Essential Cell Biology 5th edition

590 CHAPTER 17 Cytoskeleton
Figure 17–24 Many hairlike cilia project
from the surface of the epithelial cells
that line the human respiratory tract. In
this scanning electron micrograph, thick
tufts of cilia can be seen extended from
these ciliated cells, which are interspersed
with the dome-shaped surfaces of
nonciliated epithelial cells. (Reproduced
from R.G. Kessel and R.H. Kardon, Tissues
and Organs. San Francisco: W.H. Freeman
& Co., 1979.)
5 µm
the Golgi membranes pull the Golgi apparatus along microtubules in the
opposite direction, inward toward the nucleus (Figure 17–21C). In this
way, the regional differences
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in
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these internal membranes—crucial for
their respective functions—are created and maintained.
When cells are treated with colchicine—a drug that promotes microtubule
disassembly—both the ER and the Golgi apparatus change their
location dramatically. The ER, which is physically connected to the
nuclear envelope, collapses around the nucleus; the Golgi apparatus,
which is not attached to any other organelle, fragments into small vesicles,
which then disperse throughout the cytoplasm. When the colchicine
is removed, the organelles return to their original positions, dragged by
motor proteins moving along the re-formed microtubules.
power stroke
Figure 17–25 A cilium beats by
performing a repetitive cycle of
movements, consisting of a power stroke
followed by a recovery stroke. In the fast
power stroke, the cilium is fully extended
and fluid is driven over the surface of the
cell; in the slower recovery stroke, the
cilium curls back into position with minimal
disturbance to the surrounding fluid. Each
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cycle typically requires 0.1–0.2 second and
generates a force parallel to the cell surface.
Cilia and Flagella Contain Stable Microtubules Moved
by Dynein
We mentioned earlier that many microtubules in cells are stabilized
through their association with other proteins and therefore do not show
dynamic instability. Cells use such stable microtubules as stiff supports in
the construction of a variety of polarized structures, including motile cilia
and flagella. Cilia are hairlike structures about 0.25 μm in diameter, covered
by plasma membrane, that extend from the surface of many kinds
of eukaryotic cells. Each cilium contains a core of stable microtubules,
arranged in a bundle, that grow from a cytoplasmic basal body, which
serves as an organizing center (see Figure 17–11D).
Motile cilia beat in a whiplike fashion, either to move fluid over the surface
of a cell or to propel single cells through a fluid. Some protozoa, for
example, use cilia to collect food particles, and others use them for locomotion.
On the epithelial cells lining the human respiratory tract (Figure
17–24), huge numbers of beating cilia (more than a billion per square
centimeter) sweep layers of mucus containing trapped dust particles and
dead cells up toward the throat, to be swallowed and eventually eliminated
from the body. Similarly, beating cilia on the cells of the oviduct
wall create a current that helps to carry eggs away from the ovary. Each
cilium acts as a small oar, moving in a repeated cycle that generates the
movement of fluid over the cell surface (Figure 17–25).
The flagella (singular flagellum) that propel sperm and many protozoa
are usually very much longer than cilia are. They are designed to move