membranes, that have specific functions in cellular
metabolism. They are also shown in Fig. 3–2.
The endoplasmic reticulum(ER) is an extensive
network of membranous tubules that extend from
the nuclear membrane to the cell membrane. Rough
ER has numerous ribosomes on its surface, whereas
smooth ER has no ribosomes. As a network of inter-
connected tunnels, the ER is a passageway for the
transport of the materials necessary for cell function
within the cell. These include proteins synthesized by
the ribosomes on the rough ER, and lipids synthesized
by the smooth ER.
Ribosomesare very small structures made of pro-
tein and ribosomal RNA. Some are found on the sur-
face of rough ER, while others float freely within the
cytoplasm. Ribosomes are the site of protein synthesis.
The proteins produced may be structural proteins
such as collagen in the skin, enzymes, or hormones
such as insulin that regulate cellular processes. These
proteins may function within the cell or be secreted
from the cell to be used elsewhere in the body.
Our protein molecules are subject to damage, and
some cellular proteins, especially regulatory proteins,
may be needed just for a very short time. All such
proteins must be destroyed, and this is the function
of proteasomes. A proteasomeis a barrel-shaped
organelle made of enzymes that cut protein molecules
apart (protease enzymes). Proteins that are to be des-
troyed, that is, those no longer needed or those that
are damaged or misfolded, are tagged by a protein
called ubiquitin (sort of a cellular mop or broom) and
carried into a proteasome. The protein is snipped
into peptides or amino acids, which may be used again
for protein synthesis on ribosomes. Proteasomes are
particularly important during cell division and during
embryonic development, when great changes are tak-
ing place very rapidly as cells become specialized.
Many of our cells have secretory functions, that is,
they produce specific products to be used elsewhere in
tissues. Secretion is one task of the Golgi apparatus,
a series of flat, membranous sacs, somewhat like
a stack of saucers. Carbohydrates are synthesized
within the Golgi apparatus, and are packaged, along
with other materials, for secretion from the cell.
Proteins from the ribosomes or lipids from the
smooth endoplasmic reticulum may also be secreted in
this way. To secrete a substance, small sacs of the
Golgi membrane break off and fuse with the cell
membrane, releasing the substance to the exterior of
the cell. This is exocytosis, exomeaning “to go out”
of the cell.
Mitochondria are oval or spherical organelles
bounded by a double membrane. The inner mem-
brane has folds called cristae. Within the mitochon-
dria, the aerobic(oxygen-requiring) reactions of cell
respiration take place. Therefore, mitochondria are
the site of ATP (and hence energy) production. Cells
that require large amounts of ATP, such as muscle
cells, have many mitochondria to meet their need for
energy. Mitochondria contain their own genes in a
single DNA molecule and duplicate themselves when
a cell divides. An individual’s mitochondrial DNA
(mDNA) is of maternal origin, that is, from the mito-
chondria that were present in the ovum, or egg cell,
which was then fertilized by a sperm cell. The mito-
chondria of the sperm cell usually do not enter the
ovum during fertilization, because they are not found
in the head of the sperm with the chromosomes (see
Fig. 20–1).
Lysosomesare single-membrane structures that
contain digestive enzymes. When certain white blood
cells engulf bacteria, the bacteria are digested and
destroyed by these lysosomal enzymes. Worn-out cell
parts and dead cells are also digested by these enzymes.
This is a beneficial process, and is necessary before tis-
sue repair can begin. But it does have a disadvantage in
that lysosomal digestion contributes to inflammation
in damaged tissues. An excess of inflammation can
start a vicious cycle, actually a positive feedback mech-
anism, that results in extensive tissue damage.
Many of our cells are capable of dividing, or repro-
ducing, themselves. Centrioles are a pair of rod-
shaped structures perpendicular to one another,
located just outside the nucleus. Their function is to
organize the spindle fibers during cell division. The
spindle fibers are contracting proteins that pull the
two sets of chromosomes apart, toward the ends of the
original cell as it divides into two new cells. Each new
cell then has a full set of chromosomes.
Ciliaand flagellaare mobile thread-like projec-
tions through the cell membrane; each is anchored by
a basal body just within the membrane. Ciliaare usu-
ally shorter than flagella, and an individual cell has
many of them on its free surface. The cilia of a cell
beat in unison and sweep materials across the cell sur-
face. Cells lining the fallopian tubes, for example, have
cilia to sweep the egg cell toward the uterus. The only
human cell with a flagellumis the sperm cell. The fla-
gellum provides motility, or movement, for the sperm
cell.
Microvilliare folds of the cell membrane on the
free surface of a cell. These folds greatly increase theCells 51