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Cilia and Flagella

Teacher/Instructor Patrick Roisen
Patrick Roisen

M.Ed., Stanford University
Winner of multiple teaching awards

Patrick has been teaching AP Biology for 14 years and is the winner of multiple teaching awards.

Cilia and flagella are tube-like appendages which allow for motion in eukaryotic cells. If a cell has a single appendage, which often looks tail-like, it is called a flagellum, but if it has many, they are called cilia. The motion of the cilia or flagellum moves the liquid outside the cell and if the cell is not anchored, it can "swim." A commonly recognized example is the "tail" on a sperm cell, which is actually a flagellum.

One of the more easily recognized cell structures in any cell is cilia or flagella. Now when you look at them it's very easy to get them kind of confused when you're first studying but the basics differences pretty easy to spot cause essentially, with cilia you'll have lots of them but individual they're small compared to the flagella where you'll have many sorry you'll have long ones but not too many of them.

If we take a quick look over here. Here is an actual picture of two cells and you can see there's lots of these short little projections that are sticking out of the cell whereas here with this cell has a few flagella but those flagella are much longer than the individual ones that are called cilia.

Now in general their function is the same. Both of them are used for moving the cell or moving things around the cell. Now with flagella, with the prokaryotes a there's weirdness about them in that their flagella rotate whereas ours undulate.

If we take a look at this picture here we see an ecoli bacteria and you can see the rotating shapes here of the flagella they basically operate kind of like an [upward] motor, whereas our flagella instead of operating like that ours undulates to create what's called a sinusoidal motion. Now you may be wondering, how's that possible? How's that achieved? Well if we take a look at the basic structure of any cilia or flagella you'll see that they are essentially this column of microtubules that are covered in membrane and if you look at the shaft of the flagella you'll see you have these pairs of microtubules there's a set of 9 of them. In the center you'll have a 2 individual microtubules this is sometimes referred to as the “9+2” arrangement. Down at the bottom you'll again have your 9 set but instead of having pairs you'll have groups of 3 microtubules so that's called 9 again but because there's no microtubules on the center this is called the “9+0” arrangement.

And the basic way this work is that each pair of microtubules takes a turn going up a little bit and so you'll get this up up up up up up rotating and if you've ever seen those weird little wobbly guys who are set up to advertise and attract attention at a car dealership, that's kind of what a flagella will look like if you could see it at the size of scale. Cilium don't flap around nearly as much because they're somewhat shorter but they'll still tend to do that wave like motion. And that's how cilia and flagella work.