Transcript for:
Exploring Cell Structure and Function

Hey everyone, Dr. D here, and in this video we are going to be covering chapter 6 from our Biology Campbell 12th edition textbook. This is a tour of the cell, so let's go ahead and get started. Hi Dr. D, Dr. D explains stuff. All right, chapter 6, a tour of the cell.

This is a fun chapter where we get to talk about the basic unit of life, which is a living cell. Remember that you cannot be simpler than a cell and typically be considered a living life form. There are three domains of life and they're all based on at least one cell, right?

The archaea, the bacteria, and the eukarya. Here is a single cell eukarya. This is paramecium and that is what life is based on. So we're going to learn about the structure and function of the parts of the cell. Remember that a cell is usually too small to be seen by the human eye.

Here's the resolution limit of the eye. Did you know that your eye can resolve things as small as about 200 micrometers? That's about 0.2 millimeters.

Think about 0.2 millimeters. That's about as small an object as your human eye can see. However, look at this.

Most organisms are much smaller than 0.2 millimeters, right? They're much smaller than 200 micrometers. So most plant and animal cells are here in the 50 micrometer range.

And then most bacteria and archaea are down here in the low 1, 2, 3, 4, 5 micrometer range. So you need a light microscope in order to see cells. LM stands for light microscope.

And this is why we use microscopes in the lab. We cannot resolve cells with our naked eye. And again, eukaryotic cells are much larger than prokaryotic cells, right?

Like 50 times larger, 10 to 50 times larger on average. Now, here's some basic features of all cells. All cells have a plasma membrane, also known as the cytoplasmic membrane or the cell membrane.

All cells have the cytosol, which is the fluid portion of the cytoplasm, right? The cytoplasm or cytosol. All cells have at least one chromosome.

Did you know that archaea and bacteria typically only have one chromosome? However, eukaryotes have multiple chromosomes. Chromosomes are continuous pieces of genomic double-stranded DNA that carry genes. And all cells have ribosomes.

Ribosomes are organelles, though they are not membrane-bound organelles, so all cells have them. And ribosomes are responsible for making proteins, synthesizing proteins. Now, again, prokaryotic cells are the simpler cells. Remember, Two domains of life comprise the prokaryotic cells, the archaea and the bacteria.

These cells have no nucleus. Instead, their DNA, they do have DNA. Remember, all cells have at least one chromosome of DNA.

Their DNA is an unbound region called the nucleoid. So if you were pointing at the DNA of a prokaryotic cell, such as a bacteria, you would point... to the DNA and call it nucleoid, right?

So basically the DNA is floating around in the cytoplasm. It is not enveloped in a double membrane like the nucleus of eukaryotes. So, and there's also, in addition to no membrane bound nucleus, there's also no membrane bound organelles, right?

There are no organelles that have their own membranes, such as, I'll give you some examples, just to run off some examples for you. The, the, mitochondria, the chloroplasts, the Golgi apparatus, the endoplasmic reticulum. You may have heard of some of these.

Well, prokaryotes have none of those because those are all membrane-bound organelles, right? They do have ribosomes though, right? Prokaryotes do have ribosomes because those are not membrane-bound organelles. Now, what else do we have? A cytoplasm.

Remember with the cytosol, this cytoplasm is found inside of the cell. Here is a typical bacterial cell. Here is a, just a typical cell. You see this stringy stuff inside? That's the single chromosome.

Remember, most bacteria have one chromosome. And if you were pointing at this chromosome, this DNA, remember you would call that the nucleus. The nucleoid, the DNA is floating around in the cytoplasm.

It is not separated by a membrane. And then you do have cytoplasm, right? This is the fluid portion of the cell, the cytoplasm.

And then you also have a plasma membrane. Remember, all cells have a plasma membrane. Okay, those are structures that you have, you know, inside of prokaryotic cells.

And ribosomes, see the little tiny dots here? These little tiny dots represent the ribosomes as well. So those are typical structures inside of a prokaryotic cell.

And although there are many other structures which you will learn about when you move on to microbiology class in the future, you're going to learn about all these other structures as well. For example, the cell wall of the bacteria, you know, these little short hairs called fimbri, those long whip-like structures called flagella. But we're not going to touch on those in this class.

Now moving on to eukaryotic cells. Remember, eukaryotic cells are characterized by having DNA inside of a nucleus. This means that the chromosomes are bounded by a double membrane. So it's two phospholipid bilayers, two membranes surrounding your DNA in a eukaryotic cell. So that's why they call it a double membrane.

It's a double phospholipid bilayer. Now, You also have membrane-bound organelles, and the main membrane-bound organelle is called the nucleus. That's the main membrane-bound organelle. But of course, you have all those other membrane-bound organelles, which I mentioned earlier. Remember, you have the mitochondria, plants have chloroplasts as well.

You can have endoplasmic reticulum, and we're going to introduce all of those. Don't worry if those sound confusing right now. We're going to introduce all of those coming up in this chapter.

And of course, you have a cytoplasm, you have ribosomes, etc. And remember, because the DNA is inside of a double membrane, the DNA is separated from the cytoplasm. It's not floating around in the cytoplasm because it's inside of a nuclear envelope, right?

That nuclear envelope means the double membrane of the nucleus. And eukaryotic cells... are generally much larger than prokaryotic cells. If you've watched, or if you've paid attention in lab when we did microscopy, remember we looked at larger cells like diatoms and such and pond water. Those large eukaryotic cells are so easy to see with lower magnification with our microscopes.

But if you recall, the prokaryotic cells, such as the bacteria, were really hard to see. And we needed to go all the way down to, you know, one... 1000x total magnetic magnification with oil to see those those cells because they're just so small compared to the eukaryotic cells remember uh 10 to 50 times larger you know are the eukaryotic cells than the prokaryotic cells now you may be wondering why cells are so small and this is because metabolic requirements set the upper limits on cell size The surface area to volume ratio of the cell is critical.

As a cell increases in size, its volume grows proportionately more than its surface area. And this is a problem, okay? The smaller a cell is, the larger its surface area per volume ratio.

And this allows nutrients to come into the cell and waste to exit the cell. with ease, right? But as the cell itself grows larger and larger in size, this surface area to volume ratio drops, and that makes it very difficult to get nutrients into the cell, get waste out of the cell, get things around the cell.

And this probably explains why there's an upper limit on cell size, why cells can't be like basketball size, for instance. It just wouldn't be feasible. And remember, the main function of a plasma membrane, the cell membrane which all cells have, is to be a selectively permeable barrier, allowing oxygen to get in, nutrients to get in, as well as waste to get out. And again, the smaller the cell, the easier this is to accomplish because of that large surface area to volume ratio.

Here's a closer look at the plasma membrane. The plasma membrane, remember from a previous chapter, is a phospholipid bilayer. Remember the phospholipids we learned about with the hydrophilic heads and the two hydrophobic tails?

Well, these phospholipids are the ones that are the most important. Phospholipids form a bilayer, right? Remember, the heads point out of the cell where there's water, and the other layer's heads point into the cell where there is water as well in the cytoplasm, and the tails are sequestered in this hydrophobic region, this region where there should not be any water.

Now, look at this. We're going to learn about what these proteins are doing, but these purple structures are... proteins that float around in that plasma membrane, in that cytoplasmic membrane.

And these proteins all have different functions. So these are the membranes of the cell. So when I say the plasma membrane, it kind of looks like this.

If I say the membrane of the nucleus, it looks like this. If I say the membrane of the mitochondria, it looks like this. Okay, so anytime you hear the term membrane in N cell, it doesn't matter if it's the plasma membrane or any other membrane.

It's a phospholipid bilayer, and it likely also has important proteins in it that do different jobs, okay? So this is what we're talking about when we're talking about the membranes of a cell. Now look at this typical eukaryotic animal cell and compare it to what you saw.

with the prokaryotic cell. Remember the prokaryotic cell? It was so simple.

I mean, it had a cytoplasm, ribosomes, had a nucleoid, cytoplasmic membrane, but no membrane-bound organelles. But I want to show you this. Look at this.

This is a eukaryotic cell, an animal cell. Animal cells have a plasma membrane, as all cells do. See this membrane surrounding the whole cell? That's the plasma membrane.

Again, it's a phospholipid bilayer. But notice how animal cells do not have any kind of cell wall. Okay, make sure you know that animal cells do not have a cell wall.

Inside, you have that fluid, right? You have the cytoplasm. Obviously, there's cytoplasm in all cells. And then...

This big purple structure right here, this is the nucleus. And notice that it is a double membrane. Remember I said that the nucleus is a double membrane.

There's one membrane, there's the other. And keep in mind that each one of these membranes is a phospholipid bilayer. So you have two phospholipid bilayers that make up the nucleus. And the DNA would be inside of the nucleus, right? The chromosomes are inside of the nucleus.

This is what I mean by saying that in eukaryotes, the DNA is not in the cytoplasm. The cytoplasm is the fluid kind of out here in the cell, right? The DNA is inside of the double membrane of the nucleus, and that double membrane is called the nuclear envelope.

So when I say nuclear envelope, I'm talking about this double membrane of the nucleus. And look at this blue membrane here. You see this blue membrane?

It's highly folded. The term for that is convoluted. This is a highly convoluted membrane. And again, this membrane here, when I say membrane, it's also made out of phospholipid bilayer.

So just think of all these lines that I'm outlining here. All of this is phospholipid bilayer, okay? And this is known as the rough.

endoplasmic reticulum, which we're going to learn a lot about in a minute. Okay. And then outside of that, you have more endoplasmic reticulum. This right here, this is the smooth endoplasmic reticulum. And by the way, they call it the endoplasmic reticulum because it's reticulated.

