If you're seeing this message, it means we're having trouble loading external resources on our website.

If you're behind a web filter, please make sure that the domains *.kastatic.org and *.kasandbox.org are unblocked.

Main content

Introduction to cilia, flagella and pseudopodia

Amoebas use pseudopods, or "false feet", for movement and capturing prey, demonstrating the complexity of cellular structures. Unicellular organisms like Oxytricha trifallax use cilia, or hair-like structures, for movement and have unique mating processes. Chlamydomonas, a green algae, uses a flagellum, or tail-like structure, for movement. These examples highlight the intricate machinery of cells. Created by Sal Khan.

Want to join the conversation?

  • male robot hal style avatar for user Noah
    Is amoeba a multicellular or unicellular ?
    (8 votes)
    Default Khan Academy avatar avatar for user
    • duskpin ultimate style avatar for user Zelong Huang
      Amoeba are unicellular because they only have one cell.
      Humans, on the other hand, are multicellular because we have approximately 37 trillion cells!
      The living things we could see are multicellular as we cannot see cells with our eyes. Unicellular organisms can be seen with microscopes.
      (14 votes)
  • male robot hal style avatar for user Noah
    My teacher says that amoeba eats bacteria by the help of its expansion of pseudopod Here Sal says that it is pseudopod but my ma'am says and pronounces it as pseudopodiam , is that also right ?
    (10 votes)
    Default Khan Academy avatar avatar for user
  • mr pants green style avatar for user High Ground
    D-did he just say cells changing each-others DNA is... romantic?
    (13 votes)
    Default Khan Academy avatar avatar for user
  • aqualine ultimate style avatar for user ValentineRose529
    At -, Sal says that the Oxytricha trifilax will "mate" by merging with another one when it's stressed. But then, wouldn't that mean that the amount of Oxytricha trifilax is slowly decreasing? Even if there are tens of millions, if it creates a new one from two old ones, then one day wouldn't there be only one left as they all mate with each other until there's one left over?
    (8 votes)
    Default Khan Academy avatar avatar for user
  • blobby green style avatar for user Parsa Payandeh
    What is the actual difference between cytoplasm and cytosol?They both seem to be the same thing.But Sal said they are not.I have gotten so confused!
    (2 votes)
    Default Khan Academy avatar avatar for user
    • male robot donald style avatar for user Tybalt
      From my understanding...

      Cytoplasm--The material within the cell (aside from the nucleus), refers to both the gel substance and the organelles suspended within it
      Cytosol--The gel component of cytoplasm; does NOT refer to organelles that are suspended within

      You can also think about it this way: Imagine a cell being represented as a block of Jell-o with chunks of fruit inside of it. The cytosol would be comparable to a piece of just the Jell-o, while the cytoplasm would be a piece of Jell-o with some fruits (organelles) inside of it.

      Does this help?
      (17 votes)
  • blobby green style avatar for user Neelshan123
    You say it is unicellular and then that it is eukaryotic. However eukaryotes are multicellular, therefore that would make this a prokaryotic cell, right?
    (7 votes)
    Default Khan Academy avatar avatar for user
  • blobby green style avatar for user Veeabriol1892
    We all know that Flagella and cilla is an organelle but why does flagella is not an organelle when it comes to a prokaryotic bacteria?
    (7 votes)
    Default Khan Academy avatar avatar for user
  • purple pi pink style avatar for user Xallarxo
    If an organism has many cells of only one type, i.e. with no cellular differentiation, is it considered multicellular or unicellular?
    (2 votes)
    Default Khan Academy avatar avatar for user
    • piceratops ultimate style avatar for user FrozenPhoenix45
      It would still be considered multicellular, since the prefix "multi" means many; the type of cell doesn't affect the number of them.

      However, bear in mind that such an organism, at least to my knowledge, does not, and cannot exist. All multicellular organisms have cell differentiation so each type of cell can keep them alive in different ways. So I do not believe it is possible for a multicellular organism to exist with no cell differentiation. However, if you do further research and discover otherwise, then I would be happy to be proven wrong.

