Cellular Structure, Proteins, and Metabolism
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).
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
, 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.
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.
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
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 (
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.
are composed of the contractile protein
and make up a major portion of the cytoskeleton in all
are most extensively developed
Rod- or oval-shaped body
surrounded by two membranes. Inner
membrane folds into matrix of the
mitochondrion, forming cristae.
Major site of ATP production,
utilization, and CO
Contains enzymes active in Krebs cycle
and oxidative phosphorylation.
Lumen of rough
Electron micrograph courtesy of K. R. Porter.