It's highly convoluted. You see how it twists and turns, right? That's what they're talking about there. And then look at this green member. membrane here, this green membrane, these look like stacks of flattened sacs right here.

See right here, there's stacks of flattened sacs. And these are called the Golgi bodies or the Golgi apparatus, the Golgi bodies of the Golgi apparatus. And we're going to learn about that in a little bit. And look at these kidney bean shaped looking organelles that have their own membrane too. These are called the mitochondria, right?

So Here's a mitochondria. Here's another mitochondria. Here's a mitochondria.

Okay. All of these organelles, by the way, that I've mentioned so far are membrane-bound organelles. The nucleus, the rough endoplasmic reticulum, the smooth endoplasmic reticulum, the Golgi apparatus, the mitochondria, the...

lysosomes, the paroxysome, all of these are membrane-bound organelles, which are found in eukaryotic cells, but not one of these is found in a prokaryotic cell. Does that make sense? Now, here's another really interesting organelle that only, only animal cells have, and this here is called the centriole, the centriole, okay?

This little bundle. called the centriole. And here in your textbook, it's called the centrosome, but those are both terms for centrioles. It's a pair of centrioles, sometimes called the centrosome. Okay.

And we're going to learn exactly what that is in a little bit as well. We're going to learn all these structures. We're going to learn about the flagellum, which by the way, not all cells have a flagellum. Not all eukaryotes have the flagellum.

Okay. But we're going to learn about the flagellum. We're going to learn about all these organelles. We're going to learn about everything here.

And again, this is a typical eukaryotic cell. And just by looking at this, you can tell that by having all these bulky membrane-bound organelles in the cell, you're going to be much larger than a prokaryotic cell, which lacks these membrane-bound organelles, right? Do you remember the prokaryotic cell was far simpler, right?

I mean, look how much... fewer structures are inside. You've got the nucleoid, you've got some ribosomes, you've got the cytoplasm, you've got a cell membrane and a cell wall, and you know, there's very little other bulky stuff inside of the cell. This is why bacterial cells, you see here, this bacterial cell, that distance right there is 0.5 micrometers, right, whereas the eukaryotic cell is a much, much larger, usually about 50 micrometers.

in diameter. So cells are just much more complex when eukaryotic organelles are bulky and they're much more complex. Now next let me show you a plant cell.

There are so many similarities between animal cells and plant cells. However, there are some differences I want to share with you as well. So look here.

This is a typical plant cell. Notice how there are so many structures that are similar to the animal cell because they're both eukaryotic, but there are some differences. First of all, look at this giant mass in the center of the cell. Look at this large membrane. This is called the central vacuole, and we're going to talk about what that does in a bit.

But this is known as the central vacuole of the plant cell. Plant cells tend to have a central vacuole, which is a membrane full of fluid. Okay?

Also notice these green organelles. You see these green organelles here? These are what makes plants green.

These are the chloroplasts. They're responsible for photosynthesis, right? Capturing sunlight and using it to produce sugar, right? So here you have those chloroplasts.

And remember, all cells have a plasma membrane, right? So... This plant cell is no different.

It has a thin plasma membrane, but what is this thick layer outside right here? This thick green layer is the cell wall, right? Plants do have a cell wall outside of the plasma membrane. Okay, now real quick, I just want to tell you something.

So again, plant cells have a central vacuole, okay? Plant cells have chloroplasts, but this is a typical misconception. Students often say that animal cells have mitochondria while plant cells have chloroplasts, and that's partially true, but actually it's false.

Animal cells have mitochondria, but plant cells have, look at this, plant cells have both, okay? Plant cells have both mitochondrion and chloroplasts, okay? So don't forget that just because you're a plant cell doesn't mean you don't have mitochondria, okay?

This is really important to understand. And here's another misconception with students all the time, okay? Students will say, Okay.

Animal cells have a plasma membrane, a cell membrane, but plant cells have a cell wall. See how that's false? Because again, it's not and or, it's both, right?

Plant cells do have a plasma membrane, just like animal cells do, just like all cells do. Remember, all cells have a plasma membrane, but plant cells have a cell wall in addition. Does that make sense?

Okay, so that's good to understand. And conversely, I just want to share, do you see any centrioles here? Remember the centrosome bundle I showed you just a little bit ago in the animal cell?

That's right. Plant cells lack centrosomes or centrioles, okay? Plant cells lack them. But again, let me share that animal cells such as this one do have ribosomes. Remember, because all cells have ribosomes.

Plant cells have ribosomes. All cells have ribosomes, but those are not membrane-bound organelles. Now it's the nucleus of the eukaryotic cell that contains most of the DNA of that cell, and it's those ribosomes that use the information on DNA, specifically the gene information, in order to make proteins.

We're going to talk all about how the genes information is used. to make proteins in the cell when we learn about a concept called gene expression later on. Again, the nucleus contains most of the cell's genes and is usually the most conspicuous organelle.

It's the most obvious organelle. The nuclear envelope, remember that double membrane, encloses the nucleus, separating the DNA from the cytoplasm. The nuclear envelope is a double membrane. Each membrane consists of a phospholipid bilayer.

Remember, every time I say membrane in a cell, you should think phospholipid bilayer. They're essentially all membranes are made of the same stuff, phospholipid bilayers. All right, now let's explore the nucleus and its envelope in detail.

I just want to point out structures of the nucleus that we need to know about. So let's take a look at first, first let's look at the nuclear envelope. Remember the nuclear envelope? If we zoom in on this little box right here, you can see that there is not just one layer, one membrane, but there's a double membrane in that nuclear envelope. It's a double phospholipid bilayer.

And If we look down here, you can see all these little pores. You see these little reddish color pores in the nucleus? Those are called pores, pore complexes.

These pore complexes, or also known as nuclear pores, form holes in the nucleus in that double membrane, allowing substances to go in and out of the nucleus. There can also sometimes be ribosomes associated with the nuclear envelope as well. All right.

Now, inside of that nuclear envelope are the chromosomes. Your chromosomes are floating around inside of the nucleus, inside of the nuclear envelope. Humans happen to have 46 chromosomes, and your chromosomes are continuous DNA.

However, did you know that your chromosomes aren't just made of DNA? They aren't just naked DNA? Let me show you something.

Here you can see DNA in blue. See the blue string right here? That blue string represents your chromosome.

That represents your double-stranded DNA. However, look closely. The DNA appears to be wrapped around this purple structure, right?

And that's actually... how your chromosomes exist. Your chromosomes are not just DNA, the blue string.

Your chromosomes consist of DNA, the blue string, as well as these purple structures, which the DNA DNA wraps around, and the DNA wraps around almost two times around these purple structures. So what are these purple structures? These are known as histone proteins. So your chromosomes exist as not just DNA, but DNA that's wrapped around numerous of these histone proteins.

And together, together, the DNA and the histone proteins are called chromatin. So for instance, if I asked you, What are your chromosomes comprised of? You would say, oh, the chromosomes consist of chromatin, which is a combination of DNA wrapped around histone proteins. And we're gonna learn more about these histone proteins soon.

There's actually eight of these histone proteins in what's called an octamer, and that is a type of quaternary structure. We're gonna learn more about that later on. So anyway, the DNA wraps around the histone proteins. And this is kind of what your chromatin looks like, the chromatin which makes up your chromosomes.

All right, what else do we need to know about the nucleus? Look at this dark spot. I want to draw your attention to this dark spot here. That dark spot is not DNA. That dark spot is its own little organelle.

It's like a sub-organelle called the nucleolus. And the nucleolus is the site of a type of RNA called... ribosomal RNA or rRNA. And this is the organelle responsible for synthesizing rRNA. And we're going to talk about rRNA next when we talk about ribosomes.

But for now, I just want you to know that the rRNA, which makes up part of the ribosome, is synthesized here in the nucleolus. Okay. Lastly, Lastly, I want to show you one other structure associated with the nucleus, and that's this fibrous network. Look at this fibrous network of proteins. This fibrous network of proteins, which supports the nuclear shape, the nuclear envelope, is called the nuclear lamina.

And we're going to learn about the nuclear lamina near the end of this chapter. The nuclear lamina is made up of... proteins, cytoskeletal proteins called intermediate filaments.

Okay, so these intermediate filaments, these nuclear lamina network, it coats the nuclear envelope and it supports the nuclear envelope structure. Next, let's discuss ribosomes. Remember that all cells have ribosomes, including prokaryotic cells, and ribosomes are essential because they synthesize proteins and all cells need to synthesize proteins by using the directions provided on their genes. And ribosomes are complexes that are made up of, remember ribosomal RNA or rRNA, which we know was synthesized where?

That's right, Wicket, star student over here. The ribosomal RNA is synthesized in the nucleolus of the nucleus. That's where this part of the ribosome was made. So the ribosome is made up of ribosomal RNA as well as protein.

So you can think of it as an organelle that is made of part RNA and part protein. And the ribosomes, again, their job is to build proteins and they build proteins in two main locations. In the cytosol, these are known as free ribosomes, and on the outside of the endoplasmic reticulum, the rough endoplasmic reticulum, or the nuclear envelope. Sometimes they can build proteins associated with the nuclear envelope as well.

So let me show you what I'm talking about. Here, you can see that in the animal cell. Remember when we talked about the animal cell?

See all these tiny dots? I know it's hard to see, but I'm going to put my magnifier on here so you can see what I'm talking about. These little tiny dots everywhere in the cytoplasm, these little dots represent the free ribosomes in the cytoplasm.

