Chapter 3
Cells: The Living Units
87
3
apparatus can become part of the plasma membrane, secretory
vesicles, or lysosomes.
Check Your Understanding
17.
Which organelle is the major site of ATP synthesis?
18.
What are three organelles involved in protein synthesis and
how do these organelles interact in that process?
19.
Compare the functions of lysosomes and peroxisomes.
For answers, see Appendix H.
Cytoskeleton
Name and describe the structure and function of
cytoskeletal elements.
Te
cytoskeleton
, literally, “cell skeleton,” is an elaborate net-
work of rods running through the cytosol and hundreds of ac-
cessory proteins that link these rods to other cell structures. It
acts as a cell’s “bones,” “muscles,” and “ligaments” by supporting
cellular structures and providing the machinery to generate var-
ious cell movements. Te three types of rods in the cytoskeleton
are
microfilaments
,
intermediate filaments
, and
microtubules
.
None of these is membrane covered.
Microfilaments
Te thinnest elements of the cytoskeleton,
microfilaments
(mi
0
kro-fil
9
ah-ments), are semiflexible strands
of the protein
actin
(“ray”)
(Figure 3.23a)
. Each cell has its
own unique arrangement of microfilaments, so no two cells are
alike. However, nearly all cells have a fairly dense cross-linked
network of microfilaments, called the
terminal web
, attached to
“pumps,” which are A±Pases that gather hydrogen ions from
the surrounding cytosol to maintain the organelle’s acidic pH.
Second, it retains the dangerous acid hydrolases while permit-
ting the final products of digestion to escape so that they can be
used by the cell or excreted. In this way, lysosomes provide sites
where digestion can proceed
safely
within a cell.
Lysosomes function as a cell’s “demolition crew” by
Digesting particles taken in by endocytosis, particularly in-
gested bacteria, viruses, and toxins
Degrading worn-out or nonfunctional organelles
Performing metabolic functions, such as glycogen break-
down and release
Breaking down nonuseful tissues, such as the webs between
the fingers and toes of a developing fetus and the uterine lin-
ing during menstruation
Breaking down bone to release calcium ions into the blood
Te lysosomal membrane is ordinarily quite stable, but it be-
comes fragile when the cell is injured or deprived of oxygen and
when excessive amounts of vitamin A are present. When lyso-
somes rupture, the cell digests itself, a process called
autolysis
(aw
0
tol
9
ĭ-sis). Autolysis is the basis for desirable destruction of
cells, as in the fourth list item above.
Homeostatic Imbalance
3.4
Lysosomes degrade glycogen and certain lipids in the brain at a
relatively constant rate. In
Tay-Sachs disease
, an inherited con-
dition seen mostly in Jews from Central Europe, the lysosomes
lack an enzyme needed to break down a glycolipid abundant
in nerve cell membranes. As a result, the nerve cell lysosomes
swell with undigested lipids, which interfere with nervous sys-
tem functioning. Affected infants typically have doll-like fea-
tures and pink translucent skin. At 3 to 6 months of age, the first
signs of disease appear (listlessness, motor weakness). Tese
symptoms progress to mental retardation, seizures, blindness,
and ultimately death within 18 months.
Golgi
apparatus
Transport
vesicle
Plasma
membrane
Secretory
vesicle
Smooth ER
Rough ER
Nuclear
envelope
Lysosome
Nucleus
Figure 3.22
The endomembrane system.
The Endomembrane System
Te
endomembrane system
is a system of organelles (most de-
scribed above) that work together mainly to (1) produce, de-
grade, store, and export biological molecules, and (2) degrade
potentially harmful substances. It includes the ER, Golgi ap-
paratus, secretory vesicles, and lysosomes, as well as the nu-
clear membrane—that is, all of the membranous organelles or
elements that are either structurally continuous or arise via
forming or fusing transport vesicles
(Figure 3.22)
. Tere are
continuities between the nuclear envelope (itself an extension of
the rough ER) and the rough and smooth ER (Figure 3.18). Te
plasma membrane, though not actually an
endo
membrane, is
also functionally part of this system.
Besides these direct structural relationships, a wide variety
of indirect interactions (indicated by arrows in Figure 3.22) oc-
cur among the members of the system. Some of the vesicles
“born” in the ER migrate to and fuse with the Golgi apparatus
or the plasma membrane, and vesicles arising from the Golgi
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