Cellular Structure, Proteins, and Metabolism
53
may also break down cell organelles that have been damaged
and no longer function normally. They play an especially
important role in the various cells that make up the defense
systems of the body (Chapter 18).
Peroxisomes
Like lysosomes,
peroxisomes
are moderately dense oval bod-
ies enclosed by a single membrane. Like mitochondria, peroxi-
somes consume molecular oxygen, although in much smaller
amounts. This oxygen is not used in the transfer of energy
to ATP, however. Instead it undergoes reactions that remove
hydrogen from organic molecules including lipids, alcohol,
and potentially toxic ingested substances. One of the reac-
tion products is hydrogen peroxide, H
2
O
2
, thus the organ-
elle’s name. Hydrogen peroxide can be toxic to cells in high
concentrations, but peroxisomes can also destroy hydrogen
peroxide and thus prevent its toxic effects. Peroxisomes are
also involved in the process by which fatty acids are broken
down into 2-carbon fragments, which the cell can then use as
a source for generating ATP.
Vaults
Vaults
are recently discovered cytoplasmic structures composed
of protein and a type of RNA called vault RNA (vRNA). These
tiny structures have been described as barrel-shaped, but also as
resembling the vaulted cathedrals found in many large buildings,
thus their name. Although the functions of vaults are not cer-
tain, studies using electron microscopy and other methods have
revealed that vaults tend to be associated with nuclear pores. This
has led to the hypothesis that vaults are important for transport
of molecules between the cytosol and the nucleus. In addition,
at least one vault protein is believed to function in regulating a
cell’s sensitivity to certain drugs. For example, increased expres-
sion of this vault protein has been linked in some studies to drug
resistance, including some drugs used in the treatment of cancer.
If true, then vaults may someday provide a target for modulating
the effectiveness of such drugs in human patients.
Cytoskeleton
In addition to the membrane-enclosed organelles, the cytoplasm
of most cells contains a variety of protein fi laments. This fi lamen-
tous network is referred to as the cell’s
cytoskeleton,
and, like
the bony skeleton of the body, it is associated with processes that
maintain and change cell shape and produce cell movements.
There are three classes of cytoskeletal fi laments, based on
their diameter and the types of protein they contain. In order
of size, starting with the thinnest, they are (1) microfi laments,
(2) intermediate fi laments, and (3) microtubules (
Figure 3–15
).
Microfi laments and microtubules can be assembled and dis-
assembled rapidly, allowing a cell to alter these components
of its cytoskeletal framework according to changing require-
ments. In contrast, intermediate fi laments, once assembled,
are less readily disassembled.
Microfi
laments
are composed of the contractile protein
actin,
and make up a major portion of the cytoskeleton in all
cells.
Intermediate fi
laments
are most extensively developed
Cristae
(inner
membrane)
Matrix
Outer
membrane
Structure:
Rod- or oval-shaped body
surrounded by two membranes. Inner
membrane folds into matrix of the
mitochondrion, forming cristae.
Function:
Major site of ATP production,
O
2
utilization, and CO
2
formation.
Contains enzymes active in Krebs cycle
and oxidative phosphorylation.
Mitochondrion
Lumen of rough
endoplasmic reticulum
Figure 3–14
Mitochondrion.
Electron micrograph courtesy of K. R. Porter.
previous page 81 Vander's Human Physiology The Mechanisms of Body Function read online next page 83 Vander's Human Physiology The Mechanisms of Body Function read online Home Toggle text on/off