And proteins can be synthesized in the cytoplasm by these free ribosomes. However, do you remember I said this convoluted membrane on the outside of the nucleus, this blue membrane. This is called the rough endoplasmic reticulum.

And notice how it's just studded outside of the endoplasmic membrane. Now, there are also ribosomes on the surface of the nucleus as well, which can synthesize proteins. So those are the main places where ribosomes exist, where they can synthesize proteins. And by the way, the rough endoplasmic reticulum, yes, it is called the rough endoplasmic reticulum because those ribosomes give it this rough appearance. Whereas remember here, this was called the smooth endoplasmic reticulum right here.

Notice how it's not studded with ribosomes, and that's how it got its name, smooth endoplasmic reticulum, because it does not have ribosomes associated with it. But what does the ribosome look like? So let's take a look.

The ribosome, again, it is made up of part rRNA, ribosomal RNA, and part protein. And it's made up of two subunits. This here is the large subunit of the ribosome, and this down here is the small subunit of the ribosome.

And the two can detach from one another. When the ribosome is not synthesizing a protein, the two subunits will detach from one another. And when the ribosome is synthesizing a protein, the two will come together.

And I kind of I kind of described this as a hamburger bun analogy. You know how hamburger buns have the large bun and the small bun? So I kind of describe it like having a hamburger bun.

You've got the small bun and the large bun. They could come apart. They could go together. And that's what the ribosome looks like.

So now you've seen firsthand the number of membranes inside of a eukaryotic cell. These membranes make up what's called the endomembrane system. Endo meaning inside.

You have all these membranes inside of the cell. These consist of the nuclear envelope, remember that double membrane, the endoplasmic reticulum, which is rough, and then you have the smooth endoplasmic reticulum, the Golgi apparatus with its flattened sacs, you have the lysosomes, You have vacuoles and the plasma membrane. Now what's interesting is only two of these are physically connected to one another.

That means that they are continuous. The two that are physically connected to one another are the nuclear envelope and the endoplasmic reticulum. Let me show you that real quick. So here you have a cutaway of that double membrane of the nuclear envelope. Remember the double membrane of the nucleus right here?

And I just wanna show you something. Look how the outer membrane of the nucleus, look at this, watch this. Yes, it is connected to that convoluted rough ER.

Remember that blue membrane outside of the nucleus called the rough ER, the rough endoplasmic reticulum? I just want you to see this. The two membranes are connected.

This means they are, that's right, wicked, they are continuous, okay? So when, if I were to ask you which membrane is continuous with the nuclear envelope, the answer would be the rough endoplasmic reticulum, okay? Now, real quick, those are the only two that are connected and continuous, okay?

The... Other membranes, these other membranes are not continuous with one another, they are not connected to one another, instead they are connected via transfer by vesicles. There's a process in the cell known as vesicular transport, and I'll explain that when we get to it. All right, now let's delve a little deeper into the endoplasmic reticulum, shortened ER. This endoplasmic reticulum, remember, it's the rough endoplasmic reticulum that is continuous with the nuclear envelope.

And it's the rough endoplasmic reticulum that is studded with ribosomes. And those ribosomes do what ribosomes do, which is synthesize protein. So the rough endoplasmic reticulum's job is to allow these ribosomes to synthesize protein. Let me show you.

The rough endoplasmic reticulum has bound Ribosomes, which secrete glycoproteins, proteins covalently bonded to carbohydrates, and then it distributes transport vesicles, secretory proteins surrounded by membranes, is a membrane factory for the cell. So it also produces membrane for the cell. Let me show you something neat. Here you can see, I'm going to show you something really neat here. Do you remember how I said earlier that the only two membranes that are continuous or connected are the nuclear envelope, this double membrane of the nucleus, which connects directly to the rough ER?

And then how did I say the rest of the membranes are connected to one another? That is right, Wicket. Star student as always by vessel. vesicles, vesicular transport, they call it.

Let me show you something. Look what, look what the, look what the membrane of the rough endoplasmic reticulum can do. Look at it. The rough endoplasmic reticulum, what happens is proteins are made into the lumen of the rough endoplasmic reticulum. And let's say those proteins need to go to other parts of the cell.

Well, those proteins will accumulate in the lumen and then the rough ER will start to kind of pinch. You see how the rough ER is pinching right here? You see that?

It's pinching. It'll keep pinching and pinching and pinching until it kind of pop, you know, pops off. It pinches off a little vesicle. Isn't that interesting?

Think of real quick. Let me let me give you an analogy. Think of soap. bubbles.

Have you ever messed around with soap bubbles? You know how sometimes you could have one big soap bubble and you could kind of mess with it and a little soap bubble pops off and floats off? Well, the membranes of a cell kind of can do the same thing, right? They can kind of behave like that.

So when this ER membrane pinches and pinches and pinches, eventually it can pinch off a little phospholipid bilayer ball called a vesicle. In this case, it's a transport vesicle because it has a destination. This little vesicle is going to go somewhere, right?

And usually, this is just foreshadowing, usually it goes to the Golgi apparatus. That's its next stop, okay? And keep in mind, what is this transport vesicle full of?

That's right. It is full of proteins. You know, you could have proteins inside of this little ball.

You can have... proteins in the membrane of this little ball. Do you remember the membrane proteins I told you about? This is getting into a little more detail than you probably need, but you know, it's probably, it's really interesting.

You remember I said there are membrane proteins like this, like proteins that float around in membranes such as the plasma membrane. These membrane proteins live inside of membranes like this, and those membrane proteins They come from the rough ER. So these little ribosomes are making proteins. Some of those proteins are membrane proteins, which live in the membrane. And some of those proteins are kind of secreted into the lumen.

That means inside of the rough ER. And those proteins are in the rough ER. So when this little membrane blebs off and... pops off, right, as a transport vesicle, that transport vesicle can have membrane proteins in the membrane.

It can have what are called secretory proteins inside. Okay, and those are the proteins that are being transported from the rough ER. Isn't that neat? So not only are proteins synthesized at the rough ER, but then those, those, those proteins can leave the rough ER and go to other destinations in the cell.

And again, usually the next stop for these transport vesicles, which are full of proteins, is the Golgi apparatus, okay? But what does the smooth ER do? Remember, there's also a smooth ER in the cell as well. Just to remind you, let's go back to show you.

Remember, the rough ER is right outside of the nucleus. It is continuous with the nuclear envelope. However, the smooth ER over here is a little further away usually.

And it is not studded with ribosomes, so it is not involved in synthesis of proteins, is it? Instead, you might be wondering, well, what does the smooth ER do if it doesn't synthesize proteins? Well, let's talk about that. The jobs of the smooth ER are to synthesize various lipids, detoxify drugs and poisons.

What's really interesting is... The smooth ER of your liver, it specializes in detoxifying drugs and poisons in your blood and store calcium ions, stores calcium ions as well. So if I were to ask you, what are the functions of the smooth ER?

These are the functions of the smooth ER. While the rough ER specializes in production of those proteins and then transporting those proteins. via transport vesicles to different parts of the cell.

And it is also a membrane factory for the cell. This is where more phospholipids come from. Next, let's talk about the Golgi apparatus. Do you remember this structure made up of flattened membranous sacs off to the side of the cell here?

This is known as the Golgi apparatus. And let's talk about what this organelle does. The Golgi apparatus, again, it consists of flattened membranous sacs called cisternae.

The Golgi apparatus, these are its functions. It modifies products of the rough ER. Remember, do you remember I told you that proteins that were synthesized at the rough ER pinched off in transport vesicles and those transport vesicles are on their way to the Golgi. So this is what this sentence means here. The Golgi also manufactures certain macromolecules and it sorts and packages materials into transport vesicles.

One thing you may want to add here is that the Golgi modifies proteins as well. So the proteins that come to the Golgi from the rough ER, those proteins are modified. by the Golgi apparatus.

So again, that Golgi apparatus, remember that was that little stack of membranous sacs in the cell. Now, remember I told you that proteins are headed towards the Golgi via transport vesicles. See the purple dots represent the proteins and the blue capsule represents the transport vesicle. This is from the, where's this from? Where's this coming from?

That's right, Wicket. It's coming from the rough ER, right? So these are the proteins that were synthesized at the rough ER.

And now they're, they're coming to visit the Golgi, you know, in this transport vesicle. And look, do you remember what I said about soap bubbles? Do you remember what I said about soap bubbles? I said, The membranes of a cell kind of behave like soap bubbles.

You know, it's a rough analogy, but it works. If I mess with a soap bubble, a big bubble, I could blub off a little tiny soap bubble, right? But what happens if a small bubble touches a larger bubble? Have you ever seen this in real life, right? Like a small soap bubble touching a larger bubble?

What can happen sometimes? Boop! Doesn't it kind of fuse? Don't the two little bubbles fuse into one big bubble? So the membranes of the cell are a lot similar.

They're very similar to that. Look, so again, this transport vesicle full of proteins from the rough ER will touch what's called the cis face. The cis face or the receiving face, this is the face that points towards the nucleus and the rough ER. The vesicle touches the cis face.

or the receiving side of the Golgi. And what does it do? Just like soap bubbles, it will fuse.

Look how the transport vesicle fused with the cis face of the Golgi. And it became one with the Golgi. In fact, the vesicle membrane became one with the Golgi.

It became part of the Golgi. And what did it deliver to the Golgi when it did that? What got delivered to the Golgi? That's right. These proteins were delivered to the Golgi and to the cisternae of the Golgi.

