilic head pointing out of the cell where there's water. And the next layer of phospholipids would have the hydrophilic heads pointing into the cell where there's also water.
Notice that, you know, if there's water outside of the cell, that's where the heads are because the heads are hydrophilic. If there's water inside of the cell, you see that's where the heads are because the heads are hydrophilic. And remember, that leaves this middle region in the membrane, right? You see this middle region with the tails?
These tails are hydrophobic because they are essentially fatty acid chains, remember, made up of just carbon and hydrogen. So they are hydrophobic. There should not be any water in here with these tails, remember that?
There shouldn't be any water in this area here where the tails are. That should be a water-free environment. And these tails are all hanging out together because of hydrophobic interactions. right?
This is known as the hydrophobic region of the membrane. Every membrane has this hydrophobic region. And by the way, do you remember that lipids are not macromolecules? Remember what that means? That means that individual lipids like this phospholipid right here, this phospholipid is not actually linked covalently to its neighbor.
These phospholipids aren't bound to one another. Does that make sense? Because these are lipids, and lipids don't get linked covalently to one another.
Instead, they're just hanging out together with these interactions, you know, these mainly hydrophobic interactions with these tails. So what that means is that the membrane phospholipids are actually not connected to their neighbors. That means that these little phospholipids are free to float around laterally.
They're not connected to their neighbor. Like, for example, you see these two neighbors right here? You see these two phospholipids right here?
They're not obligated to hang out together. This one could drift off this way, and his neighbor could drift off the other way. Kind of like if you're at a party, you know, you're schmoozing at a party. They chat for a little bit, and then they can go off in their separate directions.
Okay? This... And that is why the membrane is called a fluid membrane because it's not static.
It's not stuck in place. These phospholipids aren't glued in place. They're liquid.
They can move around. Liquid means there's movement, right? So the membrane is a fluid membrane.
These phospholipids are not obligated to hang out. In fact, they're constantly moving around and moving around. And what do I mean by moving laterally?
Moving laterally, I mean this phospholipid will just move off in this left direction. This one will move off in this right direction. That's what lateral means. kind of like on the same plane here.
Now what phospholipids won't do is like flip to the other side. I mean, they can. It's very rare, though, you know, for a phospholipid to go from this orientation down here in the bottom bilayer. And for the phospholipids to suddenly bounce like, like swap to the top layer, it can happen, but it's rare.
Okay, so this is what the membrane looks like. Remember, it's a phospholipid bilayer. And it's got a hydrophobic region, hydrophilic regions, and these make up the membranes of your cells. Now, in addition to being fluid, the membrane is also a mosaic. If you don't know what a mosaic is, take a look at your screen right here.
This is a mosaic tile piece of artwork where they've taken different pieces of tile and pieced them together in this beautiful mosaic. And mosaic means it's made up of... different different types of parts like this and so a membrane is a mosaic of all different kinds of proteins membrane proteins and other structures as well including different types of steroids and different types of sugars and things glycoproteins lipoproteins so let me show you what i'm talking about here is a better illustration of a membrane So the membrane itself is mainly comprised of these phospholipids. See all these phospholipids here in this bilayer?
That makes up the membrane, which is fluid. Remember that these little phospholipids are floating every which way, okay, like the ocean. But notice, notice, there are all these giant proteins. These purple things are proteins. And they're floating around in the membrane.
These are known as membrane proteins. And there are so many different membrane proteins. They have so many different functions. We're going to talk about what these membrane proteins do later on in this chapter.
But they have so many different jobs, right? We talked about one of the membrane proteins before, integrin. Remember integrin, which is part of the membrane. extracellular matrix, you know, but there's so many different membrane proteins and they all have jobs. And by the way, unless they're otherwise anchored to something like these membrane proteins are free to float in this ocean.
Isn't that neat? Kind of like icebergs. You know how an iceberg can float in the ocean and just float off to wherever it wants? Okay. An iceberg.
These are like icebergs. These, these proteins are like icebergs. This membrane protein, for instance, you see this one right here, this membrane protein could float off into the distance that way and its neighbor could float off this way. And like I said, unless they're anchored to something like the extracellular matrix, this must be integrin right here because look, it's got fibronectin, it's connected to the extracellular matrix and it's got, you know, it looks like it's anchored to the microfilaments inside. So if it's anchored to a bunch of stuff, then this membrane protein isn't going to float around.
But as long as it's not anchored to anything, these membrane proteins are free to float laterally. They're free to float off to wherever they want. Because remember, the membrane is fluid. And for this reason, the membrane is called a fluid mosaic model. Isn't that neat?
It's fluid because the phospholipids and the unanchored... The proteins are so free to float around wherever they want to go laterally across the membrane. And it's a mosaic of all these different proteins and other components as well. So when they describe the membrane, they describe the membrane as a fluid mosaic model.
And now you know why. And if we look at this, not only are there proteins in the membrane, but take a look. Do you remember we talked about cholesterol in a previous chapter and I told you it's a steroid protein that lives in your membranes. Well there it is.
Cholesterol is in the membrane here. You see cholesterol is a steroid hormone and here you can see that it exists in the membrane of the cell. There are so many different components. Here's a glycolipid.
A glycolipid which is a part of the membrane. That means a part lipid, part sugar, the green part sugar. The tails are lipid, so this is a glycolipid.
There's so many different characters that live in your membranes, and that's why they call it a mosaic. It's a beautiful mosaic of different components. But of course, at the heart is always the phospholipid bilayer.
Now that you know that the membrane is fluid, you should realize that it's actually quite important for the membrane to have a certain amount of fluidity. This means that the phospholipids, these proteins, they need to be able to move around at a certain pace. And what could...
mess up this fluidity? Well, when temperatures become cold, remember what happens when temperature gets lower? Things tend to slow down, don't they? Well, the membrane needs to have a way of continuing to move if the temperatures slow down, okay?
If the temperature is lower, okay? And, and what if the temperature rises significantly? That means that things kind of can become too fluid, right? Move around too fast.
And that's a problem as well. So let me tell you how the cell addresses these issues of maintaining just the right amount of membrane fluidity, whether it's getting cold or hot. Let's talk about that next. So look at this.
As temperatures cool, membranes switch from a fluid state to a more solid state, and that's not good. Remember, we don't want the membrane to freeze up. That means that the cell isn't gonna function correctly.
The temperature at which a membrane solidifies depends on the types of lipids that are in the membrane. Membranes rich in unsaturated fatty acids, remember, such as plant membranes or fish membranes with those kinks, Those unsaturated fatty acid tails on the phospholipids are more fluid than those rich and saturated fatty acids. Remember that saturated fatty acids tend to be solids at room temperature, while unsaturated fatty acids tend to be liquid at room temperature.
Hmm, makes sense, right? You have more of these kinks that keeps the membrane moving, that makes it harder for those... fatty acids to stack up and slow down.
So membranes must be fluid to work properly. So let's talk about how cells keep their membranes fluid. Now, again, on the left, you have a membrane, right, a phospholipid bilayer, and it looks like these little phospholipid tails are kinked.
You know, it looks like they're doing a little jig, a little dance, right? So The reason these phospholipids are kinked is because of the unsaturated fatty acid tails with the double bonds causing the little kink in the tails. And what's cool is this keeps the membrane fluid.
This keeps the membrane from freezing up and solidifying when the temperature becomes cold. However, look on the right. Look on the right.
When the temperature becomes cold for a saturated fatty acid, membrane like this one, see how the fatty acid tails are all straight? That causes the tails to stack nicely. When the temperature gets cold, the saturated tails pack together, and that could cause your membrane to freeze up. So we need a way to prevent this, right? Us animals, us animals with these saturated fatty acid tails, we need a way to prevent that, don't we?
Well, in comes cholesterol. Let me introduce cholesterol. Remember we talked about cholesterol.
It's a steroid hormone. You know it's got four fused carbon rings. Cholesterol is a membrane component in animal cells, you know animal cells which have saturated fatty acid tails, that has variable effects on membrane fluidity.
Cholesterol is in our membranes, that's where it lives, and its job is to maintain membrane fluidity at different temperatures. So for example, what happens at high temperature? Does the membrane move too quickly or does the membrane move too slowly?
