50
Microtubule
triplet(^) ®
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Figure 3- 12 The structure of a centriole.
called the opposite pole of the cell. They now form a sys-
tem of microtubules, which are long, hollow cylinders
made of a protein called tubulin. These fibers or micro-
tubules redistribute the duplicated chromosomes during cell
division into the appropriate new daughter cells.
Chapter 3
the cell surfaces to bring this material to the throat to be
swallowed and then discharged- from the body.
Although cilia and flagella are similar anatomically, a
flagellum is considerably longer than a cilium. A cell with
cilia will have row on row of cilia, but a cell with fla-gella
will have one (like the sperm cell) or two or four like some
single - celled protozoans.
Externally, these structures are hairlike protru-sions
from the cell membrane. Internally, they are com-posed of
nine double fibrils arranged in a cylindrical ring around two
single, central fibrils (Figure 3-13). The -microtubules or
fibrils of the flagellum arise from a struc-ture called the
basal body found just below the area from which the
flagellum protrudes from the surface of the
Peripheral
doublet^
Link between
doublets^ Plasma^
membrane
Central
singlet
Cilium or
flagellum
Cilia and Flagella
Cilia (SIL-lee-ah) and flagella (fla-JELL-ah) are cellular (^)
organelles located on the cell surface. They are com- (^)
posed of fibrils that protrude from the cell and beat or (^)
vibrate. Some single-celled organisms use these struc- (^)
tures to move through a medium. For example, Euglena (^)
has a flagellum that pulls it through the water, whereas a (^)
Paramecium is covered with row upon row of hundreds (^)
of cilia to allow it to swim in pond water. In the human (^) Plasma (^)
body, the male sperm cell is propelled by a single beat- membrane^
ing flagellum that assists it in reaching the female egg in (^)
the upper part of the fallopian or uterine tube where they (^)
unite in fertilization. Stationary cells, like those that line (^)
our respiratory tract, are covered with cilia on their free (^)
edge to move the mucus-dust package -upward across
Figure 3- 13 The internal anatomy of a cilium or
flagellum. (^)
(^) ®
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