So now that we've seen how the cell's decision-making processes actually work, all the genetics and the genetic machinery, it's time to put it all together and see how cells actually function. And this is a picture that you've seen before, I'm sure. This is the kind of thing that everybody's seen. My daughter first saw it in fourth grade, and this is something that we see pretty much every year you take biology since then.
And you can see it has all these things inside of it, a typical eukaryotic cell, in this particular case an animal cell. And everybody knows this. There's not one cell on the planet that looks like this.
This is a diagram. That's all it is. It's simply a thing that shows what's inside cells. But real cells do not look like this.
So don't get the idea this is what a cell actually is. There are some cells that may resemble some of these things, but nothing has this exact sort of composition. The key thing to remember, though, is that these structures are things that perform particular functions within the cell. They're all membrane-bound, and so they're organelles.
And that's what they're trying to show. They're simply trying to show where the organelles are in a very diagrammatic way. This animal cell is an animal cell, we know, because it doesn't have a cell wall.
If you remember, animals by definition do not have cell walls. Here you've got the plasma membrane around the outside here. This structure right here is one that I'm sure you know. Everybody always knows this one.
This is the powerhouse of the cell, the mitochondrion. And then we have inside of here these membranes. These membranes here are all contiguous, all continuous, if you wish, with the nuclear membrane.
They are all connected to it. And this one elaborates out from the nuclear membrane. You can see the bridge here. And you can see all these black dots on it.
The black dots are ribosomes that are attached to it. So this is the rough endoplasmic reticulum. You should recognize that.
We've seen that earlier in the class. Connected to it. extending out here with no ribosomes and much more tubular as opposed to flat sheets, we have the smooth endoplasmic reticulum.
And then we have this structure here, the Golgi apparatus, and then a number of other structures inside, lysosomes, peroxisomes, the cytoskeleton, and so on. Now, all of these true organelles, if you remember, are bound by a membrane. That's the definition of a true organelle.
They actually are membrane-bound. Each one performs a particular function for the cell. Not all cells have every single one of these structures.
There are many cells that, for example, don't have any mitochondria at all. There are other cells that have lots and lots of mitochondria. There are some cells that have lots of cytoskeleton.
Other cells generally have a different cytoskeleton doing different things. Like, for example, in a muscle cell, the muscle cell is almost all cytoskeleton. That's what the contractile machinery is.
Whereas a liver cell is much more like what's depicted here. All of these membranes are active. They're doing things. For example, we know what's inside the nucleus here, the DNA.
The nucleolus is inside the nucleus. What the nucleolus does is it packages up the ribosomal parts. It produces the ribosomal parts, which then go out into the cytosol and are constructed into ribosomes, as you remember from Unit 1. And then this other stuff, the rough ER and the smooth ER, are all places where proteins are synthesized in the rough ER or processed. both in the rough ER and the smooth ER. And then those proteins get packaged up, and they get released in little vesicles that could be a peroxisome, a lysosome, or any other type of vesicle that contains proteins to be processed that then go to the Golgi apparatus.
And then they go into the Golgi apparatus, the Golgi apparatus processes them some more, and then they get released elsewhere. And we've seen all of these things. There's nothing I'm teaching you right now that we haven't actually already seen.
But notice the connection between all these membranes on the inside. The nuclear membrane, the membrane that's forming the rough ER, the smooth ER, and the Golgi apparatus are all functionally connected. And so they all are part of what we call the endomembrane system. One word, endomembrane, and system is just the connection of all of these different membranous systems. Now, these are physically connected.
Rough ER, smooth ER, and nuclear membrane are all physically connected. The Golgi apparatus is not physically connected but functionally connected to it. So the endomembrane system is extremely active and it's where most of the protein processing takes place.
These other structures like the mitochondrion and so forth aren't strictly part of the endomembrane system even though they have membranes around them. And the outside, this outer membrane, is not part of the endomembrane system because it's not endo. Endo means inside. So here's a question. Which of these objects in this picture is using the most metabolic energy?
I'm going to give you a little bit of information about this. There's one structure in this picture that uses half of the energy that you eat when you're at rest. Half of the energy that your body is using at any given time is being used by this one single structure.
