A flagellum is a microscopic, hair-like structure that extends from the surface of various cells and microorganisms. Often visualized as a tiny tail, this appendage functions as a cellular motor, and its primary purpose is to generate movement. This ability, known as motility, allows a cell to navigate through a liquid environment. These structures are present across different domains of life, from single-celled bacteria to specialized cells within animals, such as sperm, enabling organisms to find nutrients, escape threats, or complete a reproductive cycle.
The Prokaryotic Propeller
In prokaryotic organisms like bacteria, the flagellum is a molecular machine that functions as a propeller. This structure is composed of three main parts: the basal body, the hook, and the filament. The basal body is the engine, a complex assembly of protein rings embedded within the cell’s membrane and wall, anchoring the structure. This motor is connected to the external filament by a flexible adapter called the hook, which acts like a universal joint to transfer torque from the motor.
The long, helical filament extending from the cell is made of a protein called flagellin. Unlike the appendages of more complex cells, the bacterial flagellum does not bend or whip; instead, it rotates. The basal body spins the filament in a corkscrew-like motion, propelling the bacterium forward at speeds up to 300 revolutions per second. This rotation is not fueled by ATP, the common energy currency of the cell, but is driven by a flow of ions like protons across the cell membrane—a mechanism known as the proton motive force.
This design is common in many bacteria, including species such as Escherichia coli and Salmonella. The arrangement of these propellers can vary significantly, providing different kinds of motility. Some bacteria have a single flagellum at one end, while others may have a tuft of flagella at a pole. Still others, like E. coli, have flagella distributed all over their surface, which can bundle together to drive coordinated movement.
The Eukaryotic Whip
The flagellum in eukaryotic cells, such as those of animals, plants, and protists, operates on different principles from its bacterial counterpart. Structurally, it is not an external part attached to the surface but an extension of the cell’s plasma membrane. Enclosed within this membrane is a core structure called the axoneme, which provides the framework for the flagellum’s movement. The axoneme has a distinctive internal architecture.
The axoneme consists of microtubules—protein filaments that are part of the cell’s cytoskeleton—organized in the “9+2” arrangement. This pattern features nine pairs of microtubules forming a circle around two single microtubules in the center. This structure is anchored at its base within the cell by a basal body, which is structurally identical to a centriole. The entire assembly is far more complex than the prokaryotic version, involving hundreds of different proteins.
Movement in the eukaryotic flagellum is generated by a bending or whipping motion, not rotation. This action is powered directly by the hydrolysis of ATP. Motor proteins called dynein arms are attached to the outer microtubule pairs. Using energy from ATP, these dynein arms “walk” along the adjacent microtubule, causing the pairs to slide past one another. Because the microtubule doublets are linked by other proteins, this sliding force is converted into a bending motion, creating the wave-like undulations seen in a swimming sperm cell or the protist Euglena.
Functions Beyond Locomotion
While generating movement is the most recognized function of the flagellum, it serves other purposes for an organism’s survival and interaction with its environment. In many organisms, the flagellum also acts as a sensory organelle. It can detect changes in the surrounding environment, such as shifts in temperature or the presence of specific chemicals, a process known as chemotaxis.
In bacteria, flagella play a role in adhesion and the formation of communities. The ability to attach to surfaces is often a first step in creating a biofilm, a structured colony of microorganisms. Flagella can facilitate this initial contact with a surface, helping bacteria to colonize environments ranging from river rocks to medical implants.
The flagellum is also necessary for sexual reproduction in many species. The motility of sperm cells in animals and some plants is entirely dependent on the propulsive force generated by their flagellum. This movement enables the sperm to travel through the female reproductive tract to reach and fertilize an egg.