Flagellar motility is the movement of cells using a flagellum, a microscopic appendage that functions like a tail. This mechanism allows organisms, from bacteria to specialized cells in humans, to navigate liquid environments. Using their flagella, these organisms can seek out nutrients, escape threats, and explore new territories.
The Prokaryotic Rotary Motor
The flagellum in prokaryotic organisms like bacteria is a rotary motor. This structure has three primary parts: the filament, the hook, and the basal body. The filament is the long, helical propeller extending from the cell’s surface and is composed of a protein called flagellin. It connects to the hook, which acts as a flexible joint to transmit torque.
The basal body anchors the apparatus in the cell envelope and is the engine of the flagellum. It consists of a series of protein rings: the MS-ring in the cytoplasmic membrane and the P and L rings in the outer layers of gram-negative bacteria. This assembly is the rotor, turning at speeds up to 300 revolutions per second. The motor’s rotation drives the corkscrew-like filament, propelling the bacterium forward.
Unlike many other cellular processes, the prokaryotic flagellar motor is not powered by ATP. Its energy is derived from the proton motive force, which is generated by the flow of protons across the cell’s inner membrane. This flow occurs through channel proteins called MotA and MotB that make up the motor’s stator. This ion flow creates the torque needed to rotate the flagellum.
The arrangement of flagella on a bacterial cell influences its movement patterns. A cell can have a single flagellum (monotrichous), a tuft at one pole (lophotrichous), or flagella distributed over its surface (peritrichous). These configurations determine how the bacterium moves and changes direction.
The Eukaryotic Whipping Motion
Eukaryotic cells, like protists and animal sperm, use a different type of flagellum. Instead of rotating, the eukaryotic flagellum moves in a whipping or wave-like motion. This structure is an extension of the cell’s cytoplasm and is enclosed by the cell membrane.
The internal structure of the eukaryotic flagellum is a cytoskeletal array called the axoneme. The axoneme has a “9+2” arrangement of microtubules, with nine pairs forming a ring around a central pair of single microtubules. This pattern provides both structural support and the framework for movement.
Movement is generated by motor proteins called dynein arms. These arms are anchored to one microtubule doublet and “walk” along the adjacent one, causing the axoneme to bend. The coordinated bending of microtubules on opposite sides of the axoneme produces the wave-like oscillations that propel the cell.
The energy source for this whipping motion is adenosine triphosphate (ATP), which the dynein motor proteins use to create mechanical force. This use of ATP contrasts with the proton-powered prokaryotic motor. It is a case of convergent evolution, where different mechanisms evolved to solve the problem of cellular locomotion.
Navigating the Environment
Organisms use their flagella for purposeful navigation, not just movement. Many motile bacteria use a strategy called chemotaxis, moving toward beneficial chemicals (attractants) and away from harmful substances (repellents). This process allows them to find food and escape toxins by reading chemical gradients.
A common mechanism for bacterial chemotaxis is the “run and tumble” strategy. A “run” is a period of smooth, straight-line swimming that occurs when the flagella rotate counter-clockwise and form a bundle to propel the cell forward. This movement can last for about a second, covering many times the cell’s length.
To change direction, the cell initiates a “tumble” by briefly switching its flagellar motors to a clockwise rotation. This reversal causes the flagellar bundle to fly apart, resulting in a random motion that reorients the bacterium. Once the flagella return to counter-clockwise rotation, a new run begins in a different direction.
Cells use receptors to detect changes in chemical concentrations over time. If a bacterium senses it is moving toward an attractant, it will suppress tumbling, leading to longer runs in that direction. If it moves toward a repellent, it tumbles more frequently, allowing it to change course. This “biased random walk” results in a net migration toward more desirable locations.
Relevance in Disease and Reproduction
Flagellar motility is a virulence factor for many bacteria. For instance, Helicobacter pylori uses its flagella to move through the stomach’s mucus lining to reach the stomach wall, where it can cause gastritis and ulcers. Similarly, pathogens like Salmonella use their flagella to swim through the intestine and invade host cells.
Flagellar motility is also important in forming biofilms. Motile bacteria can more easily colonize surfaces, which is the first step in establishing these surface-attached communities. The ability to move allows bacteria to find suitable locations to initiate a biofilm, which can be difficult to eradicate and may contribute to chronic infections.
The eukaryotic flagellum is necessary for human reproduction. The motility of a sperm cell is dependent on its flagellum, which provides the propulsive force needed to travel through the female reproductive tract and reach an egg for fertilization.
Defects in flagellar function can directly impact fertility. Some forms of male infertility are linked to impaired sperm motility, where the flagella are unable to generate the proper movement. This impairment prevents the sperm from successfully navigating to the egg.