A flagellum is a slender, whip-like appendage that extends from the surface of many different types of cells, found across all three domains of life. This structure’s primary purpose is to generate movement, acting as a cellular propeller to navigate the surrounding fluid environment. The ability to move, or motility, allows single-celled organisms to search for resources and escape danger. In multicellular organisms, it permits the movement of specialized cells. The flagellum is a complex machine whose mechanics differ significantly depending on the organism in which it is found.
The Basic Anatomy of a Flagellum
Despite the vast differences in their internal components, flagella in both simple and complex cells share a common organizational plan consisting of three general regions. The most recognizable part is the filament, the long, helical structure that extends outward from the cell surface into the external environment. This filament is responsible for physically pushing or pulling the cell through its medium.
The filament connects to a slightly wider, curved segment known as the hook or connector region. This hook acts as a universal joint, transmitting the motion generated deep inside the cell to the external filament. The entire apparatus is anchored within the cell membrane and cell wall by the basal body, which serves as the motor and provides the necessary rotational or bending force.
Two Worlds, Two Designs: Prokaryotic vs. Eukaryotic Flagella
The structure of the flagellum represents a clear example of convergent evolution, where the structures serve the same function—motility—but evolved entirely independently, resulting in two fundamentally different designs. The prokaryotic flagellum, such as in bacteria, is a relatively simple, solid filament composed almost entirely of the protein flagellin. This filament is not covered by the cell’s plasma membrane.
The prokaryotic flagellum is powered by the flow of ions, primarily protons, moving across the cell membrane down a concentration gradient, a mechanism referred to as the proton-motive force. This ion flow drives a rotary motor embedded in the basal body, causing the entire external filament to spin like a microscopic propeller at speeds up to 100 revolutions per second. This rotation results in the cell being pushed forward through the liquid medium.
In contrast, the eukaryotic flagellum, found in organisms like protozoa, algae, and human sperm, is a much larger and more complex structure encased within an extension of the cell’s plasma membrane. Internally, the core is a sophisticated arrangement of microtubules called the axoneme. This structure consists of nine pairs of microtubules arranged in a ring around two central microtubules, a configuration known as the 9+2 array.
Rather than rotating, the eukaryotic flagellum moves with a characteristic undulatory, or whip-like, bending motion. This movement is generated by the motor protein dynein, which uses the chemical energy stored in adenosine triphosphate (ATP). Dynein causes adjacent microtubule pairs to slide past one another, making the axoneme bend and creating the rhythmic wave that moves the cell.
Flagella’s Essential Roles in Biology and Disease
Flagella are involved in biological processes from cellular sensing to human fertilization and disease establishment. In eukaryotic reproduction, the flagellum is the sole means of locomotion for the sperm cell, allowing it to navigate the female reproductive tract to reach and fertilize the egg. The precise, whip-like beat of the sperm tail is necessary for generating the required force and directionality.
The flagellum’s role extends to bacterial pathogenesis, or the ability to cause disease. Flagellated gut pathogens, such as Escherichia coli and Helicobacter pylori, utilize their motility to swim effectively through the viscous mucus layer lining the digestive tract. This ability to penetrate the mucus is a prerequisite for colonizing the host’s epithelial cells and establishing an infection.
Flagella also function as sensory organelles, enabling the cell to perform a behavioral response called chemotaxis. By detecting gradients of specific chemicals in the environment, the flagellum’s motor can be precisely controlled to either increase the time spent moving toward a nutrient source or to change direction to escape a harmful toxin. This involves modulating the rotation of the prokaryotic flagellar motor, switching it between the smooth forward run and the random tumbling motion needed to reorient the cell.