Is the membrane in danger of moving too quickly at high temperature or is it in danger of moving slowly and freezing? That's right, Wicket. You know, he always comes up with the right answer. At high temperatures, At high temperatures, the membrane is moving too quickly.
Remember, because heat causes movement, right? Heat is a consequence of movement. So high temperatures, the membrane's moving too quickly.
Well, guess what cholesterol does? Cholesterol restrains the movement of the phospholipids, saying, whoa, whoa, whoa, slow down, slow down. You don't have to move so fast.
You don't have to move so fast. But conversely, look at this. At low temperature, when it's cold, right?
When it's cold, then these phospholipids start slowing down and slowing down, and they're in danger of freezing, okay? It maintains the fluidity by preventing tight packing. The cholesterol prevents those phospholipids from packing so easily, keeping the membrane fluid even when it's cold, right? So isn't that interesting?
Plants use different but related steroid lipids to buffer membrane fluidity. So plants, they don't use cholesterol. And this is why plants products are free of cholesterol. You know, if you've ever eaten any plant products or looked at the label on the back of vegetable oil, olive oil, you know, any kind of oils from plants, and you look at the nutritional facts on the back, you know, there's never any cholesterol in plant products. And that's because plants, they don't use cholesterol to buffer the fluidity of the membrane.
They use a different but related steroid, lipid. So, so plants do not have cholesterol, but animal cells do have cholesterol. And now you know why look at this. Now, you know why it's because our phospholipids, see, this is an animal phospholipid. I see only saturated fatty acid tails in our, in our lipid tails.
We are in great danger of having these saturated fats slow down and freeze. So what we have, we have this hormone steroid lipid in our membranes called cholesterol. Remember, cholesterol is a four fused ring steroid hormone, and it lives in our membranes.
And again, its job is to serve as a fluidity buffer. And what does that mean? That means when temperatures get high, it restrains the movement so it doesn't move so fast.
And when temperatures get cold... It keeps things from freezing up. And that's what buffers do.
Buffers maintain a range. Now recall that in addition to different types of steroid hormones like cholesterol in the membrane, we also have all this mosaic of different proteins in the membrane as well. And there are two major types of membrane proteins.
peripheral proteins, and integral proteins. These are the types of proteins that you would find at membranes. So let's talk about those.
All right, so take a look again at this illustration of a typical membrane, a plasma membrane. Not only do we have the phospholipid bilayer, but we have cholesterol to help to buffer the fluidity of the membrane. And remember, we have all these large purple structures.
These are membrane proteins, and we have a myriad different types of membrane proteins, all with different functions. Some of them are loosely associated with the membrane. See here? These are known as peripheral membrane proteins. They're really kind of on the inner surface of the membrane like this, and they're easily dislodged from the membrane.
However, there are... Proteins that actually kind of delve into the hydrophobic part of the membrane, aren't there? Look at this. These proteins, they exist within and among the hydrophobic parts of the tails. They could either span the entire membrane or they could exist in there with the tails, but not necessarily span the entire membrane.
These are known as integral membrane proteins. Integral membrane proteins, as their name suggests, these are the proteins that either span the whole membrane from outside all the way to inside. By the way, there's another term for that. That's called a transmembrane protein.
You see a transmembrane protein spans from outside all the way to inside like this guy right here. So all transmembrane proteins are integral membrane proteins, but not all integral membrane proteins are transmembrane proteins. Like look at this one right here. Look at this one. This protein is not a transmembrane protein because it doesn't quite, you know, it doesn't quite span from the outside all the way in, but it is integral.
because it does exist among these fatty acid tails. Does that make sense? So if you're deep in the hydrophobic part of the membrane, you're an integral membrane protein.
But if you span the entire membrane from inside to out, you're a transmembrane integral membrane protein. I hope that makes sense. Now you may be wondering, how are these proteins hanging out in the membrane?
Isn't the membrane hydrophobic? Well, yes, you're right. Let me show you.
So if we zoom in on one of these purple proteins that is a transmembrane, integral membrane protein like this purple guy right here, notice how he lives in the membrane, the phospholipid bilayer. And it is a protein because it has, oh yeah, it's a protein. It's a long chain of amino acids.
Here's the N-terminus. Here's the C-terminus. It looks like this protein is made up of 1, 2, 3, 4, 5, 6, 7 alpha helices and maybe 1 or 2 beta sheets up here.
So anyway, that's all. I digress. But this is a protein. It's a transmembrane integral membrane protein. And you can tell it really is a protein because it has a protein structure.
Now, what's interesting is, you know. need to know this for my class, but I just like to explain things as you know, you know, in case you're curious. What's really cool about these transmembrane proteins is that they are amphipathic themselves. Remember, amphipathic means a molecule with part polar and part nonpolar, or you could think of it as part hydrophilic and part hydrophobic portions.
The outside, I'm just talking about the outside. the outside of this protein, this part that has to interact with water, do you think it's non-polar? This part of the protein right here that I'm drawing a box around, this part of the protein where there's water, do you think this part of the protein is polar or non-polar?
That means hydrophilic or hydrophobic right here. That's right, Wicket! Yes, this part of the protein right here would have to be hydrophilic or polar because it lives among water, right?
Amongst the water molecules, right? But what about this part? Look, doesn't the outside of this part of the protein have to live among hydrophobic tails, right? These fatty acid tails that are made up of just carbon and hydrogen. How would you, how would you live among these tails, right?
What property does the outside of the protein have to have? that lives among these tails in the hydrophobic region. That's a tip.
Hint, hint. That's right again, Wicket. This part of the protein should be hydrophobic or nonpolar. And by this part of the protein, I just mean the surface, the surface of the protein that actually touches the tails. The inside of the protein, that could be a different story because it's not touching the tails.
And what about, again, this part of the protein down here? What about this part of the protein? That's right, Wicket. This part is hydrophilic or polar as well because this part is touching the surface of the protein at this part is touching the cytoplasm of the cell, right? The cytoplasm, which is full of water.
So the transmembrane proteins themselves are amphipathic, just like phospholipids. They have two two hydrophilic or polar sections on the inside and the outside of the protein and one hydrophobic section which is in the middle and again remember these what I mean by that is just on the surface of the protein and again this is a little more detail than you probably need this is the type of stuff you'll learn in biochemistry when you move on to biochemistry however if you're following along right now with this then pat yourself on the back because this is some in-depth biochemistry. This is some deep understanding. And it shows why it's so important to build that strong foundation. You know, when we were in chapter two talking about chemistry and I was telling you the difference between nonpolar covalent bonds and polar covalent bonds and nonpolar molecules and polar molecules and what interacts with what.
And the reason for that was so you can... understand these more complex structures and these more complex interactions like this. So pat yourself on the back if you're following along.
If not, you know, don't worry about it. This is a little more depth than you need for 1406. But I thought, you know, some people like to know this type of detail. So let's carry on. All right, at this point, you may be wondering, what is the function of all these transmembrane proteins? You know, like this one and this one, you know, what are these transmembrane proteins doing in there?
What's their function? So let's talk about that next. All right, take a look. Cell surface membrane proteins. can carry out several functions including transport, enzymatic activity, signal transduction, cell-cell recognition, intracellular joining, and attachment to cytoskeleton, and extracellular matrix.
So take a look here. These are the functions, some of the functions of those transmembrane proteins we were discussing. All right, let's look at the top left here. Some of these transmembrane Proteins serve as transporters.
Their job is to allow substances to pass through them to cross to the other side. This could be stuff coming into the cell through the transport proteins, stuff leaving the cell through transport proteins, but their job is to transport substances across the membrane. Here's another example of what transmembrane proteins can do.
They could serve as enzymes. These are protein enzymes. Remember what an enzyme does?
It is a catalyst that speeds up a reaction, such as, look at this, you see this pink thing is being broken down into the blue thing, and then the blue thing is being broken down into the green thing. You see, so enzymes speed up a reaction. They serve as catalysts.
They don't get consumed by the reaction. They catalyze. And so some of these transmembrane proteins serve as enzymes. Here's another function. These transmembrane proteins may function in cellular signaling, or what's called signal transduction.
