54
Chapter 3
in the regions of cells subject to mechanical stress (for exam-
ple, in association with desmosomes).
Microtubules
are hollow tubes about 25 nm in diam-
eter, whose subunits are composed of the protein
tubulin.
They are the most rigid of the cytoskeletal fi laments and are
present in the long processes of nerve cells, where they provide
the framework that maintains the processes’ cylindrical shape.
Microtubules also radiate from a region of the cell known as the
centrosome,
which surrounds two small, cylindrical bodies
called
centrioles,
composed of nine sets of fused microtubules.
The centrosome is a cloud of amorphous material that regu-
lates the formation and elongation of microtubules. During
cell division the centrosome generates the microtubular spin-
dle fi
bers used in chromosome separation. Microtubules and
microfi laments have also been implicated in the movements of
organelles within the cytoplasm. These fi brous elements form
tracks, and organelles are propelled along these tracks by con-
tractile proteins attached to the surface of the organelles.
Cilia,
the hairlike motile extensions on the surfaces of
some epithelial cells, have a central core of microtubules orga-
nized in a pattern similar to that found in the centrioles. These
microtubules, in combination with a contractile protein, pro-
duce movements of the cilia. In hollow organs lined with cili-
ated epithelium, the cilia wave back and forth, propelling the
luminal contents along the surface of the epithelium.
SECTION A SUMMARY
Microscopic Observations of Cells
I. All living matter is composed of cells.
II. There are two types of cells: prokaryotic cells (bacteria) and
eukaryotic cells (plant and animal cells).
Membranes
I. Every cell is surrounded by a plasma membrane.
II. Within each eukaryotic cell are numerous membrane-bound
compartments, nonmembranous particles, and fi laments,
known collectively as cell organelles.
III. A cell is divided into two regions, the nucleus and the
cytoplasm. The latter is composed of the cytosol and cell
organelles other than the nucleus.
IV. The membranes that surround the cell and cell organelles
regulate the movements of molecules and ions into and out of
the cell and its compartments.
a. Membranes consist of a bimolecular lipid layer, composed of
phospholipids with embedded proteins.
b. Integral membrane proteins are amphipathic proteins that
often span the membrane, whereas peripheral membrane
proteins are confi ned to the surfaces of the membrane.
V. Three types of membrane junctions link adjacent cells.
a. Desmosomes link cells that are subject to considerable
stretching.
b. Tight junctions, found primarily in epithelial cells, limit
the passage of molecules through the extracellular space
between the cells.
c. Gap junctions form channels between the cytosols of
adjacent cells.
Cell Organelles
I. The nucleus transmits and expresses genetic information.
a. Threads of chromatin, composed of DNA and protein,
condense to form chromosomes when a cell divides.
b. Ribosomal subunits are assembled in the nucleolus.
II. Ribosomes, composed of RNA and protein, are the sites of
protein synthesis.
III. The endoplasmic reticulum is a network of fl attened sacs and
tubules in the cytoplasm.
a. Rough endoplasmic reticulum has attached ribosomes and
is primarily involved in the packaging of proteins to be
secreted by the cell or distributed to other organelles.
b. Smooth endoplasmic reticulum is tubular, lacks ribosomes,
and is the site of lipid synthesis and calcium accumulation
and release.
IV. The Golgi apparatus modifi es and sorts the proteins that are
synthesized on the rough or granular endoplasmic reticulum
and packages them into secretory vesicles.
V. Endosomes are membrane-bound vesicles that fuse with
vesicles derived from the plasma membrane and bud off vesicles
that travel to other cell organelles.
Cytoskeletal filaments
Diameter (nm)
Protein subunit
Microfilament
7
Actin
Intermediate filament
10
Several proteins
Microtubule
25
Tubulin
Figure 3–15
Cytoskeletal fi laments associated with cell shape and motility.
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