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Flagella: Definition, Structure & Functions

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  • 0:01 What is a Flagellum?
  • 1:09 Structure & Types
  • 4:00 Functions Other Than…
  • 4:27 Lesson Summary
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Lesson Transcript
Instructor: Nicholas Gauthier
One of the more dramatic ways that single-celled organisms get around is a whip-like structure called a flagellum. Learn the ways that different organisms make use of their flagella to move around their microscopic world.

What is a Flagellum?

A flagellum is a whip-like structure that allows a cell to move. They are found in all three domains of the living world: bacteria, archaea, and eukaryota, also known as protists, plants, animals, and fungi. While all three types of flagella are used for locomotion, they are structurally very different.

Other cells have different means of locomotion. Cilia are similar to flagella in structure and function, but a cilium is shorter and moves differently. A ciliated cell usually has hundreds or thousands of cilia, which move in unison like little oars. Sometimes a cell will also use cilia to funnel food into an oral groove. Various species of paramecium employ cilia for both purposes.

Some cells get around by pushing cytoplasm into an extension of the cell membrane, forming a structure called a pseudopod. This is true of Amoebas and white blood cells in our bloodstream.

Structures and Types of Flagella

Far from being a simple hair-like structure, the eukaryotic flagellum has a complex cross-section. It is similar to a cilium in structure, though cilia generally move in a back and forth motion, as opposed to the corkscrew movement of a flagellum.

The eukaryotic flagellum is a long, rod-like structure that is surrounded by an extension of the cell membrane like a sheath. The bulk of the structure is a filament called an axoneme. Necessary materials are transported along the flagellum. The whole structure is anchored in a basal body, which is similar to a centriole in structure.

The axoneme has nine pairs of microtubules supporting it from within. These microtubule doublets surround two single microtubules. This arrangement is called the 9 + 2 structure. Eukaryotic cilia also have this structure; the cilia are simply shorter.

The nine-microtubule doublets have dynein arms that are powered by ATP. The arms cause the microtubules in each pair to slide against one another. This causes the flagellum to bend, allowing the cell to move. Radial spokes extend toward the central microtubules. Their role is not known, but they may play a role in stabilizing the flagellum.

The flagella of domains bacteria and archaea are different. They still move the cell, but they do so by rotating, rather than by bending from the inside like eukaryotic flagella.

The flagella of archaeans are superficially similar to those of bacteria. However, there are key differences. To begin with, the two types of flagella move differently. Bacterial flagella are powered by the flow of ions, usually hydrogen, but sometimes sodium. They are composed of individual fibers that rotate individually. By contrast, achaeal flagella appear to be powered by ATP, the same chemical that powers eukaryotic flagella. Archaeal fibers are also bundled and rotate as one.

Their growth and inner structure differ as well. A bacterial flagellum has a relatively hollow interior that facilitates growth at the flagellum's tip. New subunits move up through the hollow interior and attach at the end. Archaeal flagella have no such large hollow inner tube, and grow by adding subunits at the base. There are various other chemical differences as well.

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