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Muscle
types. Primitive contractile systems. Cilia and flagella
Unicellular
organisms such as the paramecium, a protozoan that lives
in freshwater ponds and streams, propel themselves by the
action of cilia. Cilia occur in large numbers and move in
a coordinated way. Ciliated cells within the vertebrate
body propel fluid and mucus along interior passages, such
as the lining of the respiratory tract.
Flagella
are structurally similar to cilia, except that they are
longer (sometimes up to 50 times longer) than cilia and
usually number only one or two per cell. Sperm cells of
most higher organisms move using flagella. Many types of
unicellular algae and protozoans use flagella in swimming
through the water.
Both
cilia and flagella contain a regular pattern of tubules
extending along their lengths; there is an outer ring of
nine pairs of tubules surrounding a central pair of tubules.
Each tubule is composed of filaments comprising a string
of globular subunits. The movement of a cilium or a flagellum
requires energy, which is obtained from the breakdown of
adenosine triphosphate (ATP), catalyzed by a protein attached
to the outer tubules, dynein.
Some
types of bacteria have flagella whose motion seems to depend
on a cellular particle called the basal body, to which the
flagellum is attached. Such flagella derive their energy
from a difference in hydrogen ion concentration across the
cell membrane.
