swimming movements (
f g. 3.16
and chapter 22, p. 837).
It is the only known ﬂ
agellum in humans.
Two types of
threadlike structures in the cytoplasm are microF laments
and microtubules. They are distinguished by protein type,
diameter, and how they assemble. Other proteins connect
these components, creating the cytoskeleton that provides
strength and the ability to resist force and maintain shape.
±or example, this action propels mucus over the lining
of the respiratory tract
(f g. 3.15)
. Chemicals in cigarette
smoke destroy cilia, which impairs the respiratory tract’s
ability to expel bacteria. Infection may result.
A ﬂ agellum is much longer than a cilium, and
a cell usually has only one. A ﬂ
agellum begins its
characteristic undulating, wavelike motion at its base.
The tail of a sperm cell is a ﬂ agellum that generates
Layer of mucus
Cilia are sweeping hairlike extensions. (
) They fringe certain cells, such as those forming the inner lining of the respiratory tract
) Cilia have a power stroke and a recovery stroke that create a “to-and-fro” movement that sweeps F
uids across the tissue surface.
Centrioles are built of microtubules and form
the spindle ±
bers that pull apart chromosome sets as cells divide.
) A transmission electron micrograph of the two centrioles in a
centrosome (120,000×). (
) The centrioles lie at right angles to one