88
UNIT 1
Organization of the Body
3
attach to desmosomes, and their main job is to act as internal
guy-wires to resist pulling forces exerted on the cell. Because the
protein composition of intermediate filaments varies in differ-
ent cell types, there are numerous names for these cytoskeletal
elements—for example, they are called neurofilaments in nerve
cells and keratin filaments in epithelial cells.
Microtubules
Te elements with the largest diameter,
micro-
tubules
(mi
0
kro-tu
9
būlz), are hollow tubes made of spherical
protein subunits called
tubulins
(Figure 3.23c). Most micro-
tubules radiate from a small region of cytoplasm near the nu-
cleus called the
centrosome
or
cell center
(see Figures 3.2, 3.25).
Microtubules are remarkably dynamic organelles, constantly
growing out from the centrosome, disassembling, and then re-
assembling at the same or different sites. Te stiff but bendable
microtubules determine the overall shape of the cell, as well as
the distribution of cellular organelles.
Mitochondria, lysosomes, and secretory vesicles attach to the
microtubules like ornaments hanging from tree branches. ±iny
protein machines called
motor proteins
(
kinesins
,
dyneins
, and
others) continually move and reposition the organelles along
the microtubules.
Motor proteins work by changing their shapes. Powered
by A±P, some motor proteins appear to act like train engines
moving substances along on the microtubular “railroad tracks.”
the cytoplasmic side of their plasma membrane (see Figure 3.28
on p. 91). Te web strengthens the cell surface, resists compres-
sion, and transmits force during cellular movements and shape
changes.
Most microfilaments are involved in cell motility (movement)
or changes in cell shape. You could say that cells move “when
they get their act(in) together.” For example, actin filaments in-
teract with another protein,
unconventional myosin
(mi
9
o-sin),
to generate contractile forces in a cell
(Figure 3.24b)
. Tis inter-
action also forms the cleavage furrow that pinches one cell into
two during cell division. Microfilaments attached to cell adhe-
sion molecules (see Figure 3.4e) are responsible for the crawl-
ing movements of amoeboid motion, and for the membrane
changes that accompany endocytosis and exocytosis. Except
in muscle cells, where they are highly developed, stable, and
long-lived, actin filaments are constantly breaking down and
re-forming from smaller subunits whenever and wherever
their services are needed.
Intermediate Filaments
Intermediate filaments
are tough,
insoluble protein fibers that resemble woven ropes. Made of
twisted units of
tetramer
(4)
fibrils
, they have a diameter be-
tween those of microfilaments and microtubules (Figure 3.23b).
Intermediate filaments are the most stable and permanent of
the cytoskeletal elements and have high tensile strength. Tey
(a)
Microfilaments
Strands made of spherical protein
subunits called actins
(b)
Intermediate filaments
Tough, insoluble protein fibers
constructed like woven ropes
composed of tetramer (4) fibrils
(c)
Microtubules
Hollow tubes of spherical protein
subunits called tubulins
Actin subunit
7 nm
10 nm
25 nm
Tetramer subunits
Tubulin subunits
Microfilaments form the blue network
surrounding the pink nucleus in this photo.
Intermediate filaments form the purple
batlike network in this photo.
Microtubules appear as gold networks
surrounding the cells’ pink nuclei in this photo.
Figure 3.23
Cytoskeletal elements support the cell and help to generate movement.
Diagrams (above) and photos (below). The photos are of fibroblasts treated to fluorescently tag
the structure of interest.
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