And then those proteins make their way, make their way to the, what's called the trans face, the trans face or the shipping side of the Golgi apparatus. And what did I say happens along the way? Remember, these proteins make their way from the cis face of the Golgi towards the trans face of the Golgi, which, which points towards the plasma membrane of the cell.

And along the way, those proteins are modified. Okay. These proteins are modified. And once those proteins are nice and modified and finalized, what can the Golgi do? Look what the Golgi does.

Look, look, look. Those finalized proteins gather here at the transface of the Golgi. And look what can happen. The transface of the Golgi will start to pinch and pinch and pinch.

And what do you think is going to happen next? What do you think happened next? That's right, Wicket.

This will... pinch off as a vesicle. Look at this. So the Golgi can form vesicles, transport vesicles, just like the rough endoplasmic reticulum can form transport vesicles to transport those proteins, those finalized proteins to some final destination, right?

This is what we mean by, remember what I said earlier, vesicular transport? See, it's transport via vesicles. See, the Golgi is not directly connected. to anything, right? It's not directly connected to the rough ER, is it?

However, it can receive vesicles full of cargo from the rough ER, and then it could ship those finalized proteins. Once it modifies those proteins, it could ship those proteins to where they need to go. It could ship these proteins to the plasma membrane, to the plasma membrane of the cell, and this little...

This little vesicle could fuse just like soap bubbles fuse and become one with the plasma membrane of the cell. It could go off and become an organelle in the cell. So this little vesicle could go off and become its own little organelle. In fact, that's how lysosomes are made. So if you ever wonder, we're going to talk about lysosomes soon, but if you ever wondered where lysosomes came from, it was a, it's basically a vesicle full of.

proteins from the Golgi that became the lysosome. Isn't that interesting? So these vesicles can either go and touch the plasma membrane and become one with the plasma membrane, or they can float off in the cytoplasm and exist inside the cytoplasm.

Basically, these proteins can go to their final destination. Isn't that cool? So again, the job of the Golgi apparatus is to receive proteins from the rough endoplasmic reticulum at the cis face, modify those proteins as they move to the trans face, and then ship those proteins off to where they need to go from the trans face by forming these secretory vesicles at the trans face.

Okay, I hope that made sense. Speaking of the lysosome, what is that? Remember when we talked about the animal cell, I said that the animal cell has an organelle inside called the lysosome. Here you can see it as this sphere they're pointing to here. That's a little organelle that's known as the digestive organelle.

Specifically, the lysosome is a membranous sac, remember it came from the Golgi actually originally, of hydrolytic enzymes that can digest macro... macromolecules. Do you remember hydrolysis? We learned about the concept of hydrolysis in the last chapter. Hydrolysis is that process of using water to break a bond and break macromolecules back down into monomers.

Remember that? So the lysosome is full of enzymes that can do hydrolysis. These are the hydrolytic enzymes.

They're the ones that break down macromolecules. So for instance, they can break down carbohydrates from polysaccharides back into monosaccharides, or they could break down proteins back into amino acids. So that's why it's known as the digestive organelle of the cell.

Think of it like the stomach of the cell, right? So lysosomal enzymes work. best in an acidic environment inside of the lysosome.

So the inside of the lysosome is acidic. It's negatively charged and has a high concentration of protons. The hydrolytic enzymes and lysosomal membranes are made by the rough ER. So first those protons Proteins are made at the rough ER.

Then they are transferred to the Golgi, right? Remember, by fusing to the cis phase of the Golgi and then becoming modified as they go to the trans phase of the Golgi. And then once they leave the Golgi, they become lysosomes. So some lysosomes probably arise by budding from the trans phase of the Golgi apparatus.

Some types of cells can engulf other cells by phagocytosis. So some cells can engulf other cells, and this forms what's called a food vacuole. Think of a membrane surrounding some kind of food. A lysosome will then fuse with the food vacuole to digest its contents.

Lysosomes also use enzymes to recycle the cell's own organelles and macromolecules, a process called autophagy. So let me show you what I'm talking about. Again, some cells can bring food particles.

This is a food particle on the outside of the cell. The food particle will touch the plasma membrane. Remember, the plasma membrane is the membrane around the cell.

And this is interesting because this triggers a process known as phagocytosis, which means bringing things into the cell by this process. Look what happens. The cell membrane, the plasma membrane, will invaginate to form what's known as a food vacuole.

And do you remember what I said about soap bubbles? When one of these membranes pinches enough, what can it do? That's right. It can pop off into a vesicle. In this case, it's a membrane surrounding food.

So it's called a food vacuole. The food vacuole will meet up with a... Lysosome, remember the lysosome is its own little vesicle full of digestive enzymes, hydrolytic enzymes.

And what happens when two soap bubbles meet? Remember, two soap bubbles meet, they can fuse into one, right? They can fuse into one. So now what have you done?

You have food and enzyme in one vesicle, right? And there you have digestion. You have now digested that food back into building blocks like amino acids and monosaccharides and such that you can use now. You can use those resources to build your own cellular components.

Isn't that cool? Now, you can also use this concept to recycle components of the cell. So look at this.

You could have a lysosome. a lysosome. And let's say, for example, you have a sick mitochondrion, right?

You have a mitochondrion that's not doing so well, and you need to recycle the mitochondrion. So what you can do, again, is fuse the lysosome with the mitochondrion, and it'll digest the mitochondrion. So this is known as autophagy. So vacuoles are defined as large vesicles.

So again, a vacuole is basically a large membrane, okay? And these large vesicles are derived from the ER and Golgi apparatus, and these vacuoles perform a variety of functions in different kinds of cells. So it all depends on the type of vacuole you're talking about. One I showed you just a second ago was the central vacuole of plant cells.

This is found in many mature plant cells. It contains a solution called sap. It is the plant cell's main repository of inorganic ions, including potassium and chloride. The central vacuole plays a major role in the growth of plant cells.

So these central vacuoles are very important in plant cells. However, there are other kinds of vacuoles as well. There are food vacuoles, which are essentially large membranes of food formed by phagocytosis. So essentially, remember this example I gave you of phagocytosis bringing in the large food particle?

This would be known as a food vacuole. You see, so it was made by phagocytosis. That's a food vacuole. Another type of vacuole, which is really interesting, and we're going to touch on this in a subsequent chapter, is a contractile vacuole. This is found in freshwater creatures, freshwater protists.

And the contractile vacuole's job is to actually squeeze excess water out of the cell. So here you can see a cartoon of the central vacuole and just how much space it takes up inside of the plant cell. Now again, I told you about the endomembrane system. Only two components of the endomembrane system are bound to one another.

They are continuous. Which two were continuous? That's right.

It was the nuclear envelope which was continuous with the rough endoplasmic reticulum. The other components are connected via vesicular transport. So here, let's take a look at that. Let's take a look at it one more time for review, okay? So take a look here.

This is the double membrane of the nuclear envelope right here. This is the nucleus right here. Notice how the outer membrane of the nuclear envelope is continuous with or connected to the rough ER. The rough ER is this convoluted blue membrane out here, which is studded with many, many, many ribosomes, all of which are producing proteins.

And those proteins are being woven into the membrane of the ER, or those proteins are being synthesized into the opening, the lumen of the ER. And then those proteins, they need to go to their final destination, right? Do you remember that?

Those proteins need to now leave the rough ER. So what happens is, take a look here. A bunch of these proteins will collect inside of the lumen of the ER or inside of the membrane of the ER. And then that membrane will pinch and pinch and pinch. And what can the membrane do once it's pinched enough?

Just like soap bubbles. It can pinch off completely as what's known as a transport vesicle here. The transport vesicle will move to the cis face of the Golgi and then fuse with the cis face of the Golgi just like soap bubbles. Now you have delivered, you have delivered proteins to the Golgi.

Those proteins make their way from the cis face to the trans face of the Golgi at which point the modified mature proteins can then pinch, pinch, pinch, and then pinch off as what's known as a secretory vesicle. Those vesicles can either linger in the cell and become their own little organelles, such as a lysosome, or those vesicles can fuse with the plasma membrane and either secrete the proteins out of the cell, or Let me share something very interesting with you. Remember I said membrane proteins can also be made? Membrane proteins could also be made up here at the rough ER?

Well, those membrane proteins could make their way through the Golgi, and those membrane proteins might be in the membrane of this secretory vesicle. So those membrane proteins, when they reach the plasma membrane like this and fuse, those become the... proteins in the membrane of the plasma membrane. Isn't that interesting?

So if there are proteins inside floating around inside of this little vesicle, those proteins are going to get secreted out of the cell. But if there are membrane proteins in the membrane of this vesicle, those end up being the membrane proteins of the cell membrane. So if you ever see a membrane of a cell, let's say they say this is a membrane, a plasma membrane of a cell. And then they point to all these different proteins that live in that membrane.

Well, guess what? Those proteins weren't made there at the plasma membrane. They were originally made at the rough ER. Then they made their way to the cis face of the Golgi.

They were modified throughout the Golgi to the trans face of the Golgi. And then they made their way to the plasma membrane via a secretory vesicle. So isn't that interesting? So now you know where all those membrane proteins come from.

Isn't that interesting? And it all has to do with... Vesicular transport of the endomembrane system.

Awesome you guys. Well I think it's about time for our first break time with Gizmo and Wicket. Let's take a break.

Let's take a break and see what these little guys are up to and we'll come back with more. Now let's turn our attention to the mitochondria, the chloroplasts, and the paroxysomes of the eukaryotic cell. Starting with the mitochondria and chloroplasts.

