66
UNIT 1
Organization of the Body
3
Homeostatic Imbalance
3.1
Definite changes occur in the glycocalyx of a cell that is be-
coming cancerous. In fact, a cancer cell’s glycocalyx may change
almost continuously, allowing it to keep ahead of immune sys-
tem recognition mechanisms and avoid destruction. (Cancer is
discussed on pp. 145–146.)
Check Your Understanding
3.
What basic structure do all cellular membranes share?
4.
Why do phospholipids, which form the greater part of
membranes, organize into a bilayer—tail-to-tail—in a watery
environment?
5.
What is the importance of the glycocalyx in cell interactions?
For answers, see Appendix H.
Cell Junctions
Although certain cell types—blood cells, sperm cells, and some
immune system cells—are “footloose” in the body, many other
types are knit into tight communities. Typically, three factors act
to bind cells together:
Glycoproteins in the glycocalyx act as an adhesive.
Wavy contours of the membranes of adjacent cells fit to-
gether in a tongue-and-groove fashion.
Special cell junctions form
(Figure 3.5)
.
Because junctions are the most important factor securing cells
together, let’s look more closely at the various types.
Tight Junctions
In a
tight junction
, a series of integral protein molecules in the
plasma membranes of adjacent cells fuse together, forming an
impermeable junction
that encircles the cell (Figure 3.5a). Tight
junctions help prevent molecules from passing through the extra-
cellular space between adjacent cells. For example, tight junctions
between epithelial cells lining the digestive tract keep digestive en-
zymes and microorganisms in the intestine from seeping into the
bloodstream. (Although called “impermeable” junctions, some
tight junctions are leaky and may allow certain ions to pass.)
Desmosomes
Desmosomes
(des
9
muh-sōmz; “binding bodies”) are
anchoring
junctions
—mechanical couplings scattered like rivets along the sides
of abutting cells to prevent their separation (Figure 3.5b). On the cy-
toplasmic face of each plasma membrane is a buttonlike thickening
called a
plaque
. Adjacent cells are held together by thin linker protein
filaments (cadherins) that extend from the plaques and fit together
like the teeth of a zipper in the intercellular space. ±icker keratin
filaments (intermediate filaments, which form part of the cytoskele-
ton) extend from the cytoplasmic side of the plaque across the width
of the cell to anchor to the plaque on the cell’s opposite side. In this
way, desmosomes bind neighboring cells together and also contrib-
ute to a continuous internal network of strong “guy-wires.”
±is arrangement distributes tension throughout a cellular
sheet and reduces the chance of tearing when it is subjected to
ATP
A pr
ot
ein (left) that spans the membrane
may pr
o
vide a hydrophilic channel across
the membrane that is selective for a
particular solut
e.
Some transport pr
ot
eins (right) hydrol
yz
e
A
TP as an energy source
to
acti
ve
ly pump
substances across the membrane
.
A membrane protein exposed to the
outside of the cell may have a binding site
that fits the shape of a specific chemical
messenger, such as a hormone.
When bound, the chemical messenger may
cause a change in shape in the protein that
initiates a chain of chemical reactions in the
cell.
Elements of the cytoskeleton (cell’s internal
supports) and the extracellular matrix
(fibers and other substances outside the
cell) may anchor to membrane proteins,
which helps maintain cell shape and fix the
location of certain membrane proteins.
Others play a role in cell movement or bind
adjacent cells together.
A membrane protein may be an enzyme
with its active site exposed to substances in
the adjacent solution.
A team of several enzymes in a membrane
may catalyze sequential steps of a
metabolic pathway as indicated (left to
right) here.
Membrane proteins of adjacent cells may
be hooked together in various kinds of
intercellular junctions.
Some membrane proteins (cell adhesion
molecules or CAMs) of this group provide
temporary binding sites that guide cell
migration and other cell-to-cell interactions.
Some glycoproteins (proteins bonded to
short chains of sugars) serve as
identification tags that are specifically
recognized by other cells.
CAMs
(f) Cell-cell recognition
(e) Intercellular joining
G
lycopr
otein
(d) Enzymatic activity
(c) Attachment to the cytoskeleton and
extracellular matrix
(b) Receptors for signal transduction
(a) Transport
Signal
Enzymes
R
eceptor
Figure 3.4
Membrane proteins perform
many tasks.
A single protein may perform a
combination of these functions.
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