This means cell signaling. So, have you ever heard of receptors or cell receptors? Receptors are oftentimes these transmembrane proteins.
And these transmembrane proteins can respond to signals in your body. For example, look at this orange structure that looks like a pizza slice. This might be a hormone in your body, such as, I don't know, adrenaline or insulin or some kind of hormone in your body, right?
And this hormone can attach to a transmembrane receptor protein. And once the receptor is bound to the hormone, it can signal into the cell to cause changes to the cell. Isn't that interesting? So your transmembrane proteins may serve as receptors, hormone receptors, signal receptors. Another function for your transmembrane proteins, they may serve as cell-to-cell recognition proteins.
Sometimes cells need to kind of communicate with one another to identify one another. And the way they identify one another is via these cell-to-cell recognition proteins. And oftentimes... These cell-to-cell recognition proteins have a sugar attached to them. So it's a transmembrane protein in purple, but it may have a specific sugar attached.
And so it becomes what's known as a glycoprotein, a protein with a sugar attached to it. And a lot of these glycoproteins, their job is to signal to other cells. So for instance, your immune cells, your immune cells might be looking for these these cell recognition proteins on the surface of your cells in order to double check and make sure that you are a cell that belongs in your particular body and not a foreign cell.
Does that make sense? Here is another purpose for these transmembrane proteins. Intercellular joining. Do you remember when we learned about different types of cell junctions, junction proteins. Remember those transmembrane proteins?
They can fasten to other adjacent cells. So in this case, this transmembrane protein is fastening, fastening to a adjacent transmembrane. protein, and that's linking the two cells together, almost like buttons, right? Like if you want to button this cell to that cell, these intercellular joining proteins can fasten the two cells together, making tissues. Isn't that neat?
Making tissues. Think, you know, your tight junction proteins that we learned about recently in the previous chapter. And then lastly, the type of function that we learned about.
The transmembrane protein could serve as either attachment to the cytoskeleton or to the extracellular matrix. We already learned about one example, right? Remember integrin? We learned about integrin, which I said is a transmembrane anchor.
It's this transmembrane protein that links, that anchors to the extracellular matrix. Remember, this would be, for example, collagen. and proteoglycan complex in green. And this little tongue looking thing is fibronectin. Okay.
So it's linking to the extracellular matrix, but it's cytoplasmic end is linking to the cortex. Remember these microfilament fibers, right? Microfilaments.
So you see, do you see why they call this transmembrane protein a anchor? It's because it's anchoring to the ECM, the extracellular matrix. And it's anchoring to the cortex, the cytoskeleton inside. So it's anchoring.
And if it's anchoring like this, then this protein complex, this integrin, is not free then to float around like an iceberg. Remember, I said that these transmembrane proteins are free to float around among the phospholipids, kind of like icebergs floating in the ocean. And that's because everything is liquid, right? The membrane is liquid.
The phospholipids are free to float around. The proteins are free to float around. But in the case of an anchor protein like this, they're not free. They're kind of fixed in place. Now, membranes also have distinct inside and outside faces.
Do you remember vesicular transport? We learned about vesicular transport. earlier in a previous chapter. This here, this convoluted membrane right here where my mouse is, this convoluted membrane is called the endoplasmic reticulum.
Remember the rough endoplasmic reticulum? This is where proteins are synthesized. The blue part of this membrane represents the inside of the phospholipids that point towards the lumen or the the inside of the ER, whereas the pink part is the part of the phospholipid bilayer that points out towards the cytoplasm.
This is what they mean by the membrane having, you know, sidedness. Now, look what happens. Transmembrane proteins like this one right here. trans membrane proteins are synthesized at the rough ER.
That's where all trans membrane proteins are synthesized, by the way. That's their origin story. Isn't that neat?
Trans membrane proteins come from the rough ER, as well as other proteins are made at the rough ER too. These are proteins that are inside of the lumen of the rough ER. And then do you remember how did these proteins leave the rough ER?
That's right, they go, yep, the pop sound. They leave because the membrane pinches off as a transport vesicle. And if we take a close look here, you can see that this is a early transmembrane protein that has an attached carbohydrate, but it's not finalized. Remember, these proteins are not finalized quite yet. And where do these vesicles go in order to get these proteins, this protein cargo finalized.
Do you remember? That's right, Wicket. They're headed to the Golgi apparatus, right?
They're headed to the cis face of the Golgi apparatus, where this transport vesicle will bind to and fuse with the cis face of the Golgi. Now look at this. I want to show you something neat.
Look, before the sugar was quite simple. See the sugar was quite simple. But now look what's happening to the sugar. The sugar is becoming modified, right?
And that's the purpose of the Golgi. Look, it looks like a like an antler, right? Like, look where my mouse is. See how it went from a simple sugar to more complex sugar. That's because the Golgi makes modifications to the protein as the proteins make their way from the cis face of the Golgi to to the trans face of the Golgi, those, those proteins are modified and finalized.
Sometimes modifications include attachment of different sugars. Sometimes, sometimes it involves cleavage of the protein, whatever it entails, the proteins become modified and finalized. And then what happens? And then what happens? And then the protein can leave the transface of the Golgi.
How? Again, remember the transface of the Golgi will pinch, pinch, pinch until it, that's right, it pinches off as another vesicle. This is known as a secretory vesicle. Here's the transmembrane protein with the finalized sugar on it. And here are all the other finalized proteins and glycolipids and such.
Remember, the blue side is the inside of the lipid, right, of the phospholipid bilayer ball. See the blue side? But then what happens?
When this Vesicle touches the plasma membrane. It fuses with the plasma membrane and becomes part of the plasma membrane. Any proteins that were on the inside of this ball get secreted out. These are known as secreted proteins.
So for instance, I'll give you an example, right? I'll give you an example. If these little proteins were insulin, Well, then by fusing this vesicle to the membrane of the cell, you've just released insulin protein out into the blood.
Isn't that interesting? So hormones get released this way. Signaling proteins get released this way through this type of fusion. But look at your transmembrane protein.
Look at the transmembrane protein. When the vesicle fuses, guess what? Now this transmembrane protein is among the proteins of the cell membrane. Isn't that cool?
So if you ever look at a cell membrane and wonder, how did all these proteins get to the cell membrane? You know, all these membrane proteins, and every time you see a phospholipid bilayer, or when they say it's the plasma membrane, and they show you all these different membrane proteins scattered about in the plasma membrane, how did they get there? They all got there with this transport, right? Isn't this interesting with the vesicular transport of the endomembrane system.
And you can see now what they mean by sidedness. The part of the protein, the part of the transmembrane protein that's facing outside of a cell ultimately was facing in the lumen of the ER at the beginning. So isn't that interesting?
So hopefully, again, a little more detail than you probably need, but it is fascinating nonetheless. Now, when we're talking about the plasma membrane of the cell, we turn our attention to the function of the plasma membrane. And the plasma membrane controls the exchange of materials between the cell and its surroundings.
Membranes exhibit selective permeability. Some substances cross more easily than others. So essentially, the job... of the plasma membrane of a cell is to be selectively permeable. You don't want just anything to wander into the cell, right?
And you don't want just anything to float out of the cell. You want to regulate cell traffic. You want to regulate substances going across the membrane.
For instance, you would want nutrients and growth factors to enter the cell, but then you would want waste and also other things that need to be secreted to exit the cell, right? This is selective permeability. This is the main purpose of the cell membrane. Now what's interesting is that some substances, they can cross the membrane willy-nilly. They don't need any assistance to cross the membrane.
These are known as the small hydrophobic or non-polar molecules. These small hydrophobic or non-polar molecules, they can dissolve in the lipid bilayer and pass right through to the other side rapidly. These include hydrocarbons such as methane, for instance, remember CH4, or ethane, C2H6.
Remember, those can just cross the membrane because they are small and hydrophobic, non-polar. Non-polar small molecules have an easy time crossing the plasma membrane, crossing through membranes. Another example, CO2 and O2. These are small non-polar gases.
They pass easily through the membrane. Those are the only types of substances that can readily cross the membrane. Small non-polar substances like these hydrocarbons.
