Flagella are whip-like appendages extending from cell surfaces. They enable cells to move through liquid environments. The term “flagellum” originates from the Latin word for “whip,” describing its motion.
Visual Characteristics and Structure
Flagella are microscopic structures, requiring advanced imaging techniques like electron microscopy for detailed visualization. Bacterial flagella are thin, helical filaments, approximately 20 nanometers thick, composed of thousands of protein subunits called flagellin. These filaments connect to a “hook,” which attaches to a “basal body” embedded within the cell membrane.
The basal body in bacterial flagella is a complex assembly of rings and a rod, serving as an anchor and containing a rotary motor. Gram-negative bacteria have four such rings: the L-ring, P-ring, M-ring, and S-ring, each associating with different cell envelope layers, while Gram-positive bacteria possess two inner rings.
The arrangement of flagella on a bacterial cell can vary:
- Monotrichous: a single flagellum
- Lophotrichous: a tuft at one pole
- Amphitrichous: one at each pole
- Peritrichous: flagella distributed across the entire cell surface
Eukaryotic flagella, in contrast, feature a more complex internal structure called an axoneme. This consists of nine pairs of microtubules surrounding two central singlet microtubules, often referred to as a “9+2” arrangement.
How Flagella Enable Movement
The primary function of flagella is to enable cell motility, allowing organisms to navigate their environments. Bacterial flagella operate like miniature propellers, rotating at speeds between 200 and 2000 revolutions per minute, generating thrust to propel the cell forward. This rotation is powered by a rotary motor within the basal body, which utilizes the flow of ions, primarily protons, across the cell membrane. Counterclockwise rotation results in smooth, forward motion, while a clockwise rotation can cause the bacterium to tumble and reorient.
Eukaryotic flagella exhibit a wave-like or whip-like motion. This bending movement is generated by the coordinated sliding of microtubules within the axoneme, driven by motor proteins called dyneins. This wave-like action enables cells to move through various liquid environments, from propelling single cells like sperm to creating fluid currents across cell surfaces. The ability to move allows cells to respond to environmental cues, such as moving towards nutrients or away from harmful substances, a process known as chemotaxis.
Where Flagella Are Found
Flagella are diverse structures found across all three domains of life: Bacteria, Archaea, and Eukaryota. In bacteria, common examples include Vibrio cholerae, which has a single flagellum, and Helicobacter pylori, which uses multiple flagella to move through the stomach lining. Archaeal flagella, while functionally similar to bacterial flagella, differ in their protein composition and assembly mechanisms.
In eukaryotes, flagella are present on various cell types, including the sperm cells of animals and certain plants like ferns and mosses. Many single-celled protists, such as Euglena and Chlamydomonas, also possess flagella for locomotion.