The flagellum, a whip-like cellular appendage, is a widespread structure found across all three domains of life: Archaea, Bacteria, and Eukarya. While sharing the common function of providing locomotion, the flagellum represents a remarkable example of convergent evolution, where distinct molecular machines evolved independently to achieve the same purpose. This organelle allows cells to propel themselves through fluid environments, determining a microorganism’s ability to survive, navigate, and colonize new niches. Understanding its intricate design reveals fundamental differences in cellular organization and energy use between prokaryotic and eukaryotic life forms.
Comparative Architecture of Prokaryotic and Eukaryotic Flagella
The physical construction of the flagellum differs profoundly between simple and complex cells, reflecting their separate evolutionary paths. A prokaryotic flagellum is a simple filament that acts like a propeller extending from the cell surface. This structure is built from a single protein called flagellin, which self-assembles into a hollow, helical tube.
This helical filament connects to a hook, which serves as a flexible universal joint, linking the filament to the motor embedded in the cell envelope. The basal body is the motor complex, consisting of a central rod surrounded by a series of protein rings. In Gram-negative bacteria, the basal body has four rings (L, P, MS, and C) that traverse the cell envelope, while Gram-positive bacteria typically possess only two rings. Flagellar assembly is external, with flagellin subunits being transported through the hollow core of the growing filament to be added at the tip.
The eukaryotic flagellum is a large, complex, membrane-bound extension of the cytoplasm. Its internal core, known as the axoneme, is built from microtubules. This internal skeleton follows a highly conserved “9+2” arrangement, featuring nine pairs of fused doublet microtubules surrounding two central single microtubules.
Attached to these doublets are numerous accessory proteins, including the motor protein dynein, which forms inner and outer arms. The entire structure is anchored inside the cell by a basal body, or kinetosome, which is structurally similar to a centriole and typically contains nine triplet microtubules in a “9+0” arrangement.
The Physics and Energetics of Motility
Prokaryotic flagella achieve locomotion through a rotary mechanism, spinning like a miniature propeller. The basal body functions as a rotary molecular motor capable of rapid rotation, sometimes exceeding 300 revolutions per second.
This rotation is not powered by adenosine triphosphate (ATP), the typical cellular energy currency. Instead, the motor is driven by the flow of ions down an electrochemical gradient across the cell membrane, known as the proton motive force (PMF). The Mot complex proteins act as the stator, using the energy released by protons (or sometimes sodium ions) flowing into the cell to generate the torque required to spin the rotor. The direction of rotation governs the cell’s movement: a counter-clockwise spin causes the cell to move forward in a smooth path called a “run,” while a clockwise spin causes the flagella to splay apart, resulting in an erratic change in direction called a “tumble.”
Eukaryotic flagella generate movement through a whip-like, undulating, or bending motion rather than rotation. This movement is driven by the motor protein dynein, which forms transient bridges between adjacent outer doublet microtubules. Dynein hydrolyzes ATP to power its conformational change, effectively “walking” along one microtubule while pulling the adjacent doublet.
Since the microtubules are physically constrained by other linkage proteins, this sliding force is converted into a localized bending of the entire axoneme. The precise coordination of dynein activity along the nine doublets and the regulatory role of the central pair of microtubules allow the cell to produce complex, rhythmic bending waves. This mechanism is highly energy-intensive, requiring the constant hydrolysis of ATP to sustain the oscillatory bending motion.
Sensory Roles and Cellular Signaling
Beyond locomotion, flagella and their structural relatives, cilia, serve as sensory and signaling organelles. In bacteria, the flagellum is an integral component of the sensory system known as chemotaxis. The cell uses chemoreceptors to monitor gradients of specific chemicals, such as attractants or repellants.
This sensory input directly regulates the flagellar motor, controlling the frequency of runs and tumbles. Moving toward an attractant suppresses tumbling, resulting in longer, straighter runs in the favorable direction. This allows the bacterium to perform a biased random walk toward nutrients or away from toxins.
In eukaryotic cells, primary cilia often function as cellular antennae, detecting and transducing external signals. Nearly every cell in the human body possesses a primary cilium, which lacks the central pair of microtubules (a “9+0” arrangement). These antennae house specialized receptors that process mechanical and chemical stimuli, playing roles in sight, smell, and mechanosensation, such as monitoring fluid flow in kidney tubules.
Flagella also play a role in the initial stages of bacterial colonization and biofilm formation. Motility is often necessary to transport the cell to a suitable surface, where the flagellum itself can sometimes act as an adhesin, facilitating initial attachment to specific host tissues or hydrophobic surfaces. After initial contact, some bacteria suppress flagellar production, transitioning from a motile, planktonic state to a sessile, surface-adhering state to begin constructing a protective biofilm community.