13 COMPARTMENTS AND MEMBRANE TRANSPORT PROCESSES
11 Charge gradients across animal cell membranes underlie electrical activity
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- The membrane potential is the potential difference (voltage) across a cell membrane.
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There
is a negative charge on the cytosolic side of the plasma membrane as
compared with the extracellular side, resulting in voltage (or
potential difference) across the membrane.
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Biologists have observed a difference in
charge between the extracellular side and the cytosol side of the
plasma membrane. This charge difference can be measured and expressed
in volts, and is called the membrane potential. The word potential
expresses the ability to do work if a change in the voltage occurs. In
this section, we will discuss how the membrane potential develops, and
some kinds of work that the cell can accomplish because of it.
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Figure 13.14
Development of a membrane potential. Ion concentrations
inside and outside the cell are expressed here in millimoles per liter,
or millimolar (mM). The membrane potential, usually expressed in
millivolts, is proportional to the charge difference across the
membrane.
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Figure 13.14 illustrates how a cell generates a charge difference across its
membrane. A cell contains much more potassium in its cytosol than is
present in its environment because it accumulates potassium ions.
Animal cells accumulate potassium by using the sodium-potassium pump;
other kinds of organisms use other ion transport proteins. Plasma
membranes are generally more permeable to potassium than to other ions,
because they typically contain a greater number of potassium channels
in the membrane. Therefore, by diffusion through ion channels, a cell
loses potassium ions faster than it gains sodium or other positively
charged ions. Each potassium ion diffusing out of the cell carries a
positive charge with it, leaving behind large numbers of negatively
charged molecules, or anions. Anions cannot diffuse through the lipid
bilayer and there are no channels for most of them. Therefore, the
inside of the cell has proportionally more negative charges than are
outside the plasma membrane.
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While potassium ions flow down the
concentration gradient, there is also a tendency for potassium ions to
be pulled into the cell by their attraction to the negative charges. At
some point, these two processes occur at the same rate. It takes less
than a thousandth of a second for this steady state to be reached, at
which point the membrane potential is – 70 millivolts (70/1000 of a
volt). (The negative sign is used because it is conventional to
consider the outside solution neutral.)
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The charge of the membrane potential does
not seem large until we realize how small the cells are and how thin
the plasma membrane is. This voltage is equivalent to 10 million volts
per meter. Increasing it by as little as a factor of two would put so
much electrical pull on the membrane that its lipid bilayer structure
would collapse.
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The membrane potential imposes the force
of an electrical field on molecules in the plasma membrane. This
electrical field can influence the shape and orientation of the
proteins, since proteins have both positively and negatively charged
chemical groups.
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Figure 13.15
The opening and closing of an ion channel can be regulated by the voltage across the membrane.
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In many kinds of cells, the electrical
field keeps channels for ions other than potassium closed. A decrease
in the electrical field — that is, a lessening of the voltage
difference across the membrane — might allow these other ion channels
to open. Ion channels whose open or closed state is influenced by the
membrane potential are called voltage-sensitive ion channels. The
voltage-sensitive ion channel in Figure 13.15
is closed when the membrane potential is – 70 mV, but is open when the
potential reduces to – 40 mV. Voltage-sensitive ion channels are
involved in generating nerve impulses, in sensing light and other
stimuli, and in triggering muscle contractions. (See Sensory systems
and The musculo-skeletal system and body movements.)
These ion channels are also involved in cell processes such as the
secretion of insulin, which we will examine in the next section.
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© Jones and Bartlett Publishers (2007)
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