The mitochondria and the chloroplasts. change energy from one form to another. The mitochondria is the site of a process known as cellular respiration. We're going to go into great detail about cellular respiration soon in a subsequent chapter, but it is essentially the metabolic process that uses oxygen to generate ATP.

You may have heard this cheesy saying in high school or in a previous bio class that mitochondria are the powerhouse of the cell. You know, that's what they're talking about. The ATP, which is the energy molecule, it's the energy currency of a cell, it's produced by the mitochondria, right? And that process of making that ATP is called cellular respiration.

Now, chloroplasts also change energy from one form to another. These chloroplasts are found in plants as well as algae. And these are sites of photosynthesis, which change sunlight energy to chemical energy in the form of sugars. So I'm going to go into some detail now about the mitochondria and the chloroplasts.

And there are so many interesting things with regard to these organelles. First of all, the mitochondria and the chloroplasts have similarities with bacteria, and it's an eerie amount of similarity to bacteria. These similarities have given rise to what is known today as the endosymbiont theory, and this theory suggests that an early ancestor of eukaryotic cells engulfed an oxygen-using non-photosynthetic prokaryotic cell, which became the mitochondria.

It eventually became the mitochondria. So the engulfed cell formed a relationship with the host cell, becoming what's called an endosymbiont, and then the endosymbiont evolved into the mitochondria that we know of today. So take a look here.

This cell at the top would represent an early eukaryotic cell, which then engulfed, remember with phagocytosis, it engulfed a early prokaryotic cell and or a, you know, bacterial cell as a food vacuole. But instead of digesting that food vacuole with the lysosome, fusing it with a lysosome and making a yummy meal out of that engulfed bacteria, they kind of lived in harmony in that what what happened over time you is that this bacteria that was engulfed was specialized, became specialized in producing ATP for the eukaryotic cell. And the eukaryotic cell provided protection as well as sugars and fats to feed to the mitochondrion. Isn't that interesting?

So they kind of formed what's known as a mutualistic symbiotic relationship. They lived together happily ever after. And over time, this bacteria became... more and more dependent on its eukaryotic host. And it underwent what's known as genome reduction, which means a lot of genes kind of, you know, went away that it didn't need, it didn't need certain genes anymore.

And so it lost its ability to grow on its own. So like, you know, if you went today and took mitochondria outside of its home outside of the eukaryotic cell and tried to grow them on their own, they wouldn't grow like bacteria anymore. That's because they've forgotten what it's like to grow on their own. They've, you know, specialized in forming ATP for the cell. And they're kind of mutually dependent now on one another.

Isn't that beautiful? So again, the mitochondria originally were free living bacteria, which were engulfed into the early eukaryotic cell. And what's really interesting is that this is also where they believe chloroplasts came from for plant cells. So a plant cell took it one step further and also engulfed what's known as a photosynthetic prokaryote or a cyanobacteria.

And that cyanobacteria was engulfed as a food vacuole. But again, instead of becoming digested by the lysosome, they lived happily ever after. And this photosynthetic bacteria...

bacteria, again, underwent genome reduction. It lost its ability to live on its own, and it specialized in doing photosynthesis for its eukaryotic host. And today we know of that organelle as the chloroplast.

And again, this chloroplast no longer can live outside of its host plant cell. This chloroplast lives in side, and it's a mutualistic symbiosis dependent relationship now, really. Isn't that interesting?

So again, the endosymbiont theory suggests that the mitochondria and the chloroplasts have this interesting origin story, if you will, that they used to be bacteria once upon a time. Isn't that interesting? I find that fascinating.

That blows my mind every time, and it blows my students'minds when I tell them. Now let me explain some evidence that suggests that this is true. Alright, here is the evidence that suggests that the endosymbiont theory is true. The chloroplasts and the mitochondria are enveloped by a double membrane, like many bacteria are. They contain free ribosomes, and these ribosomes are not like our ribosomes.

Our ribosomes... Eukaryotic ribosomes are heavier. They're known as 80S ribosomes and bacteria possess what are called 70S ribosomes. Well guess what?

Bacteria and mitochondria and chloroplasts all share the same type of ribosomes called the 70S ribosomes. Isn't that neat? So imagine this, your mitochondria have their own ribosomes inside And those ribosomes are not like your ribosomes.

Those ribosomes are slightly smaller, just like the ones in bacteria. Also, bacteria, do you remember I told you that bacteria have one chromosome? Well, bacteria do have usually one chromosome, and it's a circular piece of double-strand DNA.

And guess what? Your mitochondria, as well as the chloroplasts of plants, have a single. circularized chromosome, much like that of bacteria. Also, bacteria grow and reproduce by a process known as binary fission. These organelles, these mitochondria and chloroplasts also grow and reproduce independently in cells.

So they also grow and divide and grow and divide in half just like bacteria do. Again, I said their ribosomes are similar to bacteria. They are genetically similar to bacteria.

They've done sequence alignments of their bacterial DNA, and they've compared it to the sequence of mitochondrial DNA, as well as chloroplast DNA. And those sequences match much more than when you compare it to eukaryotic DNA. So isn't that interesting? Also, remember I said that...

The mitochondria do cellular respiration, right? They make ATP. Well, they do it the same way bacteria do. And the chloroplasts do photosynthesis. Well, they do that the same way bacteria do, photosynthetic bacteria.

It is fascinating. There's an overwhelming amount of evidence that suggests that mitochondria and chloroplasts used to be, once upon a time, long, long ago, their own free-living bacteria. So if this is true, think about this for a second.

I just want you to think about how each one of your cells contains mitochondria. And that's kind of a whole different life form that, you know, fused with us back in time. So essentially, animals, including you and me, humans, are part eukaryotic and actually part prokaryotic. We're almost like hybrid organisms. You know, I tell my students, it's like we're hybrid powered, you know, like a Prius is hybrid powered.

It's part gas or ice engine and part electric. Isn't that interesting? So you and I are part eukaryotic and part prokaryotic.

In fact, we're prokaryotic powered. Like if we had, you know, a winch, you know, like those speedsters, you know, people who, you know, race cars and stuff, they got their sticker on the back powered by. you know, this, that, or the other, you know, we'd be powered, prokaryote powered, I guess.

That would be our sticker on the back of our, I don't know, I digress. But it's just fascinating to think that our cells are part eukaryotic and part prokaryotic. We're actually hybrid organisms.

And that blows my mind every time. And it should blow your mind. Now let's talk a little bit about the structure of a mitochondria. Remember the mitochondria were found in eukaryotic cells and don't forget not just animal cells but plants also have mitochondria don't forget and remember they kind of look like little kidney bean-shaped organelles.

Well, let's talk a little bit more about their structure, shall we? So on the right, you have an actual picture with an electron microscope of a mitochondria, and you can see the general shape. But here, this cartoon helps us understand a little bit better on the left.

This is a cartoon showing you the structure of mitochondria. Now, the mitochondria has two membranes. Look, this here...

where my mouse is right now. This is known as the outer membrane of the mitochondria. And notice that the outer membrane is smooth and it covers the mitochondria.

See, this is the outer membrane of the mitochondria. And then there's a membrane inside. Look, there's a membrane inside called the inner membrane.

And I want you to notice something about the inner membrane. Look, I'm tracing it. Watch what it's doing.

What is the inner membrane doing right here? This is the inner membrane of the mitochondria. Watch. Yes, it's convoluted.

Remember that term we learned before? That means twisty, turny, convoluted. It's forming all these folds, okay? And these folds increase the surface area of the inner membrane.

And that's good because the more surface area is in here in the inner membrane, the more... We can do cellular respiration, which is make ATP, the more ATP we can make. And by the way, you see these folds right here?

There's a fold. Here's a fold. These folds have their own name.

It's called cristae, cristae. Now, there's a little bit of space, a little bit of space between the outer membrane, which is smooth, and the inner membrane, which is convoluted. And that space, that time. tiny bit of space between the two membranes is called the intermembrane space. Lastly, I want you to look at the inside of this inner membrane.

This is the fluid inside of the mitochondria here. This is called the matrix of the mitochondria. And look what's inside of the matrix.

We have these little dots. What are these little tiny dots everywhere? Those are ribosomes, right?

Remember I said that mitochondria have their own little ribosomes? And these ribosomes, believe it or not, are not like your ribosomes. The ribosomes that are in your cytoplasm, the ribosomes that are associated with the rough ER, these ribosomes are smaller and they resemble that of bacterial ribosomes, the ribosomes of bacteria. That's one of the reasons why we think, you know, the endosymbiont theory is real. And then look at this.

What did you notice here? Look at this. What is this? That's right.

This is a single circular chromosome of DNA. And this is the genome. This is the genome of the mitochondria.

And they've sequenced this genome and they see genes on it that resemble that of bacteria. Isn't that interesting? Fascinating, right?

So it's almost like its own little organism. And when it divides, it can divide on its own, and it can grow and divide. And actually, this is really neat.

They have shown this just recently. They have shown, I'm sorry, I'm just trying to position this. They have shown that these mitochondria in the cell aren't just these static things that linger around, and they're kind of anchored, but they can fuse with one another.

to form this like grid, and then they can separate from one another. And for this reason, they're kind of dynamic in how they work. It's really interesting. So they're not just static, they're not just sitting there in the cell, but they're fusing, they're separating.

And so researchers have given them the name the power grid, right? They can form a power grid. Isn't that interesting? So they're dynamic, but they're very much almost alive, right?

Because they have so many... characteristics of bacteria. However, they are no longer bacteria. They are now an organelle that, you know, we depend on for providing ATP for the cell. Now, what about chloroplasts?