Now, I usually will ask this in class, and I just invite people to sort of say what they think it is, and almost everybody says, well, the mitochondria, which is a good guess, it's an excellent guess, because the mitochondrion is associated with energy, but it's associated with energy production, not energy use. And it doesn't actually produce energy. What it does is it transforms it from one form to another that's metabolically useful. So the mitochondrion is exactly the opposite answer. It's producing the metabolic energy.
It's not using metabolic energy. The question is, what's using it? Well, the next typical guess is the nucleus, which again is an excellent guess.
We've just been talking about everything that's going on in the nucleus, and it's doing a bunch of stuff. But in fact, really what the nucleus is is a big blob of information. It doesn't really do a lot.
It produces muscular RNA. It produces other types of RNAs. It produces ribosomal structures and so forth.
But most of the actual metabolism is not occurring inside the nucleus, so that's not the answer. And then you can start to guess other things if you want after that. But the astonishing answer to most people is this, the cell membrane. is the most energetically active structure in this cell.
Half of the energy, when you're sitting at rest, if you're sitting there just watching this video, half of the energy that you used, that you ate in your last meal, is being used to power the membranes of your cells. Not the nucleus, not the mitochondria, not the cytoskeleton, and so on. If you're running or you're active, then the cytoskeleton of your muscle cells start to take up a lot of that energy or start to use it. But when you're at rest, they're not doing anything.
They're just sitting there. So that's the key. The question then is what in the world is this membrane doing with all of that energy?
Why is it that the membrane, which is just the surrounding of the cell, well no, it's not just the surrounding of the cell. It's very, very active and it's doing something extremely important and that's what we have to understand next. So we need to understand that nuclear membrane better. And we're going to start at the beginning.
What is it that the nuclear membrane is made out of? Well, it's made out of this stuff. It's made out of these kinds of molecules right here. This is a space-filling model of that molecule. Here's the...
the structural formula of the molecule, sort of. It's a simplified structural formula. And you can see it has this portion up here that they're calling the choline phosphate. And then you've got this thing that they're calling a glycerol.
And then you've got these things coming down like this. What you're looking at here is a molecule of soap. That's what this is. This is a soap molecule. In fact, you can buy a laundry detergent that's basically made out of this soap.
Turns out your membranes are made out of the exact same molecule. And the reason that your membranes have been out of this is because the molecule is what we call amphipathic, which means that it has a section here and a section here, which are chemically very, very different when they're in water, when they're put into water. If you look here, I could ask you this question. You should be able to answer it.
Will this section here, this blue, go into solution in water or not? So think about that for a moment. Will it go into solution in water? What would you need to know to know if something goes into solution in water? Well, remember.
Is water polar or nonpolar? Remember, the oxygen and hydrogen have very different electronegativities. Therefore, the electron density, if you remember from Unit 1, gets pulled towards the oxygen. You get a slight negative charge on the oxygen side and a slight positive charge on the hydrogen side.
If you look here, this nitrogen has a positive charge. And if you look here, this phosphate has a negative charge. So these are ionic.
These are ions. And water is polar. Therefore, the answer is this portion will go into solution in water, because anything that's polar or ionic, anything that has a charge, will go into solution in water. This part, then, is attached to a glycerol. A glycerol is simply a three-carbon alcohol.
And attached to the glycerol, two of the three carbons, are these long chains here. And you see there's a CH2 and then this jagged line. If you follow the jagged line down, you ask, what does that mean? Well, let's look at the jagged line as it's depicted in the space filling model. space filling model, you see a bunch of black balls and you see a bunch of white balls.
The black balls are all carbon. The white are all hydrogen. So we have carbon with, in this case, two hydrogens attached to it, and a carbon here and a carbon here.
Now remember, carbon has four valences, so it can have four covalent bonds. And so this carbon right there is completely attached. All of its covalent bonds are taken up, one by a carbon here, one by a carbon here, and then one by hydrogen, one by that hydrogen in the back. So what you're looking at here as depicted by this jagged line is what we call a hydrocarbon chain. It's simply carbon and hydrogen.