      I hope this helped! Comment if you have answer questions; I'll answer to the best of my ability.
      (6 votes)
  • blobby green style avatar for user Lizairy Negron Bachiller
    why are bactiria only unicellular and what would happen if they were multicellular
    (3 votes)
    Default Khan Academy avatar avatar for user
    • leaf red style avatar for user B L Ø Ø ᑐ | Я ᕮ ᑐ | Γ Λ L L
      Bacteria are primarily unicellular organisms because their small size allows for efficient nutrient uptake and waste exchange. Being unicellular also enables bacteria to adapt to a wide range of environments and reproduce rapidly.
      If bacteria were multicellular, several changes would occur. Firstly, the increased size of the organisms would likely limit their ability to access nutrients and exchange waste efficiently. This could hinder their survival in various environments. Additionally, multicellularity would require more complex communication and coordination among the cells, which bacteria currently do not possess.
      Furthermore, being multicellular could reduce the adaptability and reproductive capabilities of bacteria. Their rapid reproduction and genetic diversity are essential factors in their ability to evolve and survive in changing environments. Multicellularity would likely slow down these processes and limit their overall success.
      It is important to note that some bacteria do form colonies and exhibit communal behaviors, but they are still considered unicellular organisms. These colonies are not truly multicellular since the individual cells can function independently if separated.
      (3 votes)
  • winston baby style avatar for user Mason Smith
    At Sal says that there is a whole study on how the flagellum move, what is that study called?
    (4 votes)
    Default Khan Academy avatar avatar for user

Video transcript

- [Instructor] The goal of this video is to appreciate some of the structures that you see even in unicellular organisms. So this right over here is a picture of the amoeba Chaos carolinense. And what you see here is a projection coming off from the main part of the cell, and this is called a pseudopod, which is referring to it being a false foot. The pod is coming from the same root word as podiatry, which is referring to the foot. And what I really want you to appreciate, this is used by amoeba either to move around or it could be even used to attack something that it wants to engulf. And think about what it might take to be able to do this, to be able to grow this type of a pseudo foot, this type of a false foot. You need all sorts of microstructures in here that will extend or contract as necessary. And think about the machinery that you need to do that. And so the key realization is, sometimes we just imagine cells as these bags of fluid with a few things floating around. But they're these incredibly complex structures, and biologists even today don't fully understand how everything works and they're studying how these things actually come to be. Now another structure that you'll often see in unicellular organisms that either help them move around or even help move other things around are cilia. So this right over here is a picture of Oxytricha trifallax, which is a unicellular organism. It's a eukaryote. And you can clearly see these projections from its body here, these hairlike structures. Remember this is a unicellular organism. If we were to, it's actually a fairly, it's a decent sized one. That would be about, something like that would be about 30 micrometers right over there or 30 millionths of a meter or 30 thousandths of a millimeter. So small by our scale, but it's actually pretty big on the scale of it being a cell. And once again, these cilia tend to move in unison to either allow the microorganism to move around or sometimes they're used to move other things around. For examples the cells that line your lungs will have cilia that are used to move things up or down, to move some of the saliva or any particles that are in there. Now Oxytricha trifallax is particularly interesting as a eukaryote because it doesn't just have one nucleus. It can have two nuclei. And within the nucleus, it's DNA can be extremely fragmented. Most organisms have a reasonable number of chromosomes. Human beings have 23 pair. That's actually a fairly large number. Oxytricha trifallax could have thousands of chromosomes. And what's really interesting about Oxytricha trifallax is how it mates. When it is under stress, it will merge with another Oxytricha trifallax, and instead of producing another offspring, they mingle their DNA together. So by mating, they change each other's genetic makeup, which is fascinating. And depending on your perspective, highly romantic. Now another related idea is instead of having many cilia, some unicellular organisms will just have one large thing that looks like a tail that they can whip around to move. So this right over here is a commonly studied green algae. It's called Chlamydomonas, and you can see very clearly here this flagellum, this tail-like structure. And this is extremely thin. We're seeing it under a very powerful microscope right over here, but just to get a sense of scale. A micrometer here would be about that. So the width of this flagellum, flagellum would be the singular. If we were talking about many of these, we would say flagella. This is about 1/4 of a micrometer. Another way of thinking about it, you could put 4,000 of these side by side, and you would have the width of a millimeter. So extremely extremely small, but once again, it really is amazing that these what seem like simple organisms to us are actually quite complex. There's a whole study of how these flagella move around, how the cell can spin it around so it allows it to move. If you were to actually decompose what's going on in this part of the cell, it's actually quite complex. It's biological machinery going on. So once again, these cells are not these just bags of just a few things floating around. They're incredibly complex structures that we are still trying to understand.