Cilia and Flagella

Cilia and flagella are hair-like projections that extend outward from the surface of the cell, and both are involved in movement.

Cilia versus flagella

Both structures are outgrowths of the cell membrane, but they differ in size, number and the kind of movement they produce:

• Cilia (singular: cilium) are small, short structures. They work rather like oars, and their beating causes either the cell to move or the fluid surrounding the cell to move. Cilia are usually numerous and beat in a coordinated fashion.
• Flagella (singular: flagellum) are comparatively longer and fewer in number, and they are responsible for the movement of the cell itself.

Eukaryotic versus bacterial flagella

Prokaryotic bacteria also possess flagella, but a bacterial flagellum is structurally different from a eukaryotic flagellum. Although both are called by the same name and both drive movement, their internal construction is not the same, so they should not be treated as identical.

Internal structure — the axoneme and the 9+2 array

Electron microscopy shows that a cilium or flagellum is covered on the outside by the plasma membrane. Its central core is called the axoneme, and this contains a set of microtubules running parallel to the long axis of the structure. The arrangement of these microtubules is highly regular:

• There are nine doublets (pairs) of microtubules arranged radially around the outside — these are the peripheral microtubules.
• At the centre lies a pair of single microtubules — the central tubules.

Because of this pattern — nine outer doublets surrounding two central tubules — the arrangement is known as the 9+2 array.

Several connecting elements hold this array together:

• The two central tubules are joined to each other by bridges and are enclosed within a central sheath. …