Flagella are microscopic, hair-like appendages that extend from the surface of various cells, primarily serving as instruments for movement. They enable cells to navigate their environments, playing a fundamental role in the survival and propagation of diverse organisms. Flagella highlight an evolutionary solution for cellular motility in the microscopic world.
What Flagella Are
Flagella are whip-like or rotary appendages that facilitate cell motility. These protein-based structures primarily propel cells through liquid environments. Despite their common role in movement, flagella exhibit significant structural differences across different life forms, reflecting distinct evolutionary paths.
Bacterial flagella are helical filaments composed of a protein called flagellin. They consist of three main parts: the filament, a long, whip-like structure; the hook, which connects the filament to the cell; and the basal body, embedded in the cell membrane and wall. The basal body acts as a motor, anchoring and rotating the flagellum.
Eukaryotic flagella, found in animal, plant, and protist cells, are more complex cellular projections. Their core is a structure called the axoneme, which contains a bundle of nine fused pairs of microtubules surrounding two central single microtubules, known as the “9+2” arrangement. This entire structure is encased by the cell membrane. The axoneme is anchored to the cell by a basal body, which is structurally similar to a centriole.
Mechanisms of Flagellar Motion
The mechanisms by which flagella generate movement differ significantly between bacteria and eukaryotes, highlighting their distinct evolutionary origins. Bacterial flagella operate like tiny propellers, driven by a rotary motor embedded in the cell membrane. This motor rotates the helical filament, propelling the bacterium.
The energy for this rotation comes from an electrochemical gradient, specifically the flow of protons across the cell membrane. Proteins within the flagellar motor form a channel through which these ions pass, driving the rotation of the basal body. This rotary motion can be clockwise or counterclockwise, enabling bacteria to perform “runs” (straight movement) and “tumbles” (reorientation).
Eukaryotic flagella, in contrast, move with a characteristic whip-like or undulating motion. This bending is powered by the hydrolysis of adenosine triphosphate (ATP). Motor proteins called dyneins, attached to the microtubules within the axoneme, generate the force required for this bending.
Dynein arms “walk” along adjacent microtubules, causing them to slide past each other. This sliding motion, constrained by other proteins, results in the coordinated bending of the flagellum. This ATP-driven bending mechanism is distinct from the rotary motor of bacterial flagella.
Presence and Importance Across Life
Flagella are widely distributed across various domains of life, serving diverse and important functions beyond simple locomotion. In bacteria, flagella are the primary structures for movement, enabling them to navigate towards favorable environments or away from harmful substances. This motility is relevant for pathogenic bacteria like Escherichia coli and Salmonella, allowing them to colonize tissues and contribute to virulence.
Archaea also possess flagella, termed archaella, which are superficially similar to bacterial flagella but differ significantly in structure and assembly. Archaella are rotary propellers, enabling archaeal cells to swim. Despite functional similarities, archaella are simpler in protein composition compared to bacterial flagella.
In eukaryotes, flagella are found in various organisms and play roles in reproduction, feeding, and sensory perception. Human sperm rely on a single flagellum for motility, which is essential for fertilization. Many protists use flagella for movement and sometimes as sensory antennae to detect environmental changes. Flagella are also present in the gametes of some plants. Beyond individual cell movement, flagellar motion can create water currents, aiding in nutrient acquisition and circulation in organisms like sponges.