Prokaryotic cells, which include bacteria and archaea, are single-celled organisms. They do not possess cilia; instead, prokaryotes employ different types of appendages and processes to achieve movement.
What Are Cilia?
Cilia are short, hair-like cellular appendages that extend from the surface of cells, primarily found in eukaryotic organisms. These structures are composed of microtubules, which are protein filaments arranged in a specific pattern, typically nine pairs surrounding two central microtubules, known as the 9+2 arrangement. Cilia function in various ways, including enabling cellular movement, facilitating the movement of substances across cell surfaces, and acting as sensory antennas. For instance, in humans, motile cilia line the respiratory tract, sweeping away mucus and trapped particles, while non-motile cilia can serve as sensory receptors.
How Prokaryotic Cells Move
Prokaryotic cells use diverse methods to move and respond to stimuli. The most recognized structure for motility in many prokaryotes is the flagellum. Unlike cilia, prokaryotic flagella are long, whip-like appendages made of a protein called flagellin, forming a hollow, helical filament. This filament is attached to a complex motor embedded in the cell membrane and cell wall, which rotates the flagellum like a propeller, enabling the bacterium to swim through liquid environments. This rotation is often powered by the flow of ions, such as protons, across the cell membrane.
Beyond flagella, some prokaryotes utilize other structures for movement. Pili, or fimbriae, are shorter, hair-like appendages involved in twitching motility, where the cell extends a pilus, attaches it to a surface, and then retracts it, pulling the cell along. These structures are also primarily involved in adhesion to surfaces. Certain bacteria, like spirochetes, have unique internal flagella, known as axial filaments, located within the space between the cell wall and outer membrane, which allow them to move with a corkscrew-like motion. Other less common prokaryotic movements include gliding and swarming, which can involve different surface-associated mechanisms.
Distinguishing Cellular Movement
Eukaryotic flagella and cilia share a common internal structure with their microtubule arrangement, allowing them to move with a whip-like or oar-like bending motion, driven by ATP. This contrasts sharply with prokaryotic flagella, which are simpler in composition and operate through a rotary motion, powered by ion gradients rather than ATP. Structurally, eukaryotic flagella and cilia are encased by the cell’s plasma membrane and contain a complex arrangement of microtubules. In contrast, the prokaryotic flagellum is a solid protein filament that extends outside the cell membrane and lacks the internal microtubule organization found in eukaryotic structures. These differences reflect distinct evolutionary paths for cellular motility.