CO2, O2, you know, these types of substances cross the membrane easily directly through the plasma membrane, the phospholipid bilayer. However, the hydrophobic interior of the membrane impedes the passage of hydrophilic or polar molecules. So for example, sugars, water, these are polar substances. Polar molecules cannot cross the membrane.
They need help to cross the membrane. Also, ions. Remember charged ions, cations, anions, right? Neons cannot pass through the membrane, if at all. They have a very tough time crossing the plasma membrane because of that hydrophobic region.
All right, so let me show you what I'm talking about here. Look at this plasma membrane here. You've got the phospholipid bilayer. And let's turn our attention to these purple dots. These purple dots appear to be crossing the phospholipid bilayer with ease.
And that means that they have a certain property. What property is that? That's right, wicked.
These must be small nonpolar molecules such as hydrocarbons, oxygen, CO2, I don't know, N2, H2. Small nonpolar molecules. They're the ones that can easily cross. the plasma membrane.
Why? Because they have to navigate this giant hydrophobic region. Remember where these tails are?
This is hydrophobic. These tails are hydrophobic. So in order to easily cross this sea of hydrophobicity, you must also be hydrophobic.
Does that make sense? But now let me ask you this. Look at these triangles on the right.
If these triangles represent a molecule that's polar, like water or glucose, or charged like ions, you know, like sodium cation or chloride anion, can those substances cross the membrane? Can they go and just cross willy-nilly to the other side? And remember, I said no. Because they're charged or partially charged, because they're ions or polar molecules, they cannot cross the plasma membrane, because if they tried, look, if they tried, they would hit these phospholipid tails, the fatty acid tails, and that's hydrophobic, right?
And it would just bounce right back. The triangle would try to cross, but it would bounce off, like almost as though the membrane was a force field. Does that make sense?
Polar and charged molecules, polar and charged entities. try to cross the membrane, but they are repelled by those hydrophobic tails, by the hydrophobic region of the membrane. In this case, do you remember that there's a class of transmembrane proteins like this one right here?
Transmembrane proteins that serve as transporters, they have a pore on the inside that allows substances to navigate across the membrane. See? So you there is a channel that would allow polar or charged substances to cross the membrane.
So in order for a polar molecule or an ion to cross the membrane, it would need to cross the membrane through a transport protein. Okay, does that make sense? So this is the purpose of the transport membranes. And let me ask you this.
Let me test your understanding. Would you need a transport protein like the one on the right for, I don't know, oxygen to cross the membrane? That's right. The answer is no.
You would not need a transporter for oxygen. So guess what? There is no such thing as an oxygen transporter because oxygen can just go directly through the membrane.
There's no such thing as a CO2 transporter. because CO2 can cross the membrane. There is no such thing as a methane transporter, because methane can just cross the membrane. However, there are transporters for, let's say, water.
Water goes through a transporter called an aquaporin. Aqua meaning water and porin meaning pore or hole. There are transporters for ions.
These are known as ion channels. There are transporters for glucose and other polar molecules. Isn't that neat? So if polar molecules or ions need to cross the membrane, they need to do so through a transporter, a transport protein. However, if non-polar molecules such as methane or CO2 or O2 need to cross the membrane, they just do so.
There are two types of transport proteins you need to be aware of. This one here is called a channel transport protein. And a channel transport protein is simply a channel.
It's a hole. It forms a pore. Look how this is a pore. These triangles are just simply crossing this pore and entering the cell. That's known as a channel transport protein.
The other type of transport protein is a carrier transport protein. What's the difference? Well, for a carrier, the substance that's crossing, these blue dots here, are crossing the membrane. So they must be polar molecules or ions.
This blue dot, for it to interact with a carrier, what happens is the solute that's crossing the membrane physically interacts with the carrier, with the transporter. Look what's happening here. The blue dot finds a niche, an area on the carrier where it binds. This causes a conformational change in the carrier.
See how the carrier changed shape in order to accommodate the internalization of the solute of the blue dot. Did you see the difference? Did you see the difference?
A carrier transport protein physically changes shape to allow the substance to cross. It changes what's called conformation to allow the substance to cross the membrane. While a channel.
is simply a pore. That's the big difference. And those are the two types of transport proteins.
Another thing to understand about these transport proteins, whether they are channels or carriers like this one, is that they are very specific for the solute they allow to cross the membrane. So for example, if this blue dot represents glucose, then this carrier would only permit glucose to cross the membrane. Not even a isomer of glucose would be recognized by this carrier. So for example, if this was fructose, which is a glucose isomer, it would not allow fructose across. Only glucose could come across.
So the main message there being these carriers and these channels, these transport proteins are hot. highly specific for what they allow across. The sodium ion channel only allows sodium to cross the membrane. The aquaporin only permits water to cross the membrane. If it's a glucose carrier, only glucose will cross the membrane.
These are highly specific transporters. Now the next topic of discussion has to do with how substances cross the membrane. Sometimes substances kind of force their own way across the membrane, and sometimes substances kind of need to be pulled across the membrane.
Now, to understand why, we first need to understand the first example. We need to understand this concept of diffusion. Diffusion is the movement of particles of any substance so that they spread out. evenly into the available space.
Although each molecule moves randomly, diffusion of a population of molecules may be directional. So diffusion is what's known as a type of spontaneous process. This is a process that occurs on its own without you having to put in any kind of energy for it to happen.
Diffusion occurs naturally. So to explain diffusion, let's go to the board. What's diffusion? Diffusion is the spontaneous movement of molecules from areas of high concentration, like this, to areas of low concentration. These molecules will spread out from an area of high concentration.
to areas of low concentration. This happens spontaneously. This does not require any energy input. In fact, energy is released.
These molecules will move from where they are more highly concentrated to where they are less highly concentrated. Now, one thing to remember for these tonicity problems is that just remember one thing. Whatever is diffusing, whatever is diffusing is moving from where it is high in concentration to where it is lower in concentration and that is called moving down the concentration gradient.
If something is moving against diffusion that means it's going against the concentration gradient. That's just some good terminology to understand. So now let me introduce you to the concept of passive transport.
Passive transport simply means that diffusion is happening across a membrane. You see this dashed line I drew? That represents some kind of membrane, some barrier that the diffusion is going to cross, right?
Now look, in red I have solute, and in blue, the small blue dots represent the solvent, in this case water. Now where is the solute higher in concentration? on the left side or on the right side of this dashed line of the membrane down the middle.
The solute is higher on the left side in concentration. So the solute will diffuse to the right, right? These red dots will travel through these holes in the membrane towards the right.
So what we say is there's a net flow of solute to the right. And guess what? When you have Diffusion of solute across a membrane.
This is an example of passive transport, but specifically it's called dialysis. Dialysis is the diffusion of solutes such as sugar, salt, etc. across a membrane. The diffusion of solute across a membrane is dialysis, an example of passive transport. Okay, now let me ask you this.
Watch this. What if I were to make this membrane like this where I have very small pores in the membrane? Look what I've done. This is a much more fine membrane and the holes are much smaller in this membrane now. Do you think the solute can cross now that I've introduced?
smaller holes in the membrane? Unfortunately, no. At this point, our solute wants to diffuse towards the right, but it cannot, right? The membrane is too fine.
The holes are too small. And so at this point, although the solute would like to diffuse towards the right across the membrane, it is physically blocked. So in this case, we have to ask ourselves, what will happen? Well, notice that the water molecules can still cross.
The water molecules can still cross the membrane. Now, now, water molecules will cross the membrane, but remember how they want to cross, right? They want to obey the principles of diffusion.
Diffusion means... Whatever is crossing the membrane is moving from an area of high concentration of itself to an area of low concentration of itself. Anytime something is moving or diffusing, you have to ask yourself, where is the thing that's moving high in concentration?
Because that's where it's going to move from, okay? Let me show you an example. Look at this example here on the board. Where is the water concentration high?
On the left side of that membrane or on the right side of that membrane? Where is the water concentration higher? It's the right side. The water is more concentrated on the right side. How do I know that?
Well, because there's barely any solute here. There's lower levels of solute. So that means you have a higher concentration of water. And water will diffuse across the membrane and the water will move from the right side to the left side because the water wants to diffuse from where water is high in concentration to where water is lower in concentration. Remember this, this is the key to understanding all of tonicity problems.
