Biology (Holt)

The Cytoskeleton
The cytoskeleton provides the inte-
rior framework of an animal cell,
much as your skeleton provides the
interior framework of your body. The
cytoskeleton is composed of an intri-
cate network of protein fibers
anchored to the inside of the plasma
membrane. By linking one region to
another, they support the shape of
the cell, much as steel beams anchor
the sides of a building to one another.
Other fibers attach the nucleus and
other organelles to fixed locations in
the cell. Because protein fibers are
too small for a light microscope to
reveal, biologists visualize the
cytoskeleton by attaching fluorescent dyes to antibodies. An anti-
body is an immune system protein specialized to bind to one
particular kind of molecule—in this case—cytoskeleton proteins.
When the cell is examined under fluorescent light, the fibers glow
because of the fluorescent antibody attached to them.
There are three different kinds of cytoskeleton fibers: (1) long,
slender microfilaments made of the protein actin, (2) hollow tubes
called microtubulesmade of the protein tubulin, and (3) thick ropes
of protein called intermediate fibers.

Actin FibersThe actin fibers of the cytoskeleton form a network
just beneath the cell surface that is anchored to membrane proteins
embedded within the cell membrane. By contracting or expanding,
the actin fibers play a major role in determining the shape of ani-
mal cells by pulling the plasma membrane in some places and
pushing it out in others. If you examine the surface of a protist such
as the one shown in Figure 10,you will find it alive with motion.
Tiny projections extend out from the surface like fingers. Each is a
temporary projection of the plasma membrane that shoots out and
then retracts.

MicrotubulesMicrotubules within the cytoskeleton act as a highway
system for the transportation of information from the nucleus to
different parts of the cell. RNA molecules are transported along
microtubular “rails” that extend through the interior of the cell like
train tracks. The RNA molecules, in complexes with proteins, are
attached to so-called motor proteins that chug along microtubules
like locomotives on tracks. The motor proteins drag the RNA-protein
complexes along with them like freight cars.

Intermediate FibersThe intermediate fibers of the cytoskeleton
provide a frame on which ribosomes and enzymes can be confined
to particular regions of the cell. The cell can organize complex
metabolic activities efficiently by anchoring particular enzymes
near one another.

SECTION 2 Cell Features 59

Microtubules

Nucleus

Endoplasmic

reticulum

Mitochondrion

Ribosomes

Figure 9 The cytoskeleton.

The cytoskeleton’s network of

protein fibers anchors cells

organelles and other compo-

nents of the cytoplasm.

Figure 10 Cytoskeletal

projections.The multiple

spikes on the surface of this

marine amoeba are projections

of the cytoskeleton stretching

the cell membrane outward.