83
CHAPTER THREE
Cells
smooth ER and are added to proteins arriving from the
rough ER. Smooth ER is especially abundant in liver
cells that break down alcohol and drugs.
3.
Vesicles.
Vesicles (ves
ı˘-kelz) are membranous sacs
that vary in size and contents. They may form when a
portion of the cell membrane folds inward and pinches
off. As a result, a tiny, bubblelike vesicle, containing
some liquid or solid material formerly outside the cell,
enters the cytoplasm. The Golgi apparatus and ER
also form vesicles. Fleets of vesicles transport many
substances into and out of cells in a process called
vesicle traf±
cking.
4.
Golgi apparatus.
A Golgi apparatus (gol
je ap
ah-ra
tus)
is a stack of half a dozen or so fl attened, membranous
sacs called
cisternae.
This organelle re± nes, packages, and
delivers proteins synthesized on the rough ER
(f g. 3.10)
.
Proteins arrive at the Golgi apparatus enclosed
in tiny vesicles composed of membrane from the ER.
These sacs fuse to the membrane at the innermost end
of the Golgi apparatus, specialized to receive proteins.
Previously, in the ER sugar molecules were attached to
these protein molecules, forming glycoproteins.
As the glycoproteins pass from layer to layer
through the Golgi stacks, they are modi±
ed chemically.
For example, sugar molecules may be added or removed
from them. When the altered glycoproteins reach the
outermost layer, they are packaged in bits of Golgi
apparatus membrane that bud off and form transport
vesicles. Such a vesicle may then move to the cell
membrane, where it fuses and releases its contents to
the outside of the cell as a secretion. This is an example
of a process called exocytosis (see page 98). Other
vesicles may transport glycoproteins to organelles in
the cell, as
f
gure 3.11
shows for the process of milk
secretion.
Some cells, including certain liver cells and white
blood cells (lymphocytes), secrete glycoprotein molecules
as rapidly as they are synthesized. However, certain other
cells, such as those that manufacture protein hormones,
release vesicles containing newly synthesized molecules
only when the cells are stimulated. Otherwise, the loaded
vesicles remain in the cytoplasm. (Chapter 13, p. 493
discusses hormone secretion.)
Secretory vesicles that originate in the ER not only
release substances outside the cell, but also provide new
cell membrane. This is especially important during cell
growth.
5.
Mitochondria.
Mitochondria (mi
to-kon
dre-ah) are
elongated, fl
uid-± lled sacs 2–5 µm long. They often
move slowly in the cytoplasm and can divide. A
mitochondrion contains a small amount of DNA that
encodes information for making a few types of proteins
and specialized RNA. However, most proteins used in
mitochondrial functions are encoded in the DNA of the
nucleus. These proteins are synthesized elsewhere in
the cell and then enter the mitochondria.
elongated canals, and fl
uid-±
lled vesicles
(f
g. 3.9)
.
These parts are interconnected, and they interact with
the cell membrane, the nuclear envelope, and certain
other organelles. ER is widespread in the cytoplasm,
providing a tubular transport system for molecules
throughout the cell.
The ER participates in the synthesis of protein and
lipid molecules. These molecules may leave the cell as
secretions or be used within the cell for such functions as
producing new ER or cell membrane as the cell grows.
The outer membranous surface of some ER is
studded with many ribosomes that give the ER a
textured appearance when viewed with an electron
microscope. Such endoplasmic reticulum is termed
rough ER.
The ribosomes of rough ER are sites of protein
synthesis. The proteins then move through the tubules
of the endoplasmic reticulum to the Golgi apparatus for
further processing.
ER that lacks ribosomes is called
smooth ER
g.
3.9). It contains enzymes important in synthesizing
lipids, absorbing fats from the digestive tract, and
breaking down drugs. Lipids are synthesized in the
Attachment
(rolling)
Adhesion
White blood cell
Integrin
Blood vessel
lining cell
Carbohydrates
on capillary wall
Selectin
Exit
Adhesion
receptor proteins
Splinter
FIGURE 3.8
Cellular adhesion molecules (CAMs) direct white blood
cells to injury sites, such as this splinter.
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