Whatever is crossing the membrane is moving from where it is high in concentration to where it is lower in concentration. This means if the red solute is the thing that's diffusing, it's going to go from where it is high in concentration on the left. to where it is low in concentration on the right.
But if it's the water that's diffusing, because the pores are too small now, the water will move from where water is more concentrated on the right to where water is less concentrated on the left. All right, welcome back from the board. So again, diffusion is the movement of particles or a substance from high concentration to low. And this is a spontaneous. natural movement.
And remember, if you have a membrane of some sort, and if the solute, if the solute can cross the membrane, then the solute will cross from a area of high concentration of solute, in this case on the left, and there'll be a net movement to the right until you have equilibrium, right? So this is an example of dialysis, right? Dialysis. The solute is crossing the membrane from an area of high concentration of solute to low concentration of solute.
And remember, if you have two different solutes, if you have an orange solute versus a purple solute, if you have two different solutes, each will diffuse down its own respective concentration gradient. So for instance, Because the orange dots are more highly concentrated on the left, these solute will net diffuse to the right. However, because the purple solute, the purple dots are more concentrated on the right, they will net diffuse to the left until equilibrium is reached. How do you know equilibrium is reached?
There will be an equal concentration of purple and orange. orange solute on each side of the membrane. That's when equilibrium is reached. And do you remember these are both examples of passive transport.
The diffusion of a substance across a biological membrane is passive transport. You have to have diffusion occurring across a membrane for it to qualify as passive transport because no energy is expended by the cell. And remember this, if the membrane is too fine to permit the solute to cross the membrane, then remember only the water will cross from where water is high concentration to where water is lower in concentration.
Look at this example of a tube that's shaped like a U. Let me zoom in on this tube. that's shaped like a U here.
Now the green dots represent solute and that solute let's say is sugar molecules. The green dots are sugar and the blue substance is water and this is a tube shaped like a U and there is a membrane in the middle of the tube at the bottom here. See this dashed line is a membrane and If we zoom in on this membrane, you can tell that it's too fine for the solute, the sugar, to cross the membrane. So if you recall from the board, this is an example of an osmosis problem, right? The sugar cannot cross the membrane.
This is not a dialysis problem because the sugar cannot cross the membrane. Only the water is able to cross the membrane. In this case, this is an example of osmosis. And remember, in osmosis, water is diffusing from where water is higher in concentration to where water is lower in concentration.
So ask yourself this, first of all. Just ask yourself this. Where is the water higher in concentration?
On the left, where there is low sugar concentration. Or on the right where there is higher sugar concentration, which side is the water more concentrated? Is the water more pure?
That's right, wicket. It's on the left. The water is more concentrated on the left, and the water is the only one who can cross the membrane. So which way is the water going to move?
Which way is the water going to diffuse? That's right. The water will diffuse from the left side of the membrane, and it will diffuse toward the right.
And look at here. Let me show you what happens. Whoops. Let me show you.
Yes, the water will net diffuse to the right. In fact, the level of the water will increase on the right, and this will continue until you reach equilibrium. Equilibrium means where the solute concentration is equal on both sides of the membrane.
This is osmosis. Again, osmosis is the diffusion of water across a membrane, while dialysis... was the diffusion of solute across the membrane.
Both of these are vital for in understanding how substances cross the membrane of living cells. Now speaking of the living cell and how osmosis can affect living cells, we turn our attention to a concept called tonicity. Tonicity is the ability of a surrounding solution.
To cause a cell to either gain or lose water, tonicity depends on the concentration of solutes in this solution that cannot cross the membrane relative to that inside of the cell. If the solution has a higher concentration of these solutes, then the inside of the cell, water will tend to leave the cell and vice versa. So before we delve deep into what tonicity means, and how tonicity can affect cells.
I think it's time for break time. Let's take a break time with Gizmo and Wicket and see what these little guys are up to. And when we come back, we're going to talk at the board about how tonicity affects cells.
So what is tonicity? Tonicity refers to the ability for a surrounding solution to cause a cell to either gain water volume or lose water volume. It's simple as that.
Now what could drive this phenomenon? It all has to do with the solute concentration of the cell versus the environment. Every cell has roughly 0.9% salt. And that's, I'm going to write NACL.
That's the concentration of salt in each animal cell. Now, when in our bodies, the environment outside of our cells matches the salt environment inside of our cells. So your blood, The fluids of your blood are also at 0.9% salt.
This means that the salt concentration inside of your cells matches the salt concentration in the fluid that's outside of your cells. And this is good because water enters the cell and water can leave the cell. This is a very important point.
In these tonicity problems, It's not the salt that enters the cell with diffusion. It's not the salt that leaves the cell. These are osmosis problems. Water is what's diffusing across the membrane. Okay, I need you to remember that whenever you're seeing these, it's not the solute that crosses the membrane.
It is the solvent. It is the water that crosses the membrane. And it crosses, obeying the principles of diffusion.
there's osmosis going on. That means that water is diffusing across the membrane. Now look at this.
If the salt concentration inside of the cell matches the salt concentration outside of the cell that means that not only is the salt concentration equal inside and outside, but the water concentration is also equal inside the cell versus outside of the cell. So for every one water molecule, H2O, for every one water molecule that enters the cell, about one water molecule. leaves the cell.
This causes the cell to neither gain water volume or lose net water volume. The cell will remain what's called normal. This is a happy and normal animal cell. And this is known as an isotonic environment.
Iso meaning equal. The tonicity is equal. inside of the cell and outside of the cell.
Because the water concentration inside matches the water concentration outside, there is no net flow of water. The cell will neither gain water nor lose water. And animal cells, like our blood cells, prefer isotonic solutions.
This is when our cells are normal. Next, I'm going to show you what happens when we do not have an equal concentration of solute inside and outside of the cell. Now, let's look at the animal cell again.
Remember, animal cells have roughly 0.9% salt concentration inside of them, right? Now, what if you placed an animal cell in a solution, an environment that had way less salt? Let's pretend there was no salt. Let's say there was zero.
0.0% salt, right? So there's no salt in the environment anymore, right? Now you have obviously deviated well outside of the isotonic range, right? There is no salt concentration outside. In fact, I drew the salt as the little red dots here.
In this case, remember the salt is not what's diffusing across the membrane. If the salt could diffuse, The salt would move from area of high concentration of salt and it would leave the cell, right? The salt would leave the cell.
But remember, in these tonicity problems, salt doesn't move. Salt can't move. The water is what's diffusing.
The water is what's moving, okay? So, you have to ask yourself, again, you just have to do this and you'll never make a mistake with these. You just have to ask yourself, what... can move in these tonicity problems?
It's the water that moves, right? So if water is what's moving, water will move from where water is high concentration to where water is lower in concentration. Now look at this.
If there is 0.0% salt concentration outside of the cell, where is the water concentration higher? Inside of the cell where there's salt? or outside that a cell where there's zero salt.
It has to be outside right because if there is 0.0% salt that means there is 100% water right 100% H2O. So what you need to understand is that water will diffuse from where water is high concentration outside into the cell water will net diffuse into the cell. This will cause the cell to swell up.
Okay, the cell will swell and eventually the cell will lice. That means that the cell will inflate to the point where it pops. Okay, the cell will rupture.
The cell will lice. Lice means to break. The cell starts gaining water volume, water volume, water volume, water volume until it pops.
Okay. That's what happens in this type of situation. Again, this means a situation where the salt concentration or the solute concentration is lower outside than inside.
This is also known as a hypo. Hypo means low tonic, hypotonic environment, low solute environment. okay think of that as hypotonic means low solute concentration if the environment is hypotonic to the cell then the cell will gain water volume it will swell up and it will lice i have a trick to remembering this it's the funniest thing ever the way i remember these is hypo kind of sounds like hippo right like a hippopotamus hippopotamus looks all swollen up you know and so You know, as silly as it is, to this day it helps me remember these tonicity problems. All I remember is hypo sounds like hippo, so my cells look like hippos.
They swell up in a hypotonic environment. Well, what would cause that? From there, I ask, okay, what would cause my cells to swell up?
