Flagella are microscopic, hair-like appendages that extend from the surface of some cells. They enable movement through liquid environments or help cells interact with their surroundings. These structures are important for many microorganisms and specialized cells. Flagella play a role in processes like finding nutrients, escaping harmful conditions, and aiding in reproduction.
Cellular Propellers
Flagella are widely recognized for their role in cell locomotion, often compared to propellers or whips that drive cells through liquid. In bacteria, flagella function like rigid propellers, rotating to push the cell forward. This rotation is powered by a motor embedded in the cell membrane, which converts the flow of ions (like protons or sodium ions) across the membrane into mechanical work. The bacterial flagellum’s filament is a helical structure that rotates at speeds ranging from 200 to 1700 revolutions per second, depending on the species.
The direction of rotation dictates the cell’s movement; counterclockwise rotation generally results in straight swimming, while clockwise rotation can cause the cell to tumble and reorient. This allows bacteria to navigate their environment effectively. Organisms such as Escherichia coli and Salmonella utilize this mechanism for efficient movement. In contrast, eukaryotic cells, like sperm cells and some single-celled organisms, use flagella that move with a whip-like or wave-like beating motion to propel themselves. This coordinated movement is driven by the protein dynein, which facilitates the sliding of internal structures within the flagellum.
Beyond Movement: Diverse Functions
While propulsion is a primary function, flagella also serve various other roles in cellular biology. They possess sensory capabilities, acting as cellular antennae that detect changes in the external environment. For instance, flagella can sense chemical gradients, allowing cells to move towards attractants or away from repellents, a process known as chemotaxis. This sensory function involves receptors on the flagellar membrane that detect specific molecules and relay signals to the flagellar motor, influencing its rotation.
Flagella can also detect physical cues, such as surface contact, influencing cellular behavior like adhesion to surfaces. In some bacteria, the flagellum can even sense wetness. In eukaryotic cells, specialized non-motile flagella, known as primary cilia, play a role in signaling and communication. These primary cilia act as sensory organelles, transducing signals from pathways like Hedgehog and Wnt, which are involved in embryonic development and tissue maintenance.
Different Types and Their Structures
Flagella are not uniform across all life forms, exhibiting distinct structural and compositional differences between prokaryotic and eukaryotic cells. Bacterial flagella are relatively simpler, composed of a protein called flagellin, which forms a hollow helical filament about 20 nanometers in diameter. This filament connects to a hook and a basal body embedded in the cell membrane and cell wall.
Eukaryotic flagella, conversely, are more complex and larger, enclosed by the cell membrane. Their core structure, called the axoneme, consists of an organized arrangement of microtubules. This arrangement typically features nine pairs of microtubules surrounding two central single microtubules, known as the “9+2” pattern.