Flagella are hair-like appendages extending from cell surfaces, primarily enabling locomotion in diverse microorganisms, including bacteria, archaea, and some eukaryotic cells like sperm. Their design and operational mechanisms differ significantly across these domains of life, reflecting distinct evolutionary paths for cellular motility.
Rotary Motion of Bacterial Flagella
Bacterial flagella propel cells through a propeller-like rotary motion. This structure consists of a rigid, helical filament, a flexible hook connecting the filament to the cell, and a basal body embedded within the cell membrane and cell wall. The basal body functions as a molecular motor, driving the flagellar rotation.
This rotation is powered by the proton motive force (PMF), an electrochemical gradient of protons across the bacterial cell membrane. As protons flow through specific channels within the basal body’s stator units, their energy converts into mechanical rotation. This mechanism allows bacteria to achieve speeds up to 60 cell lengths per second.
Stator complexes, composed of proteins like MotA and MotB, generate the torque that drives the C-ring’s rotation. This rotation transmits through the rod and hook to the external helical filament. Rotational speed varies with the proton motive force, allowing for speed control.
Whipping Motion of Eukaryotic Flagella
Eukaryotic flagella exhibit a distinct whip-like or wave-like beating motion, also characteristic of cilia due to their shared structure. The internal framework, called the axoneme, features a “9+2” arrangement of microtubules: nine outer doublets surround a central pair of single microtubules.
Motor proteins called dyneins, anchored to the outer doublet microtubules, power this bending motion. Dynein proteins utilize energy from adenosine triphosphate (ATP) hydrolysis to “walk” along adjacent microtubules, causing them to slide past each other.
Connecting proteins within the axoneme constrain this sliding, converting the dynein-generated force into a bending motion of the flagellum. This ATP-driven bending mechanism differs from the rotary motion seen in bacterial flagella, which are powered by proton flow and operate more like a propeller.
Unique Rotation of Archaeal Flagella
Archaeal flagella are structurally and evolutionarily distinct from both bacterial and eukaryotic flagella, though they also achieve motility through rotation. While sharing rotational function with bacterial flagella, their components are more closely related to bacterial Type IV pili.
Their energy source is a primary distinction: archaeal flagella are powered by ATP hydrolysis, rather than the proton motive force used by bacterial flagella. The motor complex includes proteins like FlaI, an ATPase, which generates energy for rotation through ATP hydrolysis.
The exact mechanism of how FlaI’s ATP hydrolysis converts into rotational motion is still being investigated; however, approximately 12 ATP molecules are used per rotation in some archaeal species. Archaeal flagella can rotate both clockwise and counterclockwise, enabling changes in swimming direction.