Flagella (singular: flagellum ) are long, whip-like appendages that protrude from the surface of certain cells and are used primarily for movement (locomotion) . They are found in both prokaryotic and eukaryotic cells, but their structure and function differ significantly between the two groups.
🧫 1. Prokaryotic Flagella Structure: Composed of flagellin (a protein ). Three main parts:Basal body : Anchors the flagellum to the cell wall and plasma membrane.Hook : Curved connector between basal body and filament.Filament : Long, rotating tail made of flagellin. Powered by a proton motive force (H⁺ gradient across membrane), causing the flagellum to rotate like a propeller . Function: Enables swimming motility in liquid environments. Can rotate clockwise or counterclockwise to control movement (e.g., “run and tumble” behavior in E. coli ). 🧬 2. Eukaryotic Flagella Structure: Made of microtubules in a 9+2 arrangement :9 pairs of microtubules form a ring around 2 central microtubules. This structure is called the axoneme . Enclosed in the plasma membrane . Anchored by a basal body (similar to a centriole). Movement is generated by dynein motor proteins , which cause bending (not rotation). Function: Enables movement of single cells (e.g., sperm cells). In some unicellular eukaryotes (like Euglena ), flagella help them swim through water . 🔬 Comparison: Prokaryotic vs. Eukaryotic Flagella Feature Prokaryotic Flagella Eukaryotic Flagella Composition Flagellin protein Microtubules + dynein (9+2) Motion Rotates like a propeller Whip-like, undulating movement Power Source Proton gradient (H⁺ or Na⁺) ATP (via dynein motors) Membrane-bound? No Yes Size Thinner Thicker
🧠 Biological and Clinical Importance: Fertility : Human sperm rely on flagella for mobility; defects cause male infertility .Infections : Some bacteria use flagella to invade tissues or evade the immune system .Immune detection : Bacterial flagellin is recognized by the innate immune system as a pathogen-associated molecular pattern (PAMP).Ciliopathies : Eukaryotic flagella share a structure with cilia —dysfunction in the axoneme affects both (e.g., Primary ciliary dyskinesia ).Summary Table: Property Description Function Cell motilityFound In Bacteria, archaea, eukaryotic cells (e.g., sperm, protists) Structural Types Prokaryotic (simple, rotating) vs. Eukaryotic (complex, bending) Movement Style Rotation (prokaryotic) or wave-like (eukaryotic) Clinical Significance Impacts fertility, infection, immune response