That's it. That's all that's here. And at the very end, this last carbon to fill all the valences has three hydrogens.
Every other one has two. So these are all carbon and hydrogen. Now, the carbon and hydrogen have very similar electronegativities. Therefore, they are not polar. The electron density is not split between the carbon and hydrogen.
I'm sorry, it's not pulled to one side, it is split evenly between those two. And so there is no polarity here. These two things are therefore lipids, or if you wish, fats. So they're called fatty acids. The reason they're called acids is because this group right here, if you notice, C double bond O to another O.
Now imagine a hydrogen attached there. That makes this into something that you've seen before, a carboxyl group. Remember, carboxyls are always acids. So this is an acid attached to a fat. So this is a fatty acid.
The fatty acid then is attached to the glycerol through this. connection here, which is called an ester bond. I'm not going to worry about having you remember that until you get your organic chemistry class. But these bonds here then connect this to the glycerol, and now we end up with this big molecule that's really made of four different parts. The phosphate choline head, the glycerol, and then the two fatty acid tails.
Now this is what's key. This is what makes it amphipathic. The fatty acids are not soluble in water, but the choline phosphate is. Therefore, this can go into solution.
on this part of the molecule but will not go into solution in this part of the molecule. That's what sets up the membrane. It's very easy to make membranes.
All you have to do is take this molecule here, put it into water, and shake it up, and then you'll automatically get this structure. These structures here will form just automatically based on the physics of the situation. So you look here, what are these ovals with the plus and the minus?
We go back to this picture here. You can see the plus and the minus. So those blue ovals represent the choline phosphate heads. And then you see these red things sticking down. Each one of these choline phosphates has these two red things.
You should recognize that then as these two fatty acid tails. Now, if we look at this picture, you'll see that these sections here all lined up. All these phosphate cholines are lined up and all the fatty acid tails are lined up. And that just comes about because of the way things go into solution water. The polar parts, the ionic parts go into solution water and the nonpolar parts don't.
and they will dissolve in each other. And so the fatty acid tails then come together, dissolve each other, and you get this bilayer. You get two different layers of molecules here, one where you've got this upper layer and the fatty acid tails going down, the other one in the bottom with the fatty acid tails going up.
And this becomes a very beautiful, strong barrier to not allow water to go through it. The reason water can't go through this membrane is because in order to do so, it has to go into this area where... the nonpolar things are and it won't do that.
So this becomes what we call a phospholipid bilayer. Phospholipid because the molecules are phospholipids and it's bilayer because there's these two different layers. Now if you look at this you might think okay well there doesn't seem to be a really strong connection between the two layers.
Like if you follow the cursor right here it doesn't look like there's a strong connection there. You're absolutely right there isn't. There is no strong connection there. So that that means is that the membrane actually has these two layers that slide relative to each other like this. At least that's the major thought at the moment, that they slide like this.
And that makes this a fluid. Things that are inside here will actually move around because of that. And we have a number of drugs, barbiturates and others, that actually operate by changing the property of this membrane. It allows it to either become more rigid or shear more easily from place to place. But that's how this is all set up here.
Phospholipid bilayer is set this way. It's very easy to make. Here's the thing, though. This is not a living membrane. Living membranes are more sophisticated than this.
This is just the basis of a membrane. To make a living membrane, you have to do this. You have to put inside the membrane a number of different things. These things, for example. What you're looking at here in this picture are a bunch of proteins that are embedded in this membrane.
And they're integrated into the membrane. So therefore, they're referred to as integral membrane proteins. Every single one of these is an integral membrane protein. And as we look through here, you'll see the proteins go through, in some cases, go completely through the membrane, like this one.
This protein here is depicting what we call an ion channel. We'll talk about those later. These are both receptors.
This is a cell adhesion protein, a cell adhesion molecule right here. All of these molecules then have the same property. They're embedded in the membrane.
They go all the way through, and you have a part that sticks off to the inside. Now let me explain the picture here. What you're looking at is a gigantic cell. Now, in this whole thing, thing here like this. Imagine a cell the size of the room that you're in.
