90
UNIT 1
Organization of the Body
3
triplets
). Additionally, the cilium has flexible “wagon wheels” of
cross-linking proteins (purple in Figure 3.26b), and motor pro-
teins (green dynein arms in Figure 3.26b) that promote move-
ment of the cilium or flagellum.
Just how ciliary activity is coordinated is not fully under-
stood, but microtubules are definitely involved. Extending from
the microtubule doublets are arms composed of the motor pro-
tein dynein (Figure 3.26). Te dynein side arms of one doublet
grip the adjacent doublet, and powered by A±P, push it up, re-
lease, and then grip again. Because the doublets are physically
restricted by other proteins, they cannot slide far and instead are
forced to bend. Te collective bending action of all the doublets
causes the cilium to bend.
As a cilium moves, it alternates rhythmically between a pro-
pulsive
power stroke
, when it is nearly straight and moves in an
arc, and a
recovery stroke
, when it bends and returns to its ini-
tial position
(Figure 3.27a)
. With these two strokes, the cilium
produces a pushing motion in a single direction that repeats
some 10 to 20 times per second. Te bending of one cilium is
quickly followed by the bending of the next and then the next,
creating a current at the cell surface that brings to mind the
traveling waves that pass across a field of grass on a windy day
(Figure 3.27b).
Cilia and Flagella
Cilia
(sil
9
e-ah; “eyelashes”) are whiplike, motile cellular exten-
sions
(Figure 3.26)
that occur, typically in large numbers, on
the exposed surfaces of certain cells. Ciliary action moves sub-
stances in one direction across cell surfaces. For example, cili-
ated cells that line the respiratory tract propel mucus laden with
dust particles and bacteria upward away from the lungs.
When a cell is about to form cilia, the centrioles multiply
and line up beneath the plasma membrane at the cell’s free (ex-
posed) surface. Microtubules then “sprout” from each centri-
ole, forming the ciliary projections by exerting pressure on the
plasma membrane.
Flagella
(flah-jel
9
ah) are also projections formed by centri-
oles, but are substantially longer than cilia. Te only flagellated
cell in the human body is a sperm, which has one propulsive
flagellum, commonly called a tail. Notice that cilia
propel other
substances
across a cell’s surface, whereas a flagellum
propels the
cell itself
.
Centrioles forming the bases of cilia and flagella are com-
monly referred to as
basal bodies
(ba
9
sal) (Figure 3.26a). Te
“9
1
2” pattern of microtubules in the cilium or flagellum itself
(nine
doublets
, or pairs, of microtubules encircling one central
pair) differs slightly from that of a centriole (nine microtubule
Plasma
membrane
Outer microtubule
doublet
Dynein arms
Central
microtubule
Radial spoke
Radial spoke
TEM
TEM
Triplet
Basal body
(centriole)
Cilium
Microtubules
Plasma
membrane
Basal body
Cross-linking
proteins between
outer doublets
Cross-linking
proteins between
outer doublets
A longitudinal section of a
cilium
shows microtubules
running the length of the
structure.
The doublets also
have attached
motor proteins,
the
dynein arms.
The outer
microtubule
doublets and the
two central
microtubules are
held together by
cross-linking
proteins and
radial spokes.
A cross section through the
basal body
. The nine outer
doublets of a cilium extend into
a basal body where each
doublet joins another
microtubule to form a ring of
nine triplets.
A cross section through the
cilium
shows the ”9 + 2”
arrangement of microtubules.
TEM
(a)
(b)
Figure 3.26
Structure of a cilium.
(TEM
5
transmission electron micrograph.)
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