80
UNIT 1
Organization of the Body
3
plasma membrane “upsets” the resting membrane potential. As
we describe in later chapters, this is a normal means of activat-
ing neurons and muscle cells.
Check Your Understanding
14.
What process establishes the resting membrane potential?
15.
Is the inside of the plasma membrane negative or positive
relative to its outside in a polarized membrane?
For answers, see Appendix H.
The Plasma Membrane:
Cell-Environment Interactions
Describe the role of the glycocalyx when cells interact with
their environment.
List several roles of membrane receptors and that of
voltage-gated membrane channel proteins.
Cells are biological minifactories and, like other factories, they
receive and send orders from and to the outside community.
But
how
does a cell interact with its environment, and
what
ac-
tivates it to carry out its homeostatic functions?
Sometimes cells interact directly with other cells. However,
in many cases cells respond to extracellular chemicals, such as
hormones and neurotransmitters distributed in body fluids.
Cells also interact with extracellular molecules that act as sign-
posts to guide cell migration during development and repair.
Whether cells interact directly or indirectly, however, the
glycocalyx is always involved. Te best-understood glycocalyx
molecules fall into two large families—cell adhesion molecules
and plasma membrane receptors (see Figure 3.4, p. 66). Another
group of membrane proteins, voltage-gated channel proteins,
are important in cells that respond to electrical signals.
Roles of Cell Adhesion Molecules (CAMs)
Tousands of
cell adhesion molecules (CAMs)
are found on
almost every cell in the body. CAMs play key roles in embryonic
development and wound repair (situations where cell mobility
is important) and in immunity. Tese sticky glycoproteins (
cad-
herins
and
integrins
) act as
Te molecular “Velcro” that cells use to anchor themselves
to molecules in the extracellular space and to each other (see
desmosome discussion on pp. 66–67)
Te “arms” that migrating cells use to haul themselves past
one another
SOS signals sticking out from the blood vessel lining that
rally protective white blood cells to a nearby infected or in-
jured area
Mechanical sensors that respond to changes in local tension
or fluid movement at the cell surface by stimulating synthesis
or degradation of adhesive membrane (tight) junctions
±ransmitters of intracellular signals that direct cell migra-
tion, proliferation, and specialization
As more and more K
1
leaves the cell, the negativity of the
inner membrane face becomes great enough to attract K
1
back
toward and even into the cell (Figure 3.15
2
). At a membrane
voltage of
2
90 mV, potassium’s concentration gradient is ex-
actly balanced by the electrical gradient (membrane potential),
and one K
1
enters the cell as one leaves (Figure 3.15
3
).
In many cells, sodium (Na
1
) also contributes to the resting
membrane potential. Sodium is strongly attracted to the cell in-
terior by its concentration gradient, bringing the resting mem-
brane potential to
2
70 mV. However, potassium still largely
determines the resting membrane potential because the mem-
brane is much more permeable to K
1
than to Na
1
. Even though
the membrane is permeable to Cl
2
, in most cells Cl
2
does not
contribute to the resting membrane potential, because its con-
centration and electrical gradients exactly balance each other.
We may be tempted to believe that massive flows of K
1
ions
are needed to generate the resting potential, but this is not the
case. Surprisingly, the number of ions producing the membrane
potential is so small that it does not change ion concentrations
in any significant way.
In a cell at rest, very few ions cross its plasma membrane.
However, Na
1
and K
1
are not at equilibrium and there is some
net movement of K
1
out of the cell and of Na
1
into the cell. Na
1
is strongly pulled into the cell by both its concentration gradient
and the interior negative charge. If only passive forces were at
work, these ion concentrations would eventually become equal
inside and outside the cell.
Active Transport Maintains Electrochemical
Gradients
Now let’s look at how active transport processes maintain the
membrane potential that diffusion has established, with the re-
sult that the cell exhibits a
steady state
. Te rate of active trans-
port is equal to, and depends on, the rate of Na
1
diffusion into
the cell. If more Na
1
enters, more is pumped out. (Tis is like
being in a leaky boat. Te more water that comes in, the faster
you bail!) Te Na
1
-K
1
pump couples sodium and potassium
transport and, on average, each “turn” of the pump ejects 3Na
1
out of the cell and carries 2K
1
back in (see Figure 3.10). Because
the membrane is always 50 to 100 times more permeable to K
1
,
the A±P-dependent Na
1
-K
1
pump maintains both the mem-
brane potential (the charge separation) and the osmotic bal-
ance. Indeed, if Na
1
was not continuously removed from cells,
so much would accumulate intracellularly that the osmotic gra-
dient would draw water into the cells, causing them to burst.
As we described on p. 73, diffusion of charged particles
across the membrane is affected not only by concentration gra-
dients, but by the electrical charge on the inner and outer faces
of the membrane. ±ogether these gradients make up the
electro-
chemical gradient.
Te diffusion of K
1
across the plasma mem-
brane is aided by the membrane’s greater permeability to it and
by the ion’s concentration gradient, but the negative charges on
the cell interior resist K
1
diffusion. In contrast, a steep electro-
chemical gradient draws Na
1
into the cell, but the membrane’s
relative impermeability to it limits Na
1
diffusion.
Te transient opening of gated Na
1
and K
1
channels in the
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