Chapter 3
Cells: The Living Units
83
3
It is widely believed that mitochondria arose from bacteria that
invaded the ancient ancestors of plant and animal cells, and that
this unique merger gave rise to all complex cells.
Ribosomes
Ribosomes
(ri
9
bo-sōmz) are small, dark-staining granules
composed of proteins and a variety of RNAs called
ribosomal
RNAs
. Each ribosome has two globular subunits that fit to-
gether like the body and cap of an acorn. Ribosomes are sites
of protein synthesis, a function we discuss in detail later in this
chapter.
Some ribosomes float freely in the cytoplasm. Others are
attached to membranes, forming a complex called the
rough
endoplasmic reticulum
(see p. 84). Tese two ribosomal popula-
tions appear to divide the chore of protein synthesis.
Free ribosomes
float freely in the cytoplasm. Tey make solu-
ble proteins that function in the cytosol, as well as those im-
ported into mitochondria and some other organelles.
Cytoplasmic Organelles
Te organelles (“little organs”) are specialized cellular compart-
ments or structures, each performing its own job to maintain
the life of the cell. Some organelles, the
nonmembranous or-
ganelles
, lack membranes. Examples are the cytoskeleton, cen-
trioles, and ribosomes.
Most organelles, however, are bounded by a membrane
similar in composition to the plasma membrane. Tis mem-
brane enables the
membranous organelles
(such as peroxisomes,
lysosomes, endoplasmic reticulum, and Golgi apparatus) to
maintain an internal environment different from that of the
surrounding cytosol. Tis compartmentalization is crucial to
cell functioning. Without it, thousands of enzymes would be
randomly mixed and biochemical activity would be chaotic.
Besides providing splendid isolation for an organelle, its mem-
brane oFen unites it with the rest of an interactive intracellular
system called the
endomembrane system
(see p. 87), and the lipid
and protein makeup of the membrane allows it to recognize and
interact with other organelles. Now let’s consider what goes on
in each of the workshops of our cellular factory.
Mitochondria
Mitochondria
(mi
0
to-kon
9
dre-ah) are threadlike (
mitos
5
thread) or lozenge-shaped membranous organelles. In living
cells they squirm, elongate, and change shape almost continu-
ously. Tey are the power plants of a cell, providing most of its
A±P supply. Te density of mitochondria in a particular cell
reflects that cell’s energy requirements, and mitochondria gen-
erally cluster where the action is. Busy cells like kidney and liver
cells have hundreds of mitochondria, whereas relatively inactive
cells (such as unchallenged lymphocytes) have just a few.
A mitochondrion is enclosed by
two
membranes, each with
the general structure of the plasma membrane
(Figure 3.17)
.
Te
outer membrane
is smooth and featureless, but the
in-
ner membrane
folds inward, forming shelflike
cristae
(krĭ
9
ste;
“crests”) that protrude into the
matrix
, the gel-like substance
within the mitochondrion. Intermediate products of food fu-
els (glucose and others) are broken down to water and carbon
dioxide by teams of enzymes, some dissolved in the mitochon-
drial matrix and others forming part of the crista membrane.
As the metabolites are broken down and oxidized, some of
the energy released is captured and used to attach phosphate
groups to ADP molecules to form A±P. Tis multistep mito-
chondrial process (described in Chapter 24) is called
aerobic
cellular respiration
(a-er-o
9
bik) because it requires oxygen.
Mitochondria are complex organelles: Tey contain their
own DNA, RNA, and ribosomes and are able to reproduce
themselves. Mitochondrial genes (some 37 of them) direct the
synthesis of 1% of the proteins required for mitochondrial func-
tion, and the DNA of the cell’s nucleus encodes the remaining
proteins needed to carry out cellular respiration. When cellular
requirements for A±P increase, the mitochondria synthesize
more cristae or simply pinch in half (a process called
fission
) to
increase their number, then grow to their former size.
Intriguingly, mitochondria are similar to bacteria in the pur-
ple bacteria phylum, and mitochondrial DNA is bacteria-like.
Enzymes
Matrix
Cristae
Mitochondrial
DNA
Ribosome
Outer
mitochondrial
membrane
Inner
mitochondrial
membrane
(b)
(a)
(c)
Figure 3.17
Mitochondrion.
(a)
Diagram of a longitudinally
sectioned mitochondrion.
(b)
Close-up of a crista showing enzymes
(stalked particles).
(c)
Electron micrograph of a mitochondrion
(50,000
3
).
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