Well, water must be rushing in. What would cause water to rush in? Well, that must mean the water is more concentrated outside of the cell. That means that water is rushing into the cell, causing the cell to swell, swell, swell until it bursts open. Does that make sense?
Hopefully, that makes sense for you. I know it's silly. but believe me it helps you on your exam and you'll you'll nail it every time all right so next i will explain the last scenario the hypertonic solution and how that affects the cell this is the final scenario for our tonicity problems here we have an animal cell again remember an animal cell typically has 0.9 salt and acl inside what if i were to place my animal cell in an environment That's really high in salt.
Let's say, I don't know, something like 10% salt, 10% NACL. That is a very high concentration of salt, isn't it, compared to the cell. So remember, again, please remember that it's not the salt that's diffusing across the membrane. If the salt could diffuse, that would be a dialysis problem. The salt would...
cross from the outside into the cell. But remember, salt is not what's moving in these tonicity problems. So now you have to ask yourself, okay, it's the water that's moving across the cell membrane. And I need to figure out where is the water more concentrated?
Where is the water more pure? Where there's low salt or where there's high salt? It has to be where there's low salt concentration.
Does that make sense? The water concentration is now higher inside of the cells. So the water, the H2O, will flow mainly, there will be a net efflux of water out of the cell. Water will net leave the cell in this case.
And do you know what that causes? That causes the cell to shrivel up. The cell shrivels up like a little raisin. It just shrivels up.
like you let the air out of a balloon okay so this results in a shriveled or what's called cre-nated cell the cell loses water volume the cell shrivels or cre-nates that's the proper term for an animal cell next let's check out tonicity problems but in a plant cell and see how things may vary a little bit. All right, check it out. This is my best attempt at drawing a plant cell.
We have in a plant cell, remember, we have a very strong, thick cell wall. I drew the cell wall in green. You can see how it's a nice, thick, strong cell wall made of cellulose.
But I want you to remember that plant cells also have a plasma membrane. I drew that. As this thin blue line across the cell wall, I mean following the cell wall, you can see the plasma membrane.
And the plasma membrane kind of pushes against the cell wall. So you've got the cell wall, you've got the plasma membrane. Inside you have the central vacuole, which is this membrane that fills with fluid and has nutrients inside. And then you have the green dots represent.
chloroplasts inside of the plant cell. Alright so remember that the saltiness of cells is about 0.9 percent right so we have about 0.9 percent salt inside of the cell and remember if we match the 0.9 percent salt concentration outside of the cell this is known as what? It's known as a isotonic solution that surrounds the cell.
Now what happens with the water flow? Is there a net gain or loss of water? Remember in this case for every one water molecule that enters the cell, about one water molecule leaves the cell.
And remember it's only water that can enter the cell and leave the cell, not the salt. So the cell, the plant cell, neither gains net water volume nor does it lose net water volume. But remember, an isotonic solution was ideal for plants, for animal cells like your cells and my cells, but it is actually not ideal for plant cells. Plant cells become what's called flaccid.
Flaccid. And that's not good. That means droopy. You don't want a plant that's drooping over, right?
This is not the ideal. condition for a plant cell. All right, so look what I've done now.
I've placed my plant cell, which has 0.9% salt concentration inside, in a pure water environment. Let's place it in 100% water, 100% H2O. That means 0% NaCl, right?
percent salt concentration environment. What type of environment was that? I'll give you a second to answer. Yeah it was a hypotonic environment. Very good hypotonic low solute concentration environment.
This is known as a hypotonic environment and what happens in a hypotonic environment? Does the salt leave the cell? Remember the salt cannot move. So it's the water that enters. The water will move from outside where the water is way more concentrated.
The water will net enter the cell causing the cell to swell up. Remember this causes the cell to take up water volume. It causes the cell to swell up. However, will this plant cell lyse? Will it explode and pop?
The answer is no. because remember this thick green cell wall that plant cells have that animal cells don't that prevents the cell from swelling and by the way not only not only does this happen but the central vacuole even swells up the central vacuole swells up and it fills with pressure inside of the cell and these chloroplasts they end up moving nicely they they move to the kind of the edges of this cell and And in a normal healthy cell you can even see these chloroplasts moving around the edge of the cell. It's called cytoplasmic streaming and the cell is actually very happy. The cell loves this condition and this is known as a turgid. Turgid means high pressure.
Turgid cell. Plant cells love it. Plant cells prefer hypotonic environments where water is just rushing in. Building that pressure inside of the cell is called turgidity. This is a nice, firm, turgid cell.
This is when your plant is growing nice and straight and tall. And, you know, plants love it. Plants prefer hypotonic environments. And they love to build this pressure. And that's the way they prefer it.
So let me show you what not to do with your plant cells next. All right, now look at this. What if I were to take my plant cell and put it in a very salty environment? Let's say again 10% salt, right? 10% salt environment.
Wow, that is very salty environment. But remember the salt can't enter the cell through diffusion. Instead, the water is what's diffusing.
You have to ask yourself as always with these tonicity problems, where is the water concentration high? that's inside of the cell so the water will leave the cell the water will move from where water is high concentration inside of the cell to where the water is lower concentration outside of the cell the water will mainly leave the cell the water will net efflux out of the cell and what happens i'm going to show you let's talk about what happens what what's the consequence of this first of all your central vacuole kind of dehydrates a little bit your central vacuole gets smaller secondly this is very important remember your cell wall your cell wall is so strong it's not going to shrivel up like an animal cell membrane does but remember that plant cells have a cell membrane they do have a plasma membrane around the cell now this membrane look at this membrane in blue you see it This membrane actually does start to peel. Look, the plasma membrane peels away from the cell wall. The plasma membrane peels away.
Oh, do you guys hear that? That's Gizmo trying to get in. Gizmo, I'll let you in.
in a minute but look at this the plasma membrane the plasma membrane peels away you see what i'm doing i'm showing you that the plasma membrane is peeling away from the cell wall it's peeling away it's peeling away it can't take the cell wall with it when it peels away because the cell wall think of it as this really strong structure the cell wall is not gonna shrivel up like a raisin it might shrink a little bit it like the length and width may shrink a tiny bit but it doesn't shrivel what does shrivel is the plasma membrane peels away from the cell wall because water is rushing out of the cell the cell is getting dehydrated but the way you see it is the plasma membrane peeling away from that cell wall and you know what it takes with it remember these chloroplasts are inside of this of the plasma membrane so what happens is the chloroplasts also peel away from the cell wall because they're inside of the plasma membrane so you end up getting this look look the chloroplasts peel away from the cell wall too because they're inside of the plasma membrane it's like it's like the way i describe it is like this imagine having a cardboard box and then blowing up a balloon in the cardboard box, right? If I let the air out of that balloon, the cardboard box doesn't fold up or shrivel up. It's the balloon.
It's the balloon that peels away from the cardboard box, right? And if there's marbles in that balloon, you see the marbles represent these coroplasts. If there were marbles in that balloon and I let the air out of the balloon, Well, all those marbles would come in with the balloon, right?
And all those marbles would cluster up. And that's what happens here. The plasma membrane is peeling away from the cell wall. All these little chloroplasts are like coming with it.
And they're kind of clustering up together. And they're bunching up, right? Now, this is really bad, you know, for the cell.
The cell really does not like this. And this is called a plasmalized cell. Plasmalized.
The cell is plasmalized and this is a plasmalized cell. That's what plasmalized means. The cell is in what type of environment is this? Remember this is a hyper tonic environment that means high solute concentration environment water is mainly leaving the cell the cell is becoming dehydrated the plasma membrane peels away from the cell wall causing these chloroplasts to cluster up together and this is known as a plasmolyzed or very sick cell plant cell all right you guys i really hope this was helpful remember my trick to to you know remembering this material on the exam all i do is i recall that hypo sounds like hippo hippos are all bloated looking creatures so when i remember that hypo sounds like hippo i think okay my cells in a hypotonic environment to look like hippos the only reason to look like a hippo is water's rushing in why would water rush in well because the water must be more concentrated outside okay that's what's causing the water to rush in that means the solute concentration must be low outside does that make sense so by remembering that hypo sounds like hippo i remember all i need to you know know for these types of questions on exam and I do well and I ace you know these types of questions every time and I hope that by you using these tricks and these tips that you'll not miss any of these questions you'll get it right every time all right welcome back from the board so hopefully at this point we understand how animals fare in different types of tonic environments and how plant cells fare in different types of tonic environments so let's move on now you might be wondering how do organisms that don't have a cell wall, how do they withstand osmotic pressure?
