Many microscopic, single-celled organisms navigate their surroundings using specialized hair-like or whip-like appendages. These structures, known as flagella and cilia, extend from the cell surface and enable movement through liquid environments. They are the primary means by which these tiny life forms propel themselves. These cellular extensions are important for the survival of numerous one-celled creatures.
Unpacking the Tail’s Design
The internal architecture of these whiplike tails varies depending on the organism. Eukaryotic flagella and cilia, found in organisms with a nucleus, share a complex internal framework called an axoneme. This axoneme consists of nine pairs of microtubules arranged in a circle around two central microtubules, known as a “9+2” arrangement. This structure arises from a basal body, which anchors the flagellum or cilium within the cell and resembles a centriole.
Prokaryotic flagella, present in bacteria, exhibit a simpler yet distinct design. They are composed of three main parts: a long, helical filament that extends into the environment, a hook that connects the filament to the cell surface, and a basal body embedded within the cell envelope. Unlike eukaryotic flagella, prokaryotic flagella lack the microtubule arrangement and are made of a protein called flagellin.
The Mechanics of Movement
The propulsion generated by these structures is diverse. Eukaryotic flagella move with a wave-like or whip-like motion, pushing fluid backward and propelling the cell forward. This undulating movement, seen in organisms like Euglena, is powered by the hydrolysis of adenosine triphosphate (ATP), a molecule that stores and transfers energy within cells. The ATP fuels motor proteins, called dyneins, which cause the microtubules within the axoneme to slide past each other, creating the characteristic bending.
Eukaryotic cilia, while structurally similar to flagella, operate with a distinct two-phase stroke. They execute a stiff “power stroke” that pushes against the surrounding fluid, propelling the cell or moving substances across its surface. This is followed by a flexible “recovery stroke” where the cilium bends and retracts to minimize resistance before the next power stroke. Like eukaryotic flagella, this coordinated action is also driven by ATP.
Prokaryotic flagella, in contrast, function more like tiny propellers. They rotate rapidly, either clockwise or counter-clockwise, to push the bacterium through its medium. This rotational motion is not driven by ATP directly, but by a flow of protons across the bacterial cell membrane, creating what is known as a proton motive force. The proton motive force powers a motor at the base of the flagellum, causing the filament to spin and move the bacterium.
Diverse Users in the Microbial World
These motile appendages are widely distributed across single-celled life, serving various purposes beyond simple locomotion. Many types of bacteria, such as Escherichia coli, utilize their rotating flagella to swim towards nutrient sources or away from harmful substances, a process called chemotaxis. This directed movement is important for their survival.
Protists, a diverse group of eukaryotic microorganisms, also rely on these structures. Euglena, a common freshwater protist, uses a single, long flagellum to pull itself through water, moving towards light for photosynthesis.
Paramecium, another well-known protist, is covered in thousands of cilia, which it uses in a coordinated fashion for rapid swimming and to sweep food particles into its oral groove for feeding. Chlamydomonas, a green alga, employs two flagella for both swimming and sensing light, enabling it to position itself for photosynthesis. These examples highlight how whiplike tails aid in feeding, escaping predators, and sensing environmental cues.