Chloroplasts, these are the reason why plants are green, right? Plants are green because they contain these chloroplasts, which contain that green pigment called chlorophyll, making them green, and as well as enzymes and other molecules that function in photosynthesis. We're going to talk a lot about how photosynthesis occurs.

You're going to know exactly how photosynthesis happens. We're going to talk about that. Not only are you going to know how cell respiration happens, but we're also going to talk about photosynthesis.

You're going to be a master of all this stuff by the end of this series of videos. All right. So chloroplasts are found in the leaves and other green organs of plants, and they're also found in algae.

But let me ask you this. Let me quiz you, right? Hopefully you can beat Wicket to this answer.

Do... Photosynthetic bacteria, remember there are photosynthetic bacteria called cyanobacteria. Do photosynthetic cyanobacteria possess chloroplasts?

That's right, wicked. They do not, right? Because chloroplasts are membrane-bound organelles. And so you are not going to find chloroplasts in photosynthetic bacteria.

bacteria. In fact, the chloroplast is roughly the size of a photosynthetic bacteria, so you're not going to find that. Now, let me tell you this though.

There's a lot of structures that a photosynthetic bacteria has that a chloroplast has. For example, thylakoids, right? So the chloroplast includes thylakoids, granum, and stroma, right? These are structures that a photosynthetic bacterium has, right? And this is, again, why they think that those photosynthetic bacteria gave rise to the chloroplasts because, you know, the insides, the guts of a chloroplast look like the guts of a photosynthetic bacteria.

Isn't that interesting? So speaking of that, let's talk about that a little bit more. Again, when you have a plant cell, a plant cell, that plant cell, in addition to mitochondria, It has chloroplasts, and I just want to show you what a chloroplast looks like.

We need to break down this structure. Again, on the right, you have an actual picture of a chloroplast from an electron microscope. But on the left, we can see, you know, a better depiction of what's going on inside.

Here you have an outer membrane. This is the outer membrane of the chloroplast. This is called the inner membrane of the chloroplast.

They're both kind of smooth, but then you have these stacks of membrane. Look at this. You have these stacks of membranes.

So again, outer membrane, inner membrane, then you've got these membrane stacks. Now, each stack is called a grenum. Grenum.

That's what the stack of membranes is called. And by the way, I affectionately call these stacks pancake stacks because they always remind me of a stack of pancakes. I don't know.

That's just me. But you know, it also helps my students remember these concepts. So the stack of membranes here inside of the chloroplast is called the granum. And I call it a pancake stack.

Now, here's where it becomes important to understand the stacks. Each pancake, okay, each pancake has a name. And that name is a thylakoid.

Okay, so again, The stack of membranes, this stack is called a granum. And the plural for granum, by the way, is grana. So all of these are grana. And this stack right here is a granum.

This one pancake in the stack is called a thylakoid. And do you see the membrane right there? Do you see that membrane of the thylakoid?

That's called the thylakoid membrane. Now, the fluid directly outside of the stack You see this fluid out here, this fluid surrounding the stack? That's called the stroma. Let me show you that term.

The stroma is the fluid outside of the pancake stacks. And what about the fluid inside of the pancakes? There's fluid inside of the pancakes.

That's called the thylakoid space, okay? We need to know these terms, okay? We need to know. And by the way, what's floating around in the stroma? What's floating around in the stroma?

Remember the stroma is the fluid out right outside of the pancake stacks? Ah, ribosomes. And again, these ribosomes are not like the rest of the ribosomes in the plant cell. These are smaller ribosomes. These ribosomes are 70S ribosomes, similar to that of bacteria.

And what's this I spy right here? This is a small circularized DNA. Much like that of the nucleoid of bacterial cells. And again, They have sequenced this DNA and they have found genes that resemble that of bacteria, not that of eukaryotes, right?

And where do new chloroplasts come from? These cells grow and divide into new cells, just like bacteria do. Again, lending so much evidence and support towards the endosymbiont theory. Isn't that neat?

So if we're prokaryotes, powered because we have mitochondria, right? Plants are doubly prokaryote powered, aren't they? Because plants, plants are a eukaryotic cell, which is not just mitochondria powered, but it is also chloroplast powered.

So it's like a triple hybrid. It's like a tribrid. It's, it's amazing, right?

Are you, are you not entertained? Right? If it's like I tell my students, right? If this concept that we are all hybrid organisms of two different creatures in one, if this concept does not blow your mind, then you are going to roam around aimlessly the rest of your life with never having your mind blown.

Because if this doesn't blow your mind, I don't know what will. All right, next, let's learn about this little organelle right here that looks an awful lot like a lysosome, but it's called the paroxysome. The paroxysome is a specialized metabolic compartment bounded by a single membrane. The paroxysome contains enzymes that remove hydrogen atoms from various substances and transfer them to oxygen.

This forms hydrogen peroxide, which is by the way quite toxic to the cell. These reactions may have different functions. What are the functions of the peroxisome?

So what is the peroxisome good for? The main thing the peroxisome does is that it uses oxygen to break fatty acids into smaller molecules, which are eventually used for fuel in respiration. What does that mean? That means that... You remember how fats contain those long tails, those three fatty acid tails?

And you guys know that those fats give you a lot of energy, right? Well, the peroxisome breaks those tails down and sends them to the mitochondria, so the mitochondria can make ATP using those tails. Does that make sense?

That's kind of what it does. Now in the liver, the peroxisomes detoxify alcohol and other harmful compounds. And this is what a peroxisome looks like if we were to look at it with a electron microscope.

All right, now we're going to turn our attention to what's known as the cytoskeleton, a network of fibers that organizes structures and activities in the cell. You and I, we have a skeleton that supports our body and aids in movement, right? Our skeleton is made up of bones, right? But the cell has its own version of a support system. Though it's not made up of bones, it's made up of protein fibers.

This is the cytoskeleton, a network of protein fibers extending throughout the cytoplasm. You can see here what I'm talking about. This is the cytoskeleton. If this is a cell with the blue being the nucleus, see how these protein fibers in green and orange are spanning throughout the cell, giving the cell structure and support.

That's what we're talking about. The cytoskeleton organizes the cell structures and activities, anchoring many organelles. They can also be utilized for movement, certain types of movement as well. I'm going to... explain that in detail in just a minute, but there are three types of fibers.

The cytoskeleton is made up of three different types of fibers. These are the microtubules, the microfilaments, and the intermediate filaments. Now again, the main job of the cytoskeleton is to support the cell and maintain the shape of the cell. However, there are dynamic processes as well. Dynamic means related to movement.

The cytoskeleton can aid in movements in the cell as well. Now, what's really cool is that one of those movements involve what are known as motor proteins. And inside the cell, remember those vesicles we talked about, the vesicles that form from the rough ER and make their way to the cis face of the Golgi?

Now, students often ask, how in the world do those vesicles know where to go? How do those vesicles make their way from the... from the rough ER all the way to the cis face of the Golgi, how do they know how to get there? And it's by using the cytoskeletal network. They use, let me show you.

So imagine if the rough ER is on the left side of your screen and the cis face of the Golgi is on the right side of your screen. Well, how would I get a vesicle, this pink thing? How would I get a vesicle full of cargo, protein cargo?

From the rough ER all the way over to the right to the Golgi. Well, look, look at these. The vesicle will attach to this motorized protein.

This is known as a motor protein. And the motor protein uses the energy of the energy currency of the cell, ATP, to walk like little feet, like a little cartoon Mickey Mouse feet. All right.

And what does it walk along? It walks along a microtubule. And a microtubule is one of the three cytoskeletal fibers.

Remember, microtubules. So microtubules can serve as kind of like monorails in the cell, allowing for vesicles to make their way around the cell. See the vesicle? And here's an actual picture. with an electron microscope of a microtubule and a vesicle making its way along the microtubule.

And this is exactly how a vesicle would move from one part of the cell to the other. This is what's responsible for vesicular transport. Now again, the three types of fibers that make up the cytoskeleton include the microtubules, which are the thickest of the three, the microfilaments, which are also known as actin filaments, which are the thinnest of the three, and the intermediate filaments, which are in the middle range, as their name suggests. So again, the microtubules, the microfilaments, and the intermediate filaments, these are the three that make up the cytoskeleton. And this is a nice little table summarizing their structure, their size, what they consist of, and what they're involved in, what processes they function for.

So let's start with the microtubules. Let's break these down. What I'm going to do at this point is go cytoskeletal complex to cytoskeletal complex. We're going to break these down starting with the microtubules. Remember, We already talked about how they can serve as monorails for vesicular transport with motorized feet.

Microtubules are the thickest. Okay, they are the thickest in diameter of the cytoskeletal fibers. Microtubules are hollow rods, so they kind of look like straws. Okay, about 25 nanometers in diameter.

Microtubules are constructed of protein. protein dimers of tubulin. Remember what proteins are? A polypeptide chain?

Well, there are two tubulin proteins that come together to form a dimer, a quaternary structure, if you will. And these tubulin dimers make up the microtubules. One tubulin is called alpha tubulin, and its partner is called beta tubulin. So these alpha and beta tubulin dimers come together to form the microtubules. And what are some of the functions of the microtubules?

Shaping the cell, guiding movements of organelles and vesicles, you saw the vesicular transport, separating chromosomes during cell division. These are some of its key functions. So let's break it down.

