Page 215 - Color Atlas of Biochemistry

206 Organelles

Structure and functions B. Microtubules
Only the cell’s microtubules are shown here.
The cytoskeleton carries out three major
tasks: They radiate out in all directions from a center
near the nucleus, the centrosome.The tube-
• It represents the cell’s mechanical scaffold- shaped microtubules are constantly being
ing, which gives it its typical shape and synthesized and broken down at their (+)
connects membranes and organelles to ends. In the centriole, the (–) end is blocked
each other. This scaffolding has dynamic by associated proteins (see p. 204). The (+)
properties; it is constantly being synthe- end can also be stabilized by associated pro-
sized and brokendowntomeet the cell’s teins—e. g., when the microtubules have
requirements and changing conditions. reached the cytoplasmic membrane.
• It acts as the motor for movement of animal The microtubules are involved in defining
cells. Not only muscle cells (see p. 332), but theshape of thecelland also serveas guiding
also cells of noncontractile tissues contain tracks for the transport of organelles. To-
many different motor proteins, which they gether with associated proteins (dynein, kine-
use to achieve coordinated and directed sin), microtubules areableto carry outme-
movement. Cell movement, shape changes chanical work—e. g., during the transport of
during growth, cytoplasmic streaming, and mitochondria, the movement of cilia (hair-
cell division are all made possible by com- like cell protrusions in the lungs, intestinal
ponents of the cytoskeleton. epithelium, and oviduct) and the beating of
• It serves as a transport track within the cell. the flagella of sperm. Microtubules also play a
Organelles and other large protein com- special role in the mitotic period of cell divi-
plexes can move along the filaments with sion (see p. 394).
the help of the motor proteins.
C. Architecture
A. Microfilaments and intermediate Thecomplex structureand net-like densityof
filaments
the cytoskeleton is illustrated here using
The illustration schematically shows a detail three examples in which the cytoskeletal
of the microvilli of an intestinal epithelial cell components are visualized with the help of
as an example of the structure and function of antibodies.
the components of the cytoskeleton (see also
C1). 1. The border of an intestinal epithelial cell
Microfilaments of Factin traverse the mi- is seen here (see also B). There are micro-
crovilli in ordered bundles. The microfila- filaments (a) passing from the interior of the
ments are attached to each other by actin–as- cell out into the microvilli. The filaments are
firmly held together by spectrin (b), an asso-
sociated proteins, particularly fimbrin and vil-
lin. Calmodulin and a myosin–like ATPase con- ciated protein, and they are anchored to in-
nect the microfilaments laterally to the termediate filaments (c).
2. Only microtubules are seeninthis fibro-
plasma membrane. Fodrin, another microfila-
ment–associated protein, anchors the actin blast cell. They originate from the microtubule
organizing center (centrosome) and radiate
fibers to each other at the base, as well as
attaching them to the cytoplasmic membrane out as far as the plasma membrane.
3. Keratin filaments are visible here in an
and to a network of intermediate filaments. In epithelial cell. Keratin fibers belong to the
this example, the microfilaments have a
mainly static function. In other cases, actin is group of intermediate filaments (see pp. 70,
204; d = nucleus).
also involved in dynamic processes. These in-
clude muscle contraction (see p. 332), cell
movement, phagocytosis by immune cells,
the formation of microspikes and lamellipo-
dia (cellular extensions), and the acrosomal
process during the fusion of sperm with the
egg cell.

Koolman, Color Atlas of Biochemistry, 2nd edition © 2005 Thieme
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