Prokaryotic cells, which include bacteria and archaea, frequently possess whip-like appendages called flagella. These structures are instrumental in enabling motility, allowing these microscopic organisms to navigate their environments. Flagella provide the propulsion necessary for prokaryotes to move towards favorable conditions, such as nutrient sources, or away from harmful substances. Their presence is a defining feature for many motile prokaryotes.
What Are Flagella?
Flagella are slender, hair-like structures that extend from the cell surface, serving as a means of locomotion. These whip-like appendages are found in many bacteria and enable them to move through liquid environments. Flagella provide propulsion, allowing cells to swim. They have a long, filamentous shape.
How Prokaryotic Flagella Propel Cells
The prokaryotic flagellum is a complex molecular machine with distinct components that work together to generate movement. It consists of three primary parts: the filament, the hook, and the basal body. The filament, a rigid, helical structure made of flagellin protein subunits, extends outward from the cell and acts as the propeller. Connecting the filament to the cell body is the hook, which functions as a universal joint, allowing the filament to transmit the rotational force generated by the motor.
The basal body anchors the flagellum to the cell envelope and serves as a rotary motor. This motor is powered by a proton motive force (PMF), which is the flow of protons across the bacterial cell membrane. Proteins within the basal body, such as MotA and MotB, form a channel through which protons move, driving the rotation of the flagellum. The MS and C rings of the basal body function as the rotor, while the MotA and MotB proteins form the stator, generating the torque for rotation.
Bacterial flagella rotate like propellers, enabling the cell to swim. This rotation can occur in both counterclockwise and clockwise directions, allowing the bacterium to change direction. In a uniform environment, bacteria often exhibit a “run and tumble” motility pattern: during a “run,” flagella rotate counterclockwise, forming a bundle that propels the bacterium in a relatively straight line. When flagella switch to clockwise rotation, the bundle disengages, causing the bacterium to “tumble” and reorient randomly. This cycle allows bacteria to perform a biased random walk, moving towards attractants or away from repellents through a process called chemotaxis.
The arrangement of flagella on a bacterial cell can vary, influencing its swimming behavior.
- Monotrichous bacteria possess a single flagellum, typically at one pole.
- Amphitrichous bacteria have a single flagellum at each of two opposite ends.
- Lophotrichous bacteria feature a tuft of two or more flagella at one or both poles.
- Peritrichous bacteria have flagella distributed over their entire surface.
Each of these arrangements allows for effective propulsion, with peritrichous flagella often bundling together during runs for efficient forward movement.
Beyond Flagella: Other Forms of Prokaryotic Movement
While flagella are a common means of propulsion for prokaryotes, these organisms also exhibit diverse strategies for movement. These alternative mechanisms allow prokaryotes to thrive in various environments.
Twitching Motility (Pili)
One mechanism involves pili, hair-like appendages distinct from flagella. While primarily involved in attachment, certain type IV pili can facilitate twitching motility. In twitching motility, these pili extend, attach to a solid surface, and then retract, effectively pulling the cell forward in a jerky, crawling motion. This movement is particularly effective on solid or semi-solid surfaces rather than in liquid environments.
Spirochete Motility (Axial Filaments)
Spirochetes, spiral-shaped bacteria, utilize internal structures called axial filaments for movement. Unlike external flagella, axial filaments are located within the periplasmic space, between the inner and outer membranes of the cell. These filaments are composed of flagella-like fibrils that wrap around the cell body. The rotation of these internal filaments causes the spirochete cell to twist and move with a corkscrew-like motion. This corkscrew movement enables spirochetes to navigate efficiently through viscous environments, such as mucus or bodily fluids.
Gliding Motility
Another form of motility is gliding motility, which does not involve flagella or pili. This slower, surface-associated movement is seen in bacteria like Myxococcus xanthus and certain cyanobacteria. The exact mechanisms for gliding motility can vary among different species, but it generally involves the cell moving smoothly along a solid surface. Some proposed mechanisms include the secretion of slime, which the cell moves along, or the action of adhesion complexes that interact with the substrate. Gliding motility allows these bacteria to colonize surfaces and form communities, such as biofilms, in environments with low water content.