Flagellum, plural flagella, is a tiny, whiplike structure certain cells use to move. A cell can propel itself through liquid by moving its flagella.
A cell may have several flagella, only one flagellum, or none. The arrangement of flagella on the surface of the cell also varies. For example, some bacteria have several flagella in the same spot. Others have flagella on opposite ends of the cell. Still others have flagella spread around the cell surface.
Flagella are found among many prokaryotes—a group of organisms made up of archaea and bacteria. Flagella also are found in certain cells of eukaryotes—a group of organisms that includes animals, plants, and single-celled protozoans. For example, sperm move using flagella. However, while the flagella appear similar, they differ in structure. The flagella of eukaryotes are made of structures called microtubules. The flagella of prokaryotes lack microtubules. Eukaryotic flagella move by a flicking motion, but prokaryotic flagella rotate. The flagella of archaea and bacteria also differ. For instance, the flagella of both groups are powered by a molecular “motor” within the cell. But the motors in archaeal flagella work differently than those in bacterial flagella.
Scientists have shown that flagella evolved (developed over many generations) independently in archaea, bacteria, and eukaryotes. Research suggests they evolved in a series of steps from simpler structures.
See also Bacteria (How bacteria move); Protozoan (Flagellates).
