A flagellum is a slender, whip-like appendage that extends from the surface of certain cells, enabling them to move. Its name comes from the Latin word for “whip.” Flagella are found across the three domains of life—Bacteria, Archaea, and Eukaryota—though their specific structures, protein compositions, and propulsion mechanisms differ significantly. Despite these differences, their fundamental role remains consistent: to propel the cell through liquid or other media.
Internal Structure and Movement
A flagellum’s general architecture includes three main parts: a filament, a hook, and a basal body. The filament is the long, external part that interacts with the surrounding medium to generate propulsion. The hook acts as a flexible joint, connecting the filament to the basal body. The basal body serves as the anchor and motor, embedding the flagellum within the cell membrane and wall.
Flagellar function involves converting energy into mechanical motion to propel the cell. This can occur through either a rotary mechanism, where the filament spins like a propeller, or a whip-like, undulating motion. The basal body contains the molecular machinery that drives this movement, transferring force to the filament via the hook. This coordinated action allows cells to move effectively.
Bacterial Flagella Characteristics
Bacterial flagella are helical filaments composed primarily of a protein called flagellin. This protein forms a hollow tube, and new flagellin subunits are added at the growing tip of the filament. At the base of the flagellum, a complex rotary motor embedded in the cell envelope drives its rotation. This motor operates using the proton motive force, an electrochemical gradient of protons across the cell membrane. Protons flow through specific channels in the motor, causing it to spin at speeds that can reach several hundred revolutions per second.
The arrangement of flagella on a bacterial cell varies among species and is categorized into distinct patterns. These flagellar arrangements are crucial for bacterial motility, enabling them to move toward favorable conditions through a process known as chemotaxis.
- Monotrichous bacteria, such as Vibrio cholerae, possess a single flagellum at one end.
- Amphitrichous bacteria have a single flagellum at each of their two opposite ends.
- Lophotrichous bacteria feature a tuft of multiple flagella at one pole, as seen in Helicobacter pylori.
- Peritrichous bacteria, like E. coli, have flagella distributed across their entire surface.
Eukaryotic and Archaeal Variations
Eukaryotic flagella differ significantly from bacterial flagella in their internal structure and mechanism of movement. These flagella are complex cellular projections found in various eukaryotic cells, including sperm cells and some protozoa. Their core structure, known as the axoneme, consists of a specific “9+2” arrangement of microtubules: nine fused pairs of microtubules surrounding two central single microtubules. This microtubule arrangement is encased within the cell’s plasma membrane.
Movement in eukaryotic flagella is generated by the coordinated sliding of these microtubules, powered by the hydrolysis of adenosine triphosphate (ATP) by motor proteins called dyneins. This ATP-driven mechanism results in a characteristic whip-like or undulating motion, distinct from the rotary movement of bacterial flagella. The basal body, structurally similar to a centriole, anchors the eukaryotic flagellum to the cell and serves as the organizing center for its microtubules.
Archaeal flagella, or archaella, are superficially similar to bacterial flagella, employing a rotary motor for propulsion. However, they are evolutionarily distinct and differ significantly in protein composition and assembly. Unlike bacterial flagella made of flagellin, archaella are composed of unique proteins similar to bacterial pilins. They are also thinner than bacterial flagella and grow by adding subunits at their base, rather than the tip. Their rotary motion is powered by ATP hydrolysis, distinguishing them from proton motive force-driven bacterial flagella.