A flagellum is a lash-like appendage that protrudes from the cell body of certain prokaryotic and eukaryotic cells. The word flagellum in Latin means whip. The primary role of the flagellum is locomotion, but it also often has function as a sensory organelle, being sensitive to chemicals and temperatures outside the cell. Flagella are organelles defined by function rather than structure. Large differences occur between different types of flagella; the prokaryotic and eukaryotic flagella differ greatly in protein composition, structure, and mechanism of propulsion. However, both can be used for swimming.
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.