In animal cells, do you guys remember that in animal cells we had that structure in the cell called the centrosome, also known as the centrioles? Remember that centriole bundle, the centrosome? In animal cells, the centrosome has a pair of centrioles, each with nine triplets of microtubules arranged in a ring. Those centrosomes are made up of microtubules, these microtubules that form a pair of centrioles.

And these centrosomes play a role in this part. Look, separating chromosomes during cell division. That's their function. So if I ask you, what is the job of the centrosomes or the pair of centrioles? Well, you would say...

They function in separating chromosomes during cell division. And what are they made of? What are they made of? They are made of microtubules, okay? They are made of microtubules.

So you see, this is what I'm talking about. Remember we talked about the animal cell and I pointed out the centrosome, which is a pair of centrioles? Well, here it is. That pair of centrioles is made of microtubules.

If you look closely, you'll notice that these are tubules, microtubules, which I said were hollow straws. And they call these triplet microtubules because there's rows of three of them. See, three, three, three, three. And those three form a big circle.

This is called a bundle, a bundle. And the centrosome is made up of two bundles of centrioles, which are essentially Bundles of triplet microtubules. Isn't that neat? And what do these play a role in?

What function do they have? During cell division, they aid in separating chromosomes. They help to separate the chromosomes during cell division.

What else do microtubules do? Well, guess what? We touched on this concept of a flagella before, right?

This whip-like structure on the outside of animal cells. Flagella, as well as we've talked about cilia, right? So flagella and cilia, these are extensions of the cell for motility, right?

Motility. The flagella allows cells to swim around. Like think of a sperm cell with its long whip-like tail. That's a flagella.

Cilia are short, short beating hairs. But either way, these structures are for motility. And without microtubules, these would not function because microtubules are at the heart of the flagella and at the heart of the cilia. Many unicellular protists, remember like single cell eukaryotes, are propelled through the water by cilia or flagella.

And cilia and flagella differ in their beating patterns. So let's discuss that real quick. A flagella is a long whip-like structure. Think of the tail of a sperm cell. This is a sperm cell.

Now, the flagella beats back and forth, sort of like a fish tail, right? So there's a beating back and forth, an equal beating back and forth to swim. However, look down here at the cilia.

These are little hairs called cilia, and they beat. However, they don't beat equally back and forth. No, they do what's known as a power stroke, where they propel the sail forward, and then a recovery stroke, where they recover without moving. So think of the butterfly, you know, when you're swimming the butterfly, you do a big powerful stroke to propel you forward, but then you kind of just recover your arms without, you know.

without doing any motion. So the recovery stroke is meant to get you back in position to do another power stroke, right? You see how that's different from just an equal beating back and forth like a fishtail.

So please remember that, that the flagella beats back and forth while a cilia undergoes what's known as a power stroke. Now, again, microtubules are in the, if we were to crack open one of these flagella. And by the way, did you know flagella, the flagella is covered with plasma membrane. The plasma membrane surrounds the flagella.

But if I was to cut the plasma membrane open and take a peek inside, do you know what I would see in there? I would see microtubules. So I want to show you something.

Look at this. If I were to cut the flagella, imagine if I cut the flagella and then I peeled that plasma membrane away. Remember?

This is the plasma membrane in blue surrounding the flagella. I've cut the flagella and peeled the plasma membrane away. What do I see inside?

Look inside. What do you guys see? What do these long tubules remind you of? These straw looking things. That's right, wicket microtubules, right?

So you have, look, these are doublets, right? You've got doublet microtubules, and you actually have nine doublets surrounding two single microtubules in the center. And you always have this nine plus two arrangement, and that's why they actually call this the nine plus two arrangement.

Let me see if I can find that here for you. Here it is. The nine doublets of microtubules which are arranged in a ring with two singlet or single microtubules in the center. Here you go. So this is called the nine plus two arrangement of microtubules and this spans the inside of the flagella and cilium.

Isn't that interesting? So this So how does it work? Let me tell you how it works.

These microtubules on the right, right, these microtubules, this doublet, I should say, has little walking feet attached. Do these walking feet look familiar? These little Y-shaped structures?

These are motor proteins. Isn't that cool? So these motor proteins are called dynein.

And with the help of ATP, these little walking feet march, kind of like a millipede. You know how a millipede marches with its little millipede feet? Okay, imagine if this is a millipede on top of another millipede, okay? Now, with the help of ATP, what would these little feet do, these little Y-shaped feet? That's right, they would start marching, right?

They would start actually, like, moving forward. And with the help of ATP, these little feet start walking, but their heads are attached to this doublet. And that's what's responsible essentially for the bending of the flagella, right? I'm not going to ask you about this, but, you know, it's very interesting to understand that the beating of the flagella is thanks to these motor proteins, which are connected to the doublet microtubules. Isn't that neat?

That's how it works. Dianine has two feet that walk. along microtubules. One foot maintains contact while the other foot releases and reattaches one step further along.

The movements of the feet cause the microtubules to bend rather than slide because microtubules are held in place. Okay, isn't that interesting? So again, this is how flagella work. The flagella is the long whip-like structure outside.

The basal body is the name of the part that embeds in the cell membrane. Okay, so this is how flagella work. Isn't that neat?

And it's all thanks to microtubules, right? All right, the next cytoskeletal component are the microfilaments. The microfilaments are the thinnest. Remember, they're the thinnest of the cytoskeletal fibers.

The microfilaments are solid rods about 7 nanometer in diameter. That's tiny. They're made up of a double twisted chain. Think of two pearl necklaces twisted together of actin proteins, actin subunits.

So it's a quaternary structure of many, many, many actin proteins. If it if essentially if if microfilaments look like two. pearl necklaces that are twisted together, then each pearl is made up of the protein called actin.

And the microfilament itself is simply a quaternary structure of actin, a bunch of little actins. Now, these microfilaments are really important because they form a network of microfilaments to support the cell's shape. In fact, they form what's called the cortex.

Just inside of the plasma membrane, remember the membrane around the cell, just underneath the plasma membrane is the cortex, the cortex made up of microfilaments. And this cortex supports the cell's shape. So the way I explain it is like this. I say, imagine a tent, you know, like a sleeping tent for camping.

Now. When you buy a tent, it comes with some kind of canvas or, you know, some kind of material, but then it also comes with these rods as well, right? Now, if it didn't come with the rod, Rods, you really wouldn't be able to pitch the tent, right? You know, the rods really dictate what the tent looks like. So if I'm underneath the tent and I'm kind of pushing the rods around, well, that's what the tent is going to look like.

Does that make sense? This is essentially what microfilaments do. So if a cell looks a certain way, an animal cell, if an animal cell looks a certain way, This is because those rods, those microfilaments are underneath the cell membrane, making it look that way, right?

So a big example, a prime example of this are bundles of microfilaments and how they make up the core of microvilli of your intestinal cells. So look at this. This is one intestinal cell and your intestinal cells have these protrusions, these fingers, these, look at these little fingers. You see these little fingers?

These are protrusions of the intestinal cells. These fingers are called microvilli, and your intestines have these microvilli pointing towards the lumen of your intestine. That's where, that's where the food, you know, the, when you eat food and drink, that food and drink passes through these fingers.

And these fingers are great because they have so much surface area for absorbing all those nutrients from your food, right? So imagine when you eat food, the food funnels around, filters around these fingers. These fingers absorb that food nutrient, the fats, the sugars, all the good stuff, right? The amino acids, all that good stuff goes in here.

Now, why are the cell, why is the cell forming these fingers? Well, look right underneath. Look.

underneath the microvilli. Look inside. So you have the plasma membrane, you have the plasma membrane, and what's underneath holding it up as a finger? That's right.

Look, the microfilaments, the actin filaments, the actin filaments are pushing the membrane up, the plasma membrane up as a microvilli. Isn't that interesting? So the reason why your intestinal cells look like they have Bart Simpson's haircut, right?

They have these finger-like projections. is because the microfilaments, just like tentpoles, are pushing the plasma membrane up, increasing the surface area of those intestinal cells, allowing you to absorb more food. Isn't that, absorb more nutrients from the food. Isn't that interesting?

So, you know, this is what, this is important. Microfilaments make up the cortex of the cell. And the cortex, just like this, can dictate what the surface of the cell looks like.

Now, more things, what about other functions of microfilaments? Microfilaments are not just involved in static structure of the cell, like the microvilli, but also several dynamic processes as well, right? Dynamic meaning movement associated, right? So microfilaments function in cell motility, contain the protein myosin in addition to actin.

So let me show you something. Let me show you something. Did you know that your muscle cells, this is a muscle cell also known as a muscle fiber, your muscles form fibers, and these muscles have actin microfilaments attached to the cell membranes inside.

And these actin filaments are what allows your muscles to contract. You see this purple structure? You're going to learn this in great detail. I'm going to gloss over this, but you're going to learn this in great detail when you take anatomy and physiology. Part of it is showing how muscles work and how your muscles contract.

So don't get bogged down in the details here, but essentially this is what's happening. You don't need to know this for my class, but I'm just going to explain it. You don't have to learn it.

right now, but your, your muscle cells have microfilaments attached to each end. So, so you see this gray bar, that's one end of the cell and you see this gray bar, that's the other end of the cell. And what do you have attached to the cell membranes at each end? You have the microfilaments, remember the twisted orange, uh, twine, right? The microfilaments.

And what happens when your muscle contracts, then then this is all loose and the cell is stretched out. But when your muscle contracts, these little proteins in purple, proteins are called myosin. And these myosin, these little nubs here, here, here, here, here, these nubs start walking, marching, kind of like those walking feet of dynein.

