Cilia and Flagella: Study with Video Lessons, Practice Problems & Examples

Cilia and flagella are organelles crucial for cell movement. Cilia move in coordinated back-and-forth motions, while flagella spin like a propeller. Both structures consist of microtubules arranged in a 9+2 pattern, with dynein motor proteins facilitating movement through the sliding microtubule model. Dynein binds to microtubules, using ATP to create bending motions that propel the cell. Additionally, intraflagellar transport moves molecules within flagella, ensuring proper function and maintenance.

Structure

Hi. In this video, I'm going to be talking about cilia and flagella. So you're probably familiar with these terms from your intro class, but just to review, cilia and flagella are organelles, and they are the organelles that are really responsible for moving the entire cell. So, let's start with cilia. Cilia are actually found in groups, so multiple cilia are found together, and they exist on the plasma membrane, and they beat back and forth in one direction. And so when they do this, this can actually move the entire cell through the fluid, but it also has the ability of moving fluid. So for instance, the liquid in the extracellular environment. If all these cilia are moving back and forth in one direction, then all that liquid that's there is also going to be propelled that way. Exactly like if you were, you know, swimming or moving water in one direction, it's just going to move that way, the same thing with the cell and cilia. And then you have flagella, and these are typically found as in a single I mean, you think of sperm. Sperm is propelled through flagella. And so a single flagellum, it's that flagella is plural. And these are also found on the top of my membrane. But unlike cilia, which beat back and forth, flagella actually spin kind of like a rope. Like, if you are just to spin a rope around, it would spin like that. And this is called, like, a wave-like beating pattern. And essentially this wave-like beating pattern will eventually propel the cells, so for instance sperm or protozoa cells throughout the environment that it's in. So if you were to look at what this looks like, you have flagellum here. This is going to spin like a propeller or like a rope, sort of going around and around in circles. And then you have cilia which beat back and forth. You can see it going that way and this way. And both of these are responsible for the entire cell movement. And despite the fact that they sort of spin differently, their structure is really similar. So the structure is dependent on the arrangement of microtubules. So microtubules are arranged in what is known as a 9 + 2 isonymy. And so what this is, is this means that there are outer doublets, means that there are 9 doublets of microtubules, spaced equally around the outside and they actually look like this. So all of these are, you know, sort of or to draw this 3d, which I'm not very good at it, it would look like this. And these lines would be the microtubules and, each one of these things would be a doublet. So each one of these has a ton of microtubules in it. So they're spaced around the outside and then you have a central pair which is a doublet placed in the center. And so the reason it's given this funny a word, is just because this a word describes all the microtubules in addition to all the proteins that are associated with it. I haven't talked about any proteins that are associated, but of course there's going to be proteins. It's not just these microtubules existing in a vacuum. And so like I said, we talked about the microtubule doublets. I'll show you a better image of that in a second, but each one of these doublets is distinct. So you have this doublet and you have this doublet. So the a2, which is this one, has 13. You can tell it's this one because the circle is complete. Whereas, this one is the b because the circle is not complete. And this is because in the a tubule, what you have is you have 13, so of course it's going to be bigger, it's going to be the complete circle. And in the b, you only have 10 or 11 that fuses to the a, so it's going to be smaller because, it doesn't have 13 and it's fusing to that a doublet. And then, around the microtubule you have a you have a ball of these outer doublets, and each one of them is going to be connected through a variety of structures. So there's this thing called inter doublet links. These are going to connect adjacent doublets together. So for instance those 2, if they were next to each other and there's a particular protein called nexin and nexin is really important for this connection. So if we were to look at what this looks like, so here we have, what looks like a flagellum, but it could be a cilia. It's hard to tell, because they have this very similar structure. And so what you get is you get these outer doublets. There's 1, 2, 3, 4, 5, 6, 7, 8, 9 outer doublets and the one central pair. And you can see that each one of these is made up of a or b. A is going to be the complete one, so it's going to be the full circle here. A's here. And then if you were to look at b, what you'd see is b is going to be not the complete one. It's going to be the one that looks like it's attaching on to a. And, of course, these are all connected through different proteins. The important one is called nexin, and you can if you follow that line, you can see nexin connecting these adjacent doublets together, so it's super important. Now one thing that I haven't talked about is this structure down here. So I do want to briefly mention that. And so this structure is called a basal body, and this is the region where the base of this, these microtubules are going to grow up here and form those outer doublets and central pairs. But, the basal body itself it doesn't have that structure, it has a different structure, and that structure is it looks like this. So you can see you have these 9 triplets. You have this 1, 2, 3, and there are 9 of these triplets that go around and that's what makes up the basal body, which then goes on to make up the entire cilia or flagella. And if you were to zoom in, it's going to look like that with the 9 doublets on the outside and the central pair inside. So that's going to be the structure of cilia and flagella, so with that let's now move on.

