Flagellum

Flagellum is a whip-like structure that allows a cell to move. They are found in all three domains of the cellular world: Bacteria, Archaea, and Eukarya.

Structure

In bacteria, the flagellum is a complex rotating structure driven by a molecular motor at the base. In eukaryotes, it is a complex bending structure driven by molecular motors that slide microtubules.

Function

Flagella are primarily used for cell movement. They are not fast enough to allow bacteria and sperm to swim against a current, but they can allow a cell to move in a particular direction in a liquid medium.

Types

There are three types of flagella: bacterial, archaeal, and eukaryotic.

Bacterial

The bacterial flagellum is made up of the protein flagellin. Its shape is a 20 nanometer-thick hollow tube. It is helical and has a sharp bend just outside the outer membrane; this "hook" allows the axis of the helix to point directly away from the cell.

Archaeal

Archaeal flagella are a different structure which has a similar function and overall appearance to bacterial flagella, but are not homologous.

Eukaryotic

Eukaryotic flagella are complex cellular projections that lash back and forth. Unlike bacterial flagella, they are composed of a bundle of nine fused pairs of microtubule doublets surrounding two central single microtubules.

See Also

• Cilia
• Cell biology
• Microbiology

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