How do they go into fresh water and live in fresh water environments? Wouldn't that cause these cells to lyse, to explode? And the answer is really interesting.
Organisms that live in environments that are hypotonic, where there is water rushing into the cell, If these cells don't have cell walls, this requires a method of osmoregulation. a control of solute concentration and water balance. So for example, paramecium. Paramecium is a single cell protist with no cell wall. And sometimes paramecium finds itself in a hypotonic environment.
This means that water is rushing into the cell. And if it doesn't do something about it, remember, the cell will pop, okay? So paramecium live in hypotonic environments. But they don't pop.
They don't pop because they have a contractile vacuole to pump excess water out of the cell. We had mentioned the contractile vacuole in a previous chapter, but let me show you. This is a paramecium, a single cell creature, and inside it has what's known as a contractile vacuole.
When it finds itself in a hypotonic environment, water rushes into the cell, and the cell is at risk of lysing from osmotic stress. Instead, what the cell does is it filters that water to the contractile vacuole. The contractile vacuole does, as its name suggests, it contracts and it squeezes that water right back out.
So imagine if water is rushing in because it's a hypotonic environment, the contractile vacuole can simply squeeze that excess water back out before the cell reaches pressures sufficient to lyse the cell. Isn't that interesting? That's the purpose of the contractile vacuole. Now. what if substances need to be moved against the concentration gradient across the plasma membrane?
Like, for instance, what if the cell needs to move something out or in against the concentration gradient, against diffusion? Well, wouldn't that require energy? Because isn't that kind of going against the natural diffusion? And that's what active transport is all about.
Active transport requires energy. Active transport involves movement of a solute or a substance against the concentration gradient. Active transport requires energy, usually in the form of ATP hydrolysis, to move substances against their concentration gradient. All proteins involved in transport are carrier proteins.
Carrier proteins are capable of active transport. Active transport enables cells to maintain solute concentrations that differ from the environment. A very common example of an active transporter is called the sodium potassium pump. Now let me show you how this pump works.
Take a look here. This is the sodium potassium pump. It's a carrier molecule, and its job is to pump sodium ion, sodium cation, out of the cell, even though sodium concentration is high outside.
Do you see this? Sodium cation concentration is high outside of the cell. However, the sodium potassium pump pumps sodium ions out of the cell.
And... It does so with the help of ATP. ATP powers the sodium potassium pump to pump sodium ion out of the cell against the concentration gradient.
Remember, there's already a high concentration of sodium cation out here in the outside of the cell. And with the help of ATP, the sodium ion is pumped out of the cell. against the concentration gradient.
It's fighting diffusion. And potassium cation is then gathered and pumped into the cell. It's pumped into the cell also against the concentration gradient. There's already a high concentration of potassium inside of the cell and the potassium pump, the sodium potassium pump pumps potassium into the cell and that's all thanks to that ATP molecule. So the sodium potassium pump is an active transporter.
It's a carrier molecule. It gathers. three sodium cation and with the help of atp power it is able to pump those sodium cation out of the cell against the concentration gradient with active transport and it's capable of pumping sodium potassium ion in against the concentration gradient as well so the sodium potassium pump uses atp for active transport of three sodium cation out and active transport of two potassium cations into the cell. And this is a very commonly studied carrier active transporter.
So to review, let's review. With passive transport, remember passive transport means substances diffuse across the membrane, across a biological membrane. Passive transport does not require energy. Passive transport is spontaneous. And some substances like these purple dots are capable of transporting across the membrane directly.
Remember these molecules can cross the membrane unimpeded. So that tells you that these purple dots represent molecules that are small and nonpolar, right? These purple dots need to be small and nonpolar because they have the ability to cross the phospholipid bilayer unimpeded.
And this is known as simple diffusion or just diffusion across the membrane. This is a type of passive transport. Now look at this, the orange triangles and the blue squares, these represent other substances that want to cross the membrane as well.
However, these orange triangles and blue squares represent either polar molecules or ions, charged ions, either cations or anions. In this case, if it's a polar molecule or an ion, it needs, those substances can't cross the membrane directly like these purple dots can. They need a transporter, right? Remember, there are transporters that look like pores. These are called channel proteins.
And there are transporters that change shape in order to allow substance across. And these are called carrier proteins. Both of these are examples of transporter proteins.
And both of them allow polar or charged. substances to cross the membrane. And as long as these substances are crossing from high concentration to low, like these, you see there's a high concentration of blue rectangles to low, a high concentration of triangles to low, then it is a type of passive transport because it's simply diffusion that's powering the process. And let me ask you this, let me ask you this, Would this diffusion across the membrane occur without the help of these facilitators, these transport proteins? That's right, Wicket.
These blue rectangles and these orange triangles could not make it across the membrane if it were not from the help of these facilitators, these transport proteins. And for this reason, this is known as facilitated diffusion, not simple diffusion. Remember, simple diffusion is what nonpolar molecules used across the membrane directly.
However, facilitated diffusion is used by polar molecules and ions to cross the membrane through a facilitator, through a transport protein. OK, so that's the difference between facilitated diffusion, which requires a transporter because the molecule that's crossing is either polar or charged and just simple diffusion, which does not require a transporter because it is. allowing substances that are nonpolar to cross the membrane directly. Either way, no energy is required, right? Because diffusion is what's driving these substances in. So all of these are examples of passive transport.
All three of these are examples of passive transport because substances are moving from an area of high concentration to an area of low concentration. The only difference is these polar and charged substances They require a transporter of some sort, either a channel protein or a carrier protein, to make it across the membrane. And without that facilitation, without that facilitator, the diffusion would not occur. And so that's why it's known as facilitated diffusion. Passive transport is a type of spontaneous transport.
It does not require ATP. It does not require energy. Conversely, look on the right.
This is an example of active transport. And this is likely a depiction of your, remember your sodium potassium transporter? The dots, the circles are sodium cation and the diamonds are potassium cation. So this transporter, the sodium potassium pump, is pumping the diamonds in and the circles out. Does that make sense?
But that's against the concentration gradient. If you're pumping circles out, and there are already a lot of circles out here, then you're pumping the circles against the concentration gradient. If you're pumping the diamonds in, and there are already a high concentration of diamonds inside, then you are pumping the circles.
the diamonds against the concentration gradient. Does that make sense? Because this transporter, this carrier, is moving substances against the concentration gradient.
In that case it's a type of active transport which means it requires energy. It requires something like ATP to power the process. Now let me show you a way that active transport can affect a cell.
This here represents, in purple, represents a transport protein, an active transporter called a proton pump. A proton pump pumps protons across the membrane against the concentration gradient. That means there's already a higher concentration of protons. And remember, protons are denoted as H+.
Hydrogen ions are the same thing as protons. So a proton pump is an active transporter of protons which uses ATP energy to pump protons against the concentration gradient. This allows you to get a higher concentration of protons outside of the cell than inside of the cell. This is also known as an electrochemical gradient. Why?
Because it is, first of all, it's a chemical gradient because you have a higher concentration of protons or I should say, hydrogen ions outside than inside. So it's a chemical gradient, but it's also an electric gradient because you have a higher amount of positive charge outside than inside. This type of proton pump, because it's pumping ions against the concentration gradient across the membrane, this is going to result in a higher amount of positive charge outside of the cell.
than inside of the cell. The inside of the cell is going to be more negatively charged as a consequence of proton pumping. outside of the cell is going to develop more of a positive charge thanks to the proton pumping. And this is going to result in a term called membrane potential. Membrane potential is when you have a difference in charge across a membrane.
See here you have a more of a positive charge outside of the membrane and you develop more of a negative charge inside of the membrane. Again as a consequence of actively pumping so many protons out of the cell. The cell develops a membrane potential and cells possess membrane potentials.
