Cells: The Living Units
a pinwheel array of nine
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.
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
. Te centrosome acts as a
. It has few distinguishing marks other than a granular-
that contains paired
, small, bar-
rel-shaped organelles oriented at right angles to each other
. 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
(microtubules or microfilaments)
(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.
Microtubules and microﬁlaments function in
cell motility by interacting with motor molecules
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.
micrograph showing a cross section of a centriole (190,000
Notice that it is composed of nine microtubule triplets.