Flagella are whip-like appendages that extend from the surface of many cells, serving as sophisticated propulsion systems. These structures enable individual cells to move through their surrounding environments, a process known as cell motility. Found across a wide range of living organisms, from single-celled bacteria to animal sperm, flagella play a fundamental role in how these cells navigate and interact with their world. They generate movement, allowing cells to seek out nutrients, escape harmful conditions, or, in the case of sperm, reach an egg for fertilization.
Two Main Types of Flagella
Despite their shared purpose of enabling cellular movement, flagella are not uniform across all forms of life. Cells are broadly categorized into two main groups: prokaryotic cells, which include bacteria and archaea, and eukaryotic cells, encompassing animals, plants, fungi, and protists. Flagella found in these two domains of life possess fundamentally different structural designs and distinct mechanisms for generating motion. This divergence highlights an evolutionary distinction in how cells achieve mobility.
The Bacterial Flagellum
The bacterial flagellum is a complex molecular machine that operates as a rotary motor. This structure consists of three main parts: the filament, the hook, and the basal body. The filament, the longest part, extends helically from the cell surface and is primarily composed of protein subunits called flagellin. This hollow, tube-like structure functions like a propeller to drive the bacterium forward when rotated.
Connecting the filament to the cell body is the hook, a short, curved, and flexible segment. The hook acts as a universal joint, allowing the rotational force generated by the motor to be transmitted to the helical filament, facilitating movement.
The basal body represents the motor of the bacterial flagellum, embedded within the cell membrane and cell wall. In Gram-negative bacteria, this intricate motor typically consists of four protein rings: the L-ring, P-ring, MS-ring, and C-ring. These rings are positioned within the cell envelope, with the MS-ring serving as the rotor and the C-ring acting as a switch. Gram-positive bacteria, lacking an outer membrane, possess only the MS and C rings. A central rod connects these rings, forming the core of the motor and transmitting rotational force.
The Eukaryotic Flagellum
Eukaryotic flagella exhibit a more complex internal organization, enclosed entirely by the cell’s plasma membrane. The core of the eukaryotic flagellum is a microtubule-based structure known as the axoneme. This axoneme is characterized by a “9+2” arrangement of microtubules: nine peripheral microtubule doublets surrounding a central pair of single microtubules.
Attached to each microtubule doublet are motor proteins called dynein arms. These dynein arms use energy from ATP to slide along adjacent microtubules, causing the doublets to slide against each other. Other proteins, such as radial spokes, connect the outer doublets to the central pair, while nexin links connect adjacent outer microtubule doublets. These connecting proteins constrain the sliding motion, converting it into a bending movement of the entire axoneme.
At the base of the eukaryotic flagellum is the basal body. This structure is similar to a centriole and consists of a cylindrical array of nine triplet microtubules arranged in a “9+0” pattern. The basal body serves as a template for the assembly of the axoneme and anchors the flagellum to the cell membrane.
How Flagella Enable Movement
The distinct structures of bacterial and eukaryotic flagella lead to fundamentally different mechanisms of movement. Bacterial flagella operate like miniature propellers, driven by the rotation of their basal body motor. This motor, powered by the flow of ions across the bacterial membrane, can rotate rapidly, generating thrust that propels the bacterium through its environment. When rotating counterclockwise, bacterial flagella often bundle together to facilitate a smooth, forward “run,” while a clockwise rotation can cause the bundle to unravel, leading to a “tumbling” motion that helps the bacterium reorient.
Eukaryotic flagella, conversely, generate movement through a whip-like or wave-like bending motion. This is achieved by the coordinated sliding of the microtubule doublets within the axoneme. The dynein motor proteins, attached to the microtubules, use ATP to cause adjacent doublets to slide past one another. Because this sliding is restricted by radial spokes and nexin links, the force is converted into a characteristic bending of the flagellum, allowing the cell to “swim” or move fluid across its surface.