CILIA AND FLAGELLA

Cilia (sing., cilium; L. “eyelashes”) and flagella (sing., flagellum;
L. “small whips”) are elongated appendages on the surface of some
cells by which the cells, including many unicellular organisms,

propel themselves. In stationary cells, cilia or flagella move material
over the cell’s surface.
Although flagella are 5 to 20 times as long as cilia and move
somewhat differently, cilia and flagella have a similar structure.
Both are membrane-bound cylinders that enclose a matrix. In this
matrix is an axoneme or axial filament, which consists of nine
pairs of microtubules arranged in a circle around two central
tubules. This is called a 9 2 pattern of microtubules.
Each microtubule pair (a doublet) also has pairs of dynein
(protein) arms projecting toward a neighboring doublet and
spokes extending toward the central pair of microtubules. Cilia
and flagella move as a result of the microtubule doublets sliding
along one another.
In the cytoplasm at the base of each cilium or flagellum lies
a short, cylindrical basal body, also made up of microtubules and
structurally identical to the centriole. The basal body controls the
growth of microtubules in cilia or flagella. The microtubules in the
basal body form a 9 0 pattern: nine sets of three with none in
the middle.