A flagellum is a whip-like cellular appendage found across all three domains of life—Bacteria, Archaea, and Eukaryota. This slender, hair-like structure protrudes from the cell body and is primarily responsible for generating movement. Its core function is to provide motility, enabling single cells to propel themselves through fluid environments. The Latin root of the word, flagellum, means “whip,” accurately describing its appearance and motion. This specialized organelle is a fundamental mechanism for locomotion in microorganisms.
The Core Function: Cellular Movement
The physical mechanism for movement differs significantly between prokaryotic and eukaryotic organisms.
Prokaryotic flagella, found in bacteria, function like microscopic, rigid propellers rotating at high speeds. This rotational movement is driven by a complex motor embedded in the cell membrane and cell wall, which can spin the external filament hundreds of times per second. The energy source for this bacterial motor is the proton motive force (PMF), the electrochemical gradient created by the flow of protons across the cell membrane. As protons flow back into the cell through motor proteins, the energy released powers the flagellum’s rotation, enabling the bacterium to swim forward in a smooth “run.” The filament itself is a simple, hollow structure made of the protein flagellin.
Eukaryotic flagella, in contrast, are larger, more complex structures that move with a flexible, undulating, whip-like motion. This movement is driven by an internal structure called the axoneme, a highly organized bundle of microtubules typically arranged in a “9+2” pattern. Specialized motor proteins called dyneins use Adenosine Triphosphate (ATP) to slide these microtubules past each other. This sliding action results in the characteristic wave-like bending, which propels the cell through the fluid.
How Flagella Direct Cells
Flagella also serve as sophisticated tools for directional control, allowing cells to navigate their environment purposefully. This directional movement is known as taxis, where the cell moves toward or away from a particular stimulus. Chemotaxis, the most widely studied form, involves movement in response to chemical signals, such as nutrients or toxins.
In bacteria, flagella facilitate chemotaxis through a mechanism called “run and tumble.” When moving toward a favorable chemical gradient, the flagella bundle together and rotate counterclockwise, resulting in a smooth, straight “run.” If the cell detects a repellent, a chemical signal causes the flagellar motor to switch to clockwise rotation. This causes the flagellar bundle to fly apart, and the cell “tumbles” randomly, reorienting its direction.
The cell’s ability to sense these external conditions is mediated by specialized chemoreceptors located on the cell surface. These receptors relay information to the flagellar motor, modulating the frequency of the tumbles to bias the cell’s movement toward the attractant. Other forms of taxis, such as phototaxis (movement in response to light) or magnetotaxis (movement in response to magnetic fields), also rely on flagellar adjustments for directed navigation.
Variations Across Different Organisms
Flagella appear across all domains of life, yet their structural variations reflect different evolutionary paths.
Bacterial flagella are composed of a single protein, flagellin, and are secured by a rotary motor. The placement of these flagella can vary, from a single flagellum at one end (monotrichous) to numerous flagella covering the entire surface (peritrichous), which influences the cell’s specific swimming pattern.
Archaea also possess flagella, sometimes called archaella, which are superficially similar to bacterial flagella in their rotary motion. However, the archaeal structures are distinct in their protein composition and assembly, suggesting they evolved separately from their bacterial counterparts. They also utilize a rotary motor, though the power source in some cases may differ from the proton motive force.
A highly relevant eukaryotic example is the flagellum of the human sperm cell, which is the only flagellated cell in the male body. The flagellum’s forceful, whip-like beat is solely responsible for propelling the sperm cell toward the egg in the female reproductive tract. This movement is regulated by complex intracellular signaling, including calcium ions, which cause an asymmetrical bending pattern necessary for hyperactivation and penetration of the egg’s protective layers.