This would be the membrane of that cell, and they've cut it so you can see it. And then this is the outside, the yellow portion of the outside, the blue portion is the inside. So this portion of the protein is sticking inside the cell, and therefore we call that the intracellular domain.
We have another section sticking out, and that's the extracellular domain. And then we have the spanning or membrane domain, which is actually in the membrane itself. And you notice all four of these proteins that are depicted here.
have these three domains. You have the membrane, domain, extracellular, intracellular for all four of them. This one's different. Notice it has a membrane domain and an intracellular domain, but it does not stick out.
That's also possible. This is what is referred to as a G-protein. This particular picture is depicting a G-protein. So all of these things are in here, and they're embedded in this membrane because of the following properties.
Now remember, amino acids make up proteins. Proteins are big chains of amino acids. We've been talking about that all semester. Remember, the amino acids come in three different groups. There's a polar group, an ionic group, and a nonpolar group.
So I could ask you right now, I could ask you a question on the test and say, okay, well, the amino acids right in here in this spanning domain, in this membrane domain, do you predict or can you predict? Is it even possible to predict their property? Are they polar, ionic, or nonpolar?
The answer would be they must be nonpolar. The reason that they are in the membrane, in fact, is because they're nonpolar, they dissolve in this nonpolar fatty acid region. Well, what about these, the extracellular domain and the intracellular domain?
What are they like? Well, this is water. So this portion of the protein has to be able to go into solution in water.
Therefore, it must have a charge. Therefore, the amino acids in this region and in this region, the extra and intracellular domains, must be polar or ionic, or the majority of them must be polar or ionic. So that's what holds this structure in the membrane. It's held together by these bonds that are really holding the membrane together.
The bonds are sometimes called lipophilic bonds, and they're not true bonds, but they're lipophilic interactions is a better way to say it. And they are strong. They actually do hold the membrane together.
And you can see, because these are all soap molecules, what you're looking at really is a soap bubble with a bunch of proteins embedded in it. And in fact, that's precisely what the cell membrane acts like. It acts like a soap bubble.
Just like any other soap bubble, you take two soap bubbles, put them together, and they'll come together and form one giant soap bubble. You have another soap bubble on the inside of a bigger soap bubble, and the outside soap bubble will incorporate the inside soap bubble, and so forth. They'll do that. Cells will do that. They don't, however, because these proteins are keeping the soap bubbles from touching.
For example, the cell adhesion protein you're seeing here. So this is an intuition that I want you to start to develop. The membrane is a soap bubble.
It acts very much like a soap bubble. The difference you might be wondering, though, is this is like soap bubbles that you see out in the air and in space are very fragile things. They tend to break, and that's one of the fun things about them. That's not true of a soap bubble in water. A soap bubble in water is stabilized.
In fact, it's very, very stable. And so this is not a property that water soap bubbles have, that air soap bubbles don't have, is that they're stable, very, very stable. Now, that's not the only thing that's in living membranes, not just these proteins. Also embedded in the membranes are these little things.
They're like, in this case, yellow chili peppers, the way they drew them here. These things are cholesterol molecules. The cholesterol molecules are embedded in this portion, the lipophilic portion of the membrane, and what they do is they change the shear force required to get the membrane to move.
Now, remember, the bilayer can move. This upper part can move relative to the lower part. That makes it fluid, and we call this the fluid mosaic model because these proteins are embedded in this thing, and they are fluid.
They're moving around. In fact, you can see this picture. You see in this, they show things kind of fuzzy on the end, and that's meant to...
represent that things are moving here. And in fact, they're moving extremely fast. If you calculate the average speed of these proteins moving around the membrane, it's about 200 miles an hour. So they're moving extremely rapidly.
The energy that's pushing them is simply thermal energy in the system, heat. But that means that this membrane is extremely fluid and allows things to move around. So even though these things aren't going to come popping out of the membrane, they are moving around inside the membrane. How much they move around is determined by the cholesterol that's in this membrane.
The more cholesterol there is, typically, although this is not strictly always true, the more cholesterol there is, the more rigid that membrane tends to be. So that's the key aspect of a living membrane. It has all these different molecules embedded in it, but the main structure you can see here is that phospholipid bilayer.