Chapter 3
Cells: The Living Units
89
3
a pinwheel array of nine
triplets
of microtubules, each con-
nected to the next by nontubulin proteins and arranged to
form a hollow tube. Centrioles also form the bases of cilia and
flagella, our next topics.
Cellular Extensions
Describe how the two main types of cell extensions, cilia
and microvilli, differ in structure and function.
Others move “hand over hand” somewhat like an orangutan—
gripping, releasing, and then gripping again at a new site further
along the microtubule (Figure 3.24).
Centrosome and Centrioles
Describe the roles of centrioles in cell division and in
formation of cilia and flagella.
As mentioned, microtubules are anchored at one end in an
inconspicuous region near the nucleus called the
centrosome
or
cell center
. Te centrosome acts as a
microtubule organizing
center
. It has few distinguishing marks other than a granular-
looking
matrix
that contains paired
centrioles
, small, bar-
rel-shaped organelles oriented at right angles to each other
(Figure 3.25)
. Te centrosome matrix is best known for
generating microtubules and organizing the mitotic spindle
in cell division (see Figure 3.33). Each centriole consists of
Receptor for
motor molecule
Microtubule
of cytoskeleton
Motor molecule
(ATP powered)
Vesicle
Cytoskeletal elements
(microtubules or microfilaments)
Motor molecule
(ATP powered)
ATP
ATP
(b) In some types of cell motility, motor molecules attached to one
element of the cytoskeleton can cause it to slide over another
element, which the motor molecules grip, release, and grip at a
new site. Muscle contraction and cilia movement work this way.
(a) Motor molecules can attach to receptors on
vesicles or organelles, and carry the organelles
along the microtubule “tracks” of the cytoskeleton.
Figure 3.24
Microtubules and microfilaments function in
cell motility by interacting with motor molecules
powered
by ATP.
Centrosome matrix
(b)
(a)
Centrioles
Microtubules
Figure 3.25
Centrioles.
(a)
Three-dimensional view of a centriole
pair oriented at right angles, as they are usually seen in the cell.
The centrioles are located in an inconspicuous region to one side of
the nucleus called the centrosome, or cell center.
(b)
An electron
micrograph showing a cross section of a centriole (190,000
3
).
Notice that it is composed of nine microtubule triplets.
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