Sliding Microtubule Model

Okay. So now we're going to talk about cilia and flagella movement. The model that we need to talk about when we discuss their movement is called the sliding microtubule model, and this describes how they move. An important protein you need to know for movement is dynein. Remember, dynein is going to be a motor protein. What do motor proteins do? They attach onto microtubules and kind of walk their way around. This walking is what allows the movement of cilia and flagella. Remember, cilia and flagella are those organelles that move; they go back and forth and they spin. This movement has to be initiated by something, and this model describes it and this protein enables it.

So, how does this work? Dynein binds to these β tubules, so the B-microtubules, with the head, and then it starts moving towards the minus end. Using ATP, it’s a motor protein that utilizes ATP to move. It binds to the β-microtubules of the head, it binds to the other microtubules with its feet, and then it starts moving towards the minus end. When it moves, it ends up sliding the other tubule down and it looks like it's bending. When it bends, that is what creates the movement of the cilia or the flagella.

In summary, this model here is pretty much just about the movement of bending. The sliding microtubule model takes that dynein and it walks along, and while it's walking along, it ends up bending the cilia or the flagella, allowing it to move. There's another type of movement that we don't really talk about, and that's what's moving inside the flagella. The intraflagellar transport is if you have a molecule at the bottom of the flagella and you need it to get to the top, then intraflagellar transport is going to get it from top to bottom or bottom to top. It is going to transport it through the flagella, from one region to another. But the sliding microtubule model is talking about the whole organelle moving.

So let's look at this. We have our dynein here, and you can see it's attached to these microtubules. There are these barrier proteins that sort of keep the microtubules attached together in the same region. As they begin walking along the microtubules, these head groups stay in place. Due to the pressure of one region being attached and moving one way and another region being attached and moving the other way, it's going to slide the microtubule, hence called the sliding microtubule region for our model, and that sliding normally would just send it past each other, but because we have these barriers here, they actually end up getting stuck, and they can't slide. So, the only thing they do is they bend. Because they can't slide, they get stuck and bend, and that's exactly what happens in the sliding microtubule model. This is going to cause the entire cilia to bend throughout the entire cilia or flagella and causes it to move. That is the sliding microtubule model. So, with that, let's move on.

Microtubules are arranged in which of the following arrays?

• A. 9 + 3
• B. 9 + 2
• C. 8 + 3
• D. 8 + 2

What is the name of the structure from which cilia and flagellum grow?

• A. Outer doublet
• B. Nexin
• C. Basal Body
• D. Central Pair

Here’s what students ask on this topic:

What are the main differences between cilia and flagella?

Cilia and flagella are both organelles responsible for cell movement, but they differ in structure and function. Cilia are short, hair-like structures found in groups on the cell surface, beating back and forth in a coordinated manner to move the cell or fluid around it. Flagella, on the other hand, are longer and usually found singly, moving in a propeller-like motion to propel the cell forward. Despite these differences, both cilia and flagella share a similar internal structure, consisting of microtubules arranged in a 9+2 pattern, and rely on dynein motor proteins for movement.

How do dynein motor proteins facilitate the movement of cilia and flagella?

Dynein motor proteins are crucial for the movement of cilia and flagella. They attach to the microtubules within these organelles and use ATP to 'walk' along them. This walking action causes the microtubules to slide past each other. However, due to the presence of linking proteins like nexin, the microtubules cannot slide freely and instead bend. This bending motion is what propels the cilia and flagella, enabling the cell to move. This mechanism is known as the sliding microtubule model.

What is the 9+2 arrangement in cilia and flagella?

The 9+2 arrangement refers to the specific structure of microtubules within cilia and flagella. It consists of nine doublet microtubules arranged in a circle around two central single microtubules. Each doublet is made up of one complete microtubule (A-tubule) and one partial microtubule (B-tubule). This arrangement is crucial for the structural integrity and function of cilia and flagella, allowing them to bend and move effectively.

What is intraflagellar transport and why is it important?

Intraflagellar transport (IFT) is the process by which molecules are moved along the length of the flagella. This transport is essential for the assembly, maintenance, and function of flagella. IFT involves the movement of protein complexes along microtubules, facilitated by motor proteins like kinesin and dynein. This process ensures that necessary components are delivered to the correct locations within the flagella, allowing for proper cellular function and signaling.

What role do microtubules play in the structure and function of cilia and flagella?

Microtubules are the structural backbone of cilia and flagella, arranged in a 9+2 pattern. They provide the necessary rigidity and flexibility for these organelles to move. Microtubules also serve as tracks for motor proteins like dynein, which facilitate the sliding and bending motions required for movement. Additionally, microtubules are involved in intraflagellar transport, ensuring that essential molecules are delivered to the correct locations within the cilia and flagella.