Chapter 3
Cells: The Living Units
91
3
21.
Of microfilaments, microtubules, or intermediate filaments,
which is most important in maintaining cell shape?
22.
The major function of cilia is to move substances across the
free cell surface. What is the major role of microvilli?
For answers, see Appendix H.
The Nucleus
Outline the structure and function of the nuclear envelope,
nucleolus, and chromatin.
Anything that works, works best when it is controlled. For
cells, the control center is the gene-containing
nucleus
(
nu-
cle
5
pit, kernel). Te nucleus can be compared to a com-
puter, design department, construction boss, and board of
directors—all rolled into one. As the genetic library, it con-
tains the instructions needed to build nearly all the body’s
proteins. Additionally, it dictates the kinds and amounts of
proteins to be synthesized at any one time in response to
signals acting on the cell.
Most cells have only one nucleus, but some, including skel-
etal muscle cells, bone destruction cells, and some liver cells,
are
multinucleate
(mul
0
tĭ-nu
9
kle-āt), that is, they have many
nuclei. Te presence of more than one nucleus usually signifies
that a larger-than-usual cytoplasmic mass must be regulated.
Except for mature red blood cells, whose nuclei are ejected
before the cells enter the bloodstream, all of our body cells are
nucleated.
Anucleate
(a-nu
9
kle-āt;
a
5
without) cells cannot
reproduce and therefore live in the bloodstream for only three
to four months before they deteriorate. Without a nucleus, a cell
cannot produce mRNA to make proteins, and when its enzymes
and cell structures start to break down (as all eventually do),
they cannot be replaced.
Te nucleus, averaging 5 μm in diameter, is larger than any
of the cytoplasmic organelles. Although most o±en spherical
or oval, its shape usually conforms to the shape of the cell.
Te nucleus has three recognizable regions or structures:
the
nuclear envelope
(
membrane
),
nucleoli
, and
chromatin
(Figure 3.29a)
.
Te motile cilia just discussed are familiar to most biology
students, but many have not heard of
primary cilia
, their non-
motile cousins. Present as a single cilium on the surface of most
body cells, primary cilia function as antennae that probe the ex-
ternal environment for molecules their receptors can recognize.
Because of this ability, primary cilia can coordinate several in-
tracellular pathways that regulate embryonic development and
maintain healthy tissues later in life.
Microvilli
Microvilli
(mi
0
kro-vil
9
i; “little shaggy hairs”) are minute, fin-
gerlike extensions of the plasma membrane that project from
an exposed cell surface (Figure 3.5 top and
Figure 3.28
). Tey
increase the plasma membrane surface area tremendously and
are most o±en found on the surface of absorptive cells such as
intestinal and kidney tubule cells. Microvilli have a core of bun-
dled actin filaments that extend into the so-called
terminal web
of the cytoskeleton of the cell. Actin is sometimes a contractile
protein, but in microvilli it appears to function as a mechanical
“stiffener.”
Check Your Understanding
20.
How are microtubules and microfilaments related
functionally?
(a) Phases of ciliary motion.
(b)
Power, or
propulsive, stroke
Layer of mucus
Cell surface
Recovery stroke, when cilium
is returning to its initial position
1
2
3
4
5
6
7
Traveling wave created by the activity of
many cilia acting together propels mucus
across cell surfaces.
Figure 3.27
Ciliary function.
Microvillus
Actin
filaments
Terminal
web
Figure 3.28
Microvilli.
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