Interactions Between Cells and the Extracellular Environment 1596.4 The Membrane Potential 149
A. The cytoplasm of the cell contains negatively charged organic
ions (anions) that cannot leave the cell—they are “fixed” anions.
- These fixed anions attract K^1 , which is the inorganic ion
that can pass through the plasma membrane most easily. - As a result of this electrical attraction, the concentration
of K^1 within the cell is greater than the concentration of
K^1 in the extracellular fluid. - If K^1 were the only diffusible ion, the concentrations of
K^1 on the inside and outside of the cell would reach an
equilibrium.
a. At this point, the rate of K^1 entry (due to electrical
attraction) would equal the rate of K^1 exit (due to
diffusion).
b. At this equilibrium, there would still be a higher
concentration of negative charges within the cell
(because of the fixed anions) than outside the cell.
c. At this equilibrium, the inside of the cell would be
90 millivolts negative ( 2 90 mV) compared to the
outside of the cell. This potential difference is called
the K^1 equilibrium potential ( E (^) K ).
4. The resting membrane potential is less than E (^) K (usually
2 65 mV to 2 85 mV) because some Na^1 can also enter
the cell.
a. Na^1 is more highly concentrated outside than inside
the cell, and the inside of the cell is negative. These
forces attract Na^1 into the cell.
b. The rate of Na^1 entry is generally slow because the
membrane is usually not very permeable to Na^1.
B. The slow rate of Na^1 entry is accompanied by a slow rate of
K^1 leakage out of the cell.
The Na^1 /K^1 pump counters this leakage, thus
maintaining constant concentrations and a constant
resting membrane potential.
Most cells in the body contain numerous Na^1 /K^1
pumps that require a constant expenditure of energy.
The Na^1 /K^1 pump itself contributes to the membrane
potential because it pumps more Na^1 out than it pumps
K^1 in (by a ratio of three to two).
6.5 Cell Signaling 153
A. Cells signal each other generally by secreting regulatory
molecules into the extracellular fluid.
B. There are three categories of chemical regulation between cells.
Paracrine signaling refers to the release of regulatory
molecules that act within the organ in which they are made.
Synaptic signaling refers to the release of chemical
neurotransmitters by axon endings.
Endocrine signaling refers to the release of regulatory
molecules called hormones, which travel in the blood to
their target cells.
C. Regulatory molecules bind to receptor proteins in their target
cells.
The receptor proteins are specific for the regulatory
molecule; there may be as many as 30,000 different
types of receptor proteins for regulatory molecules in
the body.
If the regulatory molecule is nonpolar, it can penetrate
the plasma membrane; in that case, its receptor
proteins are located within the cell, in the cytoplasm or
nucleus.
If the regulatory molecule is polar, it cannot penetrate
the plasma membrane; in that case, its receptors are
located in the plasma membrane with their binding sites
exposed to the extracellular fluid.
When a polar regulatory molecule binds to its receptor
on the plasma membrane, it stimulates the release of
second messengers, which are molecules or ions that
enter the cytoplasm and produce the action of the
regulator within its target cell.
a. For example, many polar regulatory molecules bind
to receptors that indirectly activate an enzyme that
converts ATP into cyclic AMP.
b. The rise in cyclic AMP within the cell cytoplasm
then activates enzymes, and in that way carries out
the action of the regulatory molecule within the
cell.
Some plasma membrane receptor proteins are
G-protein-coupled receptors.
a. There are three G-protein subunits, designated alpha,
beta, and gamma, which are aggregated at a plasma
membrane receptor protein.
b. When the receptor is activated by binding to its
regulatory molecule, the G-proteins dissociate.
c. Then, either the alpha subunit or the beta-gamma
complex moves through the membrane to an effector
protein, which is an enzyme or an ion channel.
d. In this way, the effector protein (enzyme or ion
channel) and the receptor protein can be in different
locations in the plasma membrane.
Test Your Knowledge
The movement of water across a plasma membrane occurs by
a. an active transport water pump.
b. a facilitated diffusion carrier.
c. simple diffusion through membrane channels.
d. all of these.Review Activities
- Which of these statements about the facilitated diffusion of
glucose is true?
a. There is a net movement from the region of lower to the
region of higher concentration.
b. Carrier proteins in the cell membrane are required for this
transport.
c. This transport requires energy obtained from ATP.
d. It is an example of cotransport.