Flagella



Discovery


Flagella were first discovered by Anton van Leeuwenhoek in 1675. Leeuwenhoek used a compound light microscope, as pictured on the right. As for its name, the word flagellum (the singular form of flagella) in Latin means whip--in photos and diagrams of the flagellum like the one pictured to the right, it is apparent that the organelle resembles a little whip.

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An early European compound light microscope; similar to what Leeuwenhoek used to discover flagella

Function


The flagellum is an organelle that allows both eukaryotic and prokaryotic cells to move--it is a tool of transportation for the cell. It primarily propels cells through liquid--more often prokaryotic than eukaryotic, though (for example, Vibrio cholerae, a strain of bacteria causing cholera, usually found in water). Not only do eukaryotes use flagella to move through liquid (for example, the euglena pictured on the right), some eukaryotic cells use flagella to increase reproduction rates, while other eukaryotic (as well as bacterial) flagella are used to sense changes in the environment, such as temperature or pH disturbances.

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Vibrio cholerae’s flagellum
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A euglena’s flagellum
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An example of a bacterial cell’s flagellum ability to sense pH changes, as shown in the flagellum of a Salmonella cell.

Structure


The structure of the flagella is more complex than the organelle appears. In both eukaryotic and prokaryotic cells, the core of a flagellum is a bundle of nine pairs of microtubules; the organelle is anchored to the rest of the cell by a basal body and is enclosed in the cell envelope. When these microtubules slide in a coordinated manner, the movement of the flagellum is produced. In both types of organisms, the flagellum is made up of a special kind of protein, which is a unique aspect of the flagellum’s molecular makeup.
Although eukaryotes and prokaryotes have these structural elements in common, there are a few differences that make both types of flagella different from one another.
In eukaryotes:
* Each microtubule is made of the protein tubulin
  • Can have multiple flagella
  • Whip back and forth to produce movement
In prokaryotes:
  • Consist of a protein called flagellin
  • Flagella are helix-shaped and rotate quickly like a windmill
The flagellum’s function is related to its structure in that its main function is to allow the cell to move from one place to another in liquid. Its structure (a bundle nine pairs of microtubules at its core) produces that movement--when the microtubules slide together, the flagellum either whips back and forth or rotates like a windmill, propelling the cell through liquid.

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Prokaryotic (left) vs. Eukaryotic Flagella

What Types of Cells Have a Flagellum?


Not every cell has a flagellum--in fact, the organelle is not too common in many eukaryotes. There are some instances, though, of eukaryotic flagella. Animals’ sperm cells have flagella for the same reason bacteria and other prokaryotes do--they are needed to help propel the cell through liquid in order to reach the female’s egg. Prokaryotes (bacteria and archaebacteria) have flagella as their main mode of transportation (along with cilia). If the bacterium does not rely on air currents to blow it from place to place in order to move, it has a flagellum to propel it through liquid.

Cells that need to travel long distances (both eukaryotic and prokaryotic) have flagella. A normal red blood cell, for example, would not need a flagellum, as they can travel fast enough without the help of a transportation organelle.

Some bacteria and protists have more than one flagellum for the sole purpose of increased mobility. This allows them to move faster and more efficiently through their liquid environment.

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A sperm cell and its flagellum
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Red blood cells do not need flagella, as they can travel fast enough on their own.
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Bacteria with multiple flagella (at the bottom of each cell)

Organelles Flagella Work in Conjunction With


The flagellum is a part of the cytoskeleton--the cytoskeleton controls the movement of the cell through its environment. Because of this, the flagellum works in conjunction with the other parts of the cytoskeleton that help the cell move: the undulipodia and the cilia. Although all three of these structures may not be present in a cell (for example, how eukaryotes do not often have flagella), the ones that are present work together to create movement for the cell. They each produce movement in their own ways in order to help the cell move from one place to another in its environment--for example, if cilia and flagella are present in a cell, each tiny hair-like cilia move slightly, and together, create motion that sweeps the cell across its environment, while the flagella will either whip back and forth (in eukaryotes) or rotate quickly like a windmill (in prokaryotes) propelling the cell. Both of these organelles producing motion will move the cell through its environment. The more transportation organelles present in a cell, the more efficiently it will be able to move.

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Cytoskeleton in cells
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Undulipodia: another organelle providing transportation for the cell
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Cilia: the third organelle providing transportation for the cell

Analogy


Flagellum: Cell
Automobile: Person
The flagellum is the cell’s automobile--it is a mode of transportation that the cell controls. The cell chooses where the flagellum takes it.



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Sources

__http://jcb.rupress.org/content/jcb/91/3/125s.full.pdf__

__https://www.britannica.com/science/flagellum__

__https://biologydictionary.net/flagellum/__
__https://alevelnotes.com/Organelles-Working-Together/115__