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.
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