Chapter 3
Cells: The Living Units
79
3
2
100 millivolts (mV), depending on cell type. For this reason,
all cells are said to be
polarized
. Te minus sign before the volt-
age indicates that the
inside
of the cell is negative compared to
its outside. Tis voltage (or charge separation) exists
only at the
membrane
. If we added up all the negative and positive charges
in the cytoplasm, we would find that the cell interior is electri-
cally neutral. Likewise, the positive and negative charges in the
extracellular fluid balance each other exactly.
So how does the resting membrane potential come about,
and how is it maintained? Te short answer is that diffusion
causes ionic imbalances that polarize the membrane, and active
transport processes
maintain
that membrane potential. First,
let’s look at how diffusion polarizes the membrane.
Selective Diffusion Establishes Membrane
Potential
Many kinds of ions are found both inside cells and in the extra-
cellular fluid, but the resting membrane potential is determined
mainly by the concentration gradient of potassium (K
1
) and by
the differential permeability of the plasma membrane to K
1
and
other ions
(Figure 3.15)
. Recall that K
1
and protein anions
predominate inside body cells, and the extracellular fluid con-
tains relatively more Na
1
, which is largely balanced by Cl
2
. Te
unstimulated plasma membrane is somewhat permeable to K
1
because of leakage channels, but impermeable to the protein
anions. Consequently, K
1
diffuses out of the cell along its con-
centration gradient but the protein anions are unable to follow,
and this loss of positive charges makes the membrane interior
more negative (Figure 3.15
1
).
Check Your Understanding
10.
What happens when the Na
1
-K
1
pump is phosphorylated?
When K
1
binds to the pump protein?
11.
As a cell grows, its plasma membrane expands. Does this
membrane expansion involve endocytosis or exocytosis?
12.
Phagocytic cells gather in the lungs, particularly in the lungs
of smokers. What is the connection?
13.
Which vesicular transport process allows a cell to take in
cholesterol from the extracellular fluid?
For answers, see Appendix H.
The Plasma Membrane:
Generation of a Resting
Membrane Potential
Define membrane potential and explain how the resting
membrane potential is established and maintained.
As you’re now aware, the selective permeability of the plasma
membrane can lead to dramatic osmotic flows, but that is not its
only consequence. An equally important result is the generation
of a
membrane potential
, or voltage, across the membrane. A
voltage
is electrical potential energy resulting from the sepa-
ration of oppositely charged particles. In cells, the oppositely
charged particles are ions, and the barrier that keeps them apart
is the plasma membrane.
In their resting state, all body plasma membranes exhibit a
resting membrane potential
that typically ranges from
2
50 to
K
+
diffuse down their steep
concentration gradient (out of the cell)
via leakage channels. Loss of K
+
results
in a negative charge on the inner
plasma membrane face.
K
+
also move into the cell
because they are attracted to the
negative charge established on the
inner plasma membrane face.
A negative membrane potential
(–90 mV) is established when the
movement of K
+
out of the cell equals
K
+
movement into the cell. At this
point, the concentration gradient
promoting K
+
exit exactly opposes the
electrical gradient for K
+
entry.
Na
+
Na
+
Na
+
Na
+
Na
+
Na
+
Na
+
Na
+
Na
+
K
+
K
+
K
+
K
+
K
+
K
+
K
+
K
+
K
+
K
+
K
+
K
+
K
+
K
+
Cl
Cl
Potassium
leakage
channels
Protein anion (unable to
follow
K
+
through the
membrane)
Cytoplasm
Extracellular fluid
+
+
+
+
+
+
+
+
A
A
3
2
1
Figure 3.15
The key role of K
1
in generating the resting membrane potential.
The
resting membrane potential is largely determined by K
1
because at rest, the membrane is much
more permeable to K
1
than Na
1
. The active transport of sodium and potassium ions (in a ratio
of 3:2) by the Na
1
-K
1
pump maintains these conditions.
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