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6.9.2.8. Cilia and flagella

Cilia and flagella

The undulipodia (cilia and flagella) are mobile extensions of the plasma membrane of certain cells, consisting of microtubules.

The flagella have the function of allowing the movement of the cell, and the cilia, creating turbulence near the cell to bring food closer together.

  • The cilia are short and very numerous, covering the cell surface. Its movement is coordinated from back to front.
  • The flagella are long and few, generally only 1 or 2. Their movement is wave-like.

Although prokaryotic cells can possess flagella, their structure is totally different.

Structure and composition of cilia and flagella

Cilia and flagella are made up of:

  • The stem or axoneme. Surrounded by the plasma membrane and has inside:
    • Two central microtubules surrounded by a thin nexin sheath .
    • 9 pairs (doublets) of peripheral microtubules surrounding the central pair. In each pair, it is distinguished:
      • Microtubule A: complete, with 13 protofilaments. From this microtubule, two arms of the protein dynein emerge towards the microtubule B of the neighboring pair.
      • Microtubule B: with 10 protofilaments, it shares 3 with A.

This arrangement is called 9 + 2. The microtubule pairs are linked by nexinOther nexin fibers link each A microtubule to the central sheath.

  • Transition zone. It is the area where the change occurs between the structure (9 + 2) of the stem or axoneme with the structure (9 + 0) of the basal body or centriole. The two central microtubules disappear, and the peripheral doublets become triplets.
  • The basal corpuscle is a cylinder located at the base of the cilium or flagellum, below the plasma membrane. It has the same structure as the centrioles (9 + 0), since it lacks the central microtubule pair, and has 9  peripheral microtubule triplets .
  • Ciliary roots (not always present). They are microfilaments that emerge from the lower end of the basal corpuscle, with a contractile function, which coordinates the movement of the cilia.

By LadyofHats. Versión en español de Alejandro Porto (File:Eukaryotic cilium diagram en.svg) [Public domain], via Wikimedia Commons

Function of cilia and flagella

The cilia perform two distinct functions:

  • Move the fluid or particles located on the surface of the cilia, such as the cells of the fallopian tube and the ciliated epithelial cells of the trachea and bronchi.
  • Propel the cell through a liquid, as in the case of some protozoa.

The cilia have a pendulum motion. The cilium, initially rigid, beats vigorously and then recovers until it returns to the initial position, in the same way as a field of wheat would be beaten by the wind.

The flagella are responsible for the displacement of various types of protozoa and sperm through an undulating movement. A wave is produced at the base that propagates to the other end of the flagellum.

As in muscle contraction, the movement of cilia and flagella is due to the sliding of some peripheral doublets with respect to others. As the doublets are anchored in the basal corpuscle, sliding causes flexion of the cilium. The dynein allows the gliding of the microtubules .

Cilia and cells of the trachea

The main structures of the cells of the trachea are the cilia, since they are responsible for moving the mucus that retains the dust particles and other substances that arrive with the air that enters the lungs.

Fundamental ideas about cilia and flagella

Cilia and flagella

  • They are mobile extensions of the plasma membrane of some cells, made up of  microtubules .
  • The cilia 
    • They are short and very numerous, covering the cell surface.
    • Pendulum movement, from back to front. 
    • They move the liquid nearby or are propelled through it.
  • Fhe  flagella 
    • They are long and scarce, generally only 1 or 2.
    • Movement is wave.
  • Structure of cilia and flagella:
    • The stem or axoneme. Layout 9 + 2
    • Two central microtubules .
    • 9 pairs (doublets) of peripheral microtubules.
  • Transition zone. 
  • The basal corpuscle. It has the same structure as the  centrioles  (9 + 0), since it lacks the  central microtubule pair .
  • Ciliary roots (not always present).