Organization of the Body
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.
Te elements with the largest diameter,
būlz), are hollow tubes made of spherical
protein subunits called
(Figure 3.23c). Most micro-
tubules radiate from a small region of cytoplasm near the nu-
cleus called the
(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
others) continually move and reposition the organelles along
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
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,
to generate contractile forces in a cell
. 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.
insoluble protein ﬁbers that resemble woven ropes. Made of
twisted units of
, 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
Strands made of spherical protein
subunits called actins
Tough, insoluble protein fibers
constructed like woven ropes
composed of tetramer (4) fibrils
Hollow tubes of spherical protein
subunits called tubulins
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.
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.