Introduction to cilia, flagella and pseudopodia | Cells | High school biology | Khan Academy

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 carolinensis, and what you see here is a projection coming off from the main part of the cell. 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.

What I really want you to appreciate is that this is used by amoeba either to move around or could be even used to attack something that it wants to engulf. Think about what it might take to be able to do this. To be able to grow this type of a pseudopod, this type of a false foot, you need all sorts of microstructures in here that will extend or contract as necessary. Think about the machinery that you need to do that.

So, the key realization is that sometimes we just imagine cells as these bags of fluid with a few things floating around, but there are these incredibly complex structures. 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 will often see on 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 hair-like structures. Remember, this is a unicellular organism. It's actually a fairly decent-sized one; 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. 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 example, 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. Within the nucleus, its DNA can be extremely fragmented. Most organisms have a reasonable number of chromosomes. Human beings have 23 pairs—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 that 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. 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) is about one-fourth of a micrometer. Another way of thinking about it is you could put four thousand 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 and 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 just bags of a few things floating around; they're incredibly complex structures that we are still trying to understand.