They start marching. And when they march, they literally pull the actin microfilaments inward. So these feet march and pull the actin this way, while at the other end, the feet are marching the other way, pulling this actin the other way. And that essentially causes these walls. to come in, right?

It causes the cell to contract, right? And that's what's responsible for muscle contraction. You're going to learn all of this, I promise you, when you take anatomy and physiology, this is a big deal in anatomy physiology, you're going to learn all of this.

But what I want you to know is that without microfilaments, muscle contraction would not occur, right? Your muscles depend on actin to pull on to contract. the muscle. Does that make sense?

So you need to know that microfilaments are essential for muscle contraction. Here's another thing that microfilaments are involved in. If you've ever taken a close look at a plant cell, this is a plant cell right here. This is a one plant cell right here. It's a brick-shaped cell.

Inside of the plant cell, you'll see these little green organelles called clots. chloroplasts, which we've discussed, the chloroplasts. And a lot of times, if you really, if you really look closely, you'll see these chloroplasts moving in a circular fashion, they'll be moving around the cell, sort of like someone flushed the, you know, when you flush the toilet, things move around the bowl, right? Someone flushed this cell. And I know that's a crude analogy, but it gets the point across.

These chloroplasts look like they're circling around the cell. And this is a process known as cytoplasmic streaming in plant cells. And it's, it relies on microfilaments. So what I want you to know is that cytoplasmic streaming depends on microfilaments to work.

Next, have you ever heard about pseudopods, right? Amoeba move by forming what are known as pseudopods. Pseudopods are extensions, extensions of the cytoplasm.

Pseudopod refers to a fake foot. By extending the cytoplasm, the amoeba extends a fake foot. It then uses that fake foot to get a foothold and then at the other end, the cell can retract.

The cytoplasm can retract. This is how This is how amoeba get around. They move via pseudopod motility. It's known as amoeboid movement.

And what's interesting is that it's the actin network. It's the microfilaments that make this possible. Imagine if I'm underneath the plasma membrane here at this side of the cell and I'm building those rods, right?

I'm building those tent poles. I'm building microfilament rods. Well, if I'm building tent poles, if I'm building microfilaments and the cortex here under the cell membrane, I could push that cell membrane out, right?

I could push the cell membrane out because I'm building microfilament rods underneath this plasma membrane. Isn't that neat? So what I need you to know is that amoeboid movement or pseudopod motility depends on those actin microfilaments.

Again, sometimes they're called actin filaments. You can call them microfilaments. That's the same thing.

Lastly, let's talk about the intermediate filaments. This is the third type of cytoskeletal fiber. Remember, these are intermediate in diameter, so they're not as thick as the microfilaments, and they're thicker than the microfilaments.

They are intermediate in thickness. That's how they got their name. And these aren't nearly as interesting as the previous two we spoke about. These intermediate filaments are more permanent cytoskeleton fixtures than the other two classes. They essentially support cell shape and fix organelles in place.

They are not known for doing any dynamic process. Remember dynamic meaning movement related. Let me just refresh your memory. Look at all the movement related functions that microfilaments partake in.

And the muscle cell contraction, cytoplasmic streaming, pseudopod motility, you know, all this good stuff. with microfilaments, and then remember with the microtubules involved in flagella beating, cilia beating, involved in separation of the chromosomes during cell division. These are dynamic processes, right? So the only cytoskeletal component of the three that doesn't do anything dynamic like these is the intermediate filaments.

And there's one more thing I want you to remember about the intermediate filaments. Do you remember the nuclear lamina we discussed earlier in this chapter? I said the nuclear lamina is a network of protein fibers that supports the structure of the nucleus.

I need you to remember that that nuclear lamina is made up of none other than intermediate filaments, okay? So the intermediate filaments make up the nuclear lamina of the cell. All right, at this point, we've finished talking about the parts of the cell inside of the cell. But what about structures outside of the cell? These are known as extracellular components.

Extra means outside. So extracellular means outside of the cell, right? Most cells synthesize and secrete materials to the outside of the cell.

These materials and structures are involved in essential cellular functions. So let's talk about some of these extracellular components. We briefly touched on cell walls, right? Do all cells have cell walls? That's right, Wicket.

Not all cells have cell walls, right? Certain cells have cell walls. For instance, bacteria have cell walls.

Plants have cell walls. Several different fungi have cell walls. Okay, so certain organisms do have cell walls. And let me just give you some examples here. With prokaryotes, with bacteria, most bacteria have a cell wall of what's called...

Peptidoglycan. Most fungi have a cell wall of something called chitin. Protists can also have cell walls. Some protists have cell walls of cellulose.

Some protists have cell walls of silica. Plants have a cell wall too, right? Plants have a cell wall of cellulose, okay? We're going to learn all this later on, but just good to understand.

Now, the only one here that really is not known for having any kind of cell wall are animal cells. Animal cells do not have cell walls. Instead, animal cells have what are called extracellular matrix, an elaborate extracellular matrix, or abbreviated ECM. The ECM is made up of glycoproteins, this means proteins with sugars attached, such as Collagen, proteoglycans, and fibronectin.

And these structures are anchored to what these membrane proteins called integrins, these membrane proteins called integrins. So it kind of looks like this. Okay.

The collagen is a fiber. Remember collagen is a quaternary structure of three proteins that form alpha helices. Remember that we discussed that before.

So collagen is a quaternary structure of a protein. Here's collagen on the outside of the cell. This would be inside of the cell down here, the cytoplasm. Here's outside of the cell, the extracellular fluid out here.

Here's the plasma membrane, right? The plasma membrane is made up of a phospholipid bilayer. And these proteins, look at these membrane proteins that span the entire membrane.

So they're called transmembrane proteins because they span the entire membrane. These are your integrin anchors, the integrin. Here's an integrin protein. Here's an integrin protein.

And here's what's cool. The integrin connects to a adapter. This adapter is called fibronectin. Fibronectin connects to collagen.

See the collagen fibers. All right. And not only that, but look on the inside of the cell. The integrin also attaches to the microfilaments.

Remember the actin microfilaments on the inside of the cell. This is where those actin microfilaments are. Remember this is called the cortex of the cell.

Remember the little rods? inside of the tent. Okay.

So the integrin attaches to microfilaments on the inside of the wall, and it attaches to fibronectin and collagen on the outside of the cell. So it's a great anchor, isn't it? It's anchoring the cytoskeleton to the extracellular matrix.

And then look at this, you see all these, it looks like barbed wire moving throughout this thing, this green, green and purple barbed wire. This is your proteoglycan complex, which is made up of part sugar and part protein. So these are the components of the extracellular matrix.

And this is an important part of your cell. You know, again, animal cells, we don't have a cell wall, do we? Animals don't have cell walls. However, your cells do have this extracellular matrix, and it is important in supporting your cell's shape.

In fact, when you age, you know, when you age, you form more wrinkles and your skin becomes less elastic. Part of that is the breakdown of this extracellular matrix. Why do you think so many beauty creams have collagen in them, right?

Collagen is a component of the ECM and it helps to keep your skin youthful, right? And elastic and wrinkle-free. Isn't that interesting? Now, in multicellular creatures like you and me, We have tissues, right? Like our skin.

If you take a look at your skin, these cells, these skin cells are all attached to one another. And not only are they attached to one another, they're attached firmly and they're so tightly associated with one another. You are watertight. Even water can't squeeze through the, you know, the gaps between your skin.

So what's holding these cells together and how do these junctions work? So So let's go to the notes and let me show you something. These are known as cell junctions. Neighboring cells in tissues, organs, or organ systems often adhere, interact, or communicate through direct physical contact.

Let me show you some cell junctions in plants. These are known as plasmodesmata. Plasmodesmata are junctions in plants. They serve as channels that connect.

plant cells. Through the plasmodesmata, water and small solutes, and sometimes even proteins and RNA, can pass from one cell to another. Here you can see an electron micrograph of a plant junction, and you can see that these are plasmodesmata connecting one cell to another. And so solutes can pass, water can pass from cell to cell through these little openings.

from adjacent cells. Isn't that cool? But what about you and me? What about animals?

Well, there are three types of cell junctions that are common in animal epithelial tissues. Think of, you know, like your skin, for instance. There are tight junctions. Tight junctions are responsible for making you watertight, right? They're membranes of neighboring cells that are pressed together preventing leakage of extracellular fluid.

So these tight junctions fasten your cells together and make you water tight. Okay. Then there are desmosomes or anchoring junctions, which crudely fasten cells together into strong sheets or tissues. And then there are gap junctions or communicating junctions, which provide cytoplasmic channels between adjacent cells. So solutes can get from cell to cell.

So take a look here at this cartoon. And on the right, you have the actual electron images of these structures. Tight junctions up here. Look, tight junctions keep you watertight.

Tight junctions prevent fluid from moving across a layer of cells. Then you have the desmosomes, which fasten cells together into sheets. And then lastly, you have the gap junctions, which allow small molecules and ions to cross from cell to cell. Isn't that neat? So this is why us multicellular organisms have these sheets of watertight tissue.

And with that, we have... We have finished this chapter. What an interesting chapter, right?

We finally get to take a look at the cell, all the parts of the cell, or at least the important parts of the cell. We looked at cells that are prokaryotic and eukaryotic. We looked at structures inside of the cell, outside of the cell, between cells.

So we've gained an appreciation for the structures of a cell, and that's essentially what this chapter was about, okay? So let's move on to the next chapter and explore more details on these structures. Dr. D, Dr. D, Dr. D, Dr. D, Dr. D, Dr. D, Dr. D, Dr. D, Dr. D, Dr. D, Dr. D, Dr. D,