Flagellum

A Flagellum is a long, tail-like organelle in a cell that allows a cell to move around. It is made up of microtubules and surrounded by a plasma membrane.

In single-cell organisms flagella move like a whip to provide propulsion to the cell. As part of a larger organism, their function is often to move fluids along mucous membranes. When they perform this function, they are called cilia.

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

The internal structure of eukaryotic flagella is made up of nine microtubule doublets linked by proteins, which form a cylinder around a central pair of microtubules. There are usually only one or two flagella on a eukaryotic cell, and they move in a lashing motion like a whip. One example of this is the [sperm] cell.

Bacterial flagella have threads in a helix pattern, that rotate rather like a screw to move the organism.

Archaeal flagella appear similar to bacterial flagella, but are different in many details and considered by biologists to be an example of convergent evolution.