Flagella

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

FeatureProkaryotic FlagellaEukaryotic Flagella
CompositionFlagellin proteinMicrotubules + dynein (9+2)
MotionRotates like a propellerWhip-like, undulating movement
Power SourceProton gradient (H⁺ or Na⁺)ATP (via dynein motors)
Membrane-bound?NoYes
SizeThinnerThicker

🧠 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:

PropertyDescription
FunctionCell motility
Found InBacteria, archaea, eukaryotic cells (e.g., sperm, protists)
Structural TypesProkaryotic (simple, rotating) vs. Eukaryotic (complex, bending)
Movement StyleRotation (prokaryotic) or wave-like (eukaryotic)
Clinical SignificanceImpacts fertility, infection, immune response