Membrane potential means a difference in charge across a gradient. This results in a difference in voltage across the gradient as well. So membrane potential is a voltage difference across a membrane and this is due to a charge difference across the membrane and this is due to act transport of ions.
Active transport of ions results in membrane potential. Next, let's talk about co-transport. This is another consequence of active transport. It's really neat. Co-transport occurs when active transport of a solute indirectly drives transport of other solutes.
This is really, really interesting. And let me show you how this works. Let's go back to our proton pump example. This proton pump is pumping protons, remember H plus hydrogen ions, also known as protons. The proton pump is actively transporting protons out of the cell with the help of ATP energy.
This forms a high concentration of protons outside of the cell. And let me ask you this, let me test your understanding real quick. Can these protons, which have a positive charge, they are cations, can these protons then just simply diffuse back into the cell through the plasma membrane?
Let me think, let me ask you to think about this. Can protons simply diffuse through simple diffusion back through the membrane back into the cell? That's right, wicked. can't, right? Because they are ions.
I remember ions can't diffuse through simple diffusion across the cell. These protons, these hydrogen ions would love to diffuse back into the cell where it's lower concentration of protons. However, however, they can't cross the phospholipid bilayer because of the hydrophobic region of the membrane.
However, look at this structure up here. Look at this thing. This is known as the proton sucrose co-transporter.
This is really neat and an example of co-transport. This transporter allows sucrose to cross into the cell against the concentration gradient. That means this transporter actively transports sucrose.
into the cell where there's already a high concentration of sugar inside of the cell. And you want more sugar inside because you can use that sugar for energy. And instead of using ATP to actively transport the sugar into the cell, what happens is the sugar attaches to the transporter and it waits.
It waits for a proton to come along and force its way through the co-transporter Also forcing the sucrose across the membrane. Isn't that neat? This is co-transport. Remember what co-transport was? Co-transport occurs when active transport of a solute, remember proton pumping, indirectly drives transport of other substances, in this case, sucrose.
Isn't that neat? So this here is an example of co-transport because you are actively pumping protons and those protons are... diffusing back, diffusing back into the cell through the co-transporter and using that power of diffusion to actively transport sucrose into the cell.
Isn't that neat? So in this case, the power of the power the co-transporter is using to actively bring sucrose into the cell is being provided by diffusion of protons. Diffusion of protons is releasing more than enough energy to bring the sucrose into the cell actively.
Isn't that neat? So you don't necessarily need ATP. in order to do active processes as long as you use something that's spontaneous such as diffusion. All right and here is the final concept from the chapter.
This concept deals with bulk transport across a plasma membrane. Sometimes cells want to bring in bulky items, big items, across the membrane. Let's say an amoeba wants to bring an entire food particle into the cell or an entire bacteria into the cell to digest it.
There's no single transporter that's that big. So how would you bring in a giant food particle? Well, this is known as bulk transport, right?
And bulk transport can work both ways. You could bring large substances into the cell via bulk transport, or you could release bulky items out of the cell through bulk transport. So again, small molecules and water enter or leave the cell through the lipid bilayer or via transport proteins.
Remember the channels or the carrier proteins? That's for small molecules. But large molecules, large things such as polysaccharides, proteins, or even entire bacteria cross the membrane in bulk inside of vesicles.
So this requires vesicles. First, let's talk about a type of bulk export, right? Removing things from the cell in bulk. This is known as exocytosis.
Exo meaning outside, cyto meaning the cell. So in exocytosis, transport vesicles migrate, you know, from the inside of the cell to the membrane, fuse with it, and release their contents outside. So, for example...
Cells in the pancreas secrete insulin by exocytosis. So here you can see, let's say this is a vesicle from the Golgi. Remember the Golgi has these proteins in it. Let's say these are insulin proteins in purple.
Those insulin proteins will pinch and pinch and pinch and pinch as the Golgi apparatus membrane pinches off as a secretory vesicle. Look at this secretory vesicle. full of insulin proteins.
That secretory vesicle will go and fuse with the plasma membrane. Remember like soap bubbles, right? When the membrane touches the cell membrane, it can fuse.
And look, the contents are released out of the cell. The insulin would be released out of the cell. This would be an example of exocytosis.
We used a vesicle fusing with the plasma membrane to release in bulk lots of substances, larger substances like proteins. Does that make sense? Exocytosis requires, you know, it involves a bulk export and bulk export and bulk import, by the way, require energy to happen.
Now, what's endocytosis? Endocytosis is the opposite. It's endo means in, cyto means the cell. So endocytosis is when macromolecules are taken into the cell in vesicles.
The membrane forms a pocket that deepens and pinches off forming a vesicle around the material for transport. There are three types of endocytosis. Phagocytosis, which is considered cellular eating.
because the cell brings in bulky solid substances, penocytosis, which is considered cellular drinking because the cell brings in liquids and solutions, and receptor-mediated endocytosis, which is endocytosis triggered by surface receptors of the cell. So let's start with, you know, let's look at the different types of endocytosis in animal cells. On the left, you have phagocytosis. This is a large food particle, right? There's not a transporter that would accommodate this large food particle.
But let's say the cell wants to bring all this food into the cell so it can, you know, digest it and to get energy from this food particle. Well, what can happen when the food particle touches the plasma membrane here? The plasma membrane can invaginate. It can it can kind of fold inwards.
And remember, it can fold and fold and fold until what happens? That's right. This membrane can pop off as a little vessel. vesicle or also a vacuole, a food vacuole. And there you go.
You have internalized that food particle via bulk import. This is a type of endocytosis called phagocytosis. And what the cell would likely do next is fuse this food vacuole with a lysosome to digest this food.
Now, remember, that's not the only type of endocytosis, is it? There's also penocytosis. which is the same concept except what you're bringing in is what?
You're bringing in solution, right? And by solution, I mean not just water, but anything dissolved in water. So for example, this could be sugar water that you're bringing in.
Instead of a big bulky solid particle like this food particle, you're bringing in liquid and anything that's dissolved in the liquid like sugar or salt. So when the... This time when the plasma membrane invaginates, it can pinch off bringing in solution, bringing in liquid.
That liquid can contain sugars or salts or whatever else is dissolved in the surrounding solution. This is known as penocytosis. Peno meaning drink. Lastly, receptor-mediated endocytosis. This is simply a type of endocytosis that involves receptor on the surface of the cell.
Remember that the cell has transmembrane proteins called receptors and certain receptors can trigger endocytosis if the receptor is triggered, if the receptor binds to a specific hormone or ligand that can cause the invagination and the internalization of the coded vesicle. Again, in phagocytosis, a cell engulfs a solid particle by extending pseudopodia around it. So fake feet around it and bringing it in as a food vacuole. The vacuole fuses with a lysosome to digest the particle.
Here you can see what I'm talking about with phagocytosis. This is an amoeba. Check out this amoeba.
This amoeba is trying to eat an algae. an algal cell that's about the same size as the amoeba. It will wrap its pseudopods, see the pseudopods, its fake feet? It will wrap its pseudopods around the food particle, just like this. It will invaginate to form a food vacuole, bringing in a food vacuole into the cell.
And again, this is known as phagocytosis, okay, bringing in solid material such as food or another solid particle. Phenocytosis is when molecules are taken up when extracellular fluid is gulped into tiny vesicles. Remember, this is cellular drinking, right? Bringing in bulk liquid into the solution. Phenocytosis is a nonspecific process.
Any and all solutes are taken up into the cells. So here you can see, again, phenocytosis at work. With penocytosis, the cell membrane invaginates to bring in solution and anything that's dissolved in the solution. And remember last type, the last type of endocytosis was receptor-mediated endocytosis, where vesicle formation is triggered by a solute binding to receptors, specific receptors on the surface of the cell.
Here you can see these receptors These receptors on the cell look like little Y structures. And here the solute that binds to the Y structures are these little triangles. If these triangles are around, they will trigger the endocytosis to occur.
All right. And with that, that leads us to the end of this interesting chapter covering the cell membrane. structure and function. I hope that was informative. Let me know if you have any questions in the comment box below and I will catch you guys next time.