396
UNIT 3
Regulation and Integration of the Body
11
electrons “running around” in a living system.) As we described
in Chapter 3, there is a slight difference in the numbers of
positive and negative ions on the two sides of cellular plasma
membranes (there is a charge separation), so there is a potential
across those membranes. Te plasma membranes provide the
resistance to current flow.
Role of Membrane Ion Channels
Recall that plasma membranes are peppered with a variety of
membrane proteins that act as
ion channels
. Each of these chan-
nels is selective as to the type of ion (or ions) it allows to pass.
For example, a potassium ion channel allows only potassium
ions to pass.
Membrane channels are large proteins, o±en with several
subunits, whose amino acid chains snake back and forth across
the membrane. Some channels,
leakage
or
nongated
channels
,
are always open. Other channels are
gated
: Part of the protein
forms a molecular “gate” that changes shape to open and close
the channel in response to specific signals. Tere are three main
types of gated channels:
Chemically gated
, or
ligand-gated
,
channels
open when
the appropriate chemical (in this case a neurotransmitter)
binds
(Figure 11.6a)
.
Voltage-gated channels
open and close in response to
changes in the membrane potential (Figure 11.6b).
Mechanically gated channels
open in response to physi-
cal deformation of the receptor (as in sensory receptors for
touch and pressure).
When gated ion channels open, ions diffuse quickly across the
membrane following their electrochemical gradients, creating
Some Definitions: Voltage, Resistance, Current
Voltage
, the measure of potential energy generated by separated
charge, is measured in either
volts
(V) or
millivolts
(1 mV
5
0.001 V). Voltage is always measured between two points and is
called the
potential difference
or simply the
potential
between
the points. Te greater the difference in charge between two
points, the higher the voltage.
Te flow of electrical charge from one point to another is a
cur-
rent
, and it can be used to do work—for example, to power a flash-
light. Te amount of charge that moves between the two points
depends on two factors: voltage and resistance.
Resistance
is the
hindrance to charge flow provided by substances through which
the current must pass. Substances with high electrical resistance are
insulators
, and those with low resistance are
conductors
.
Ohm’s law
gives the relationship between voltage, current,
and resistance:
Current (
I
) =
voltage (
V
)
resistance (
R
)
Ohm’s law tells us three things:
Current (
I
) is directly proportional to voltage. Te greater the
voltage (potential difference), the greater the current.
Tere is no net current flow between points that have the
same potential, as you can see by inserting a value of 0 V into
the equation.
Current is inversely related to resistance: Te greater the re-
sistance, the smaller the current.
In the body, electrical currents reflect the flow of ions (rather
than free electrons) across cellular membranes. (Unlike the
electrons flowing along your house wiring, there are no free
Na
+
Na
+
Membrane
voltage
changes
Closed
Open
+ + +
+ + +
+ + +
+ + +
Receptor
Open in response to binding of the
appropriate neurotransmitter
Open in response to changes
in membrane potential
Na
+
K
+
K
+
Na
+
Neurotransmitter chemical
attached to receptor
Chemical
binds
Closed
Open
– – –
– – –
– – –
– – –
(a)
Chemically gated ion channels
(b)
Voltage-gated ion channels
Figure 11.6
Operation of gated channels.
(a)
A chemically gated channel permeable to
both Na
1
and K
1
, and
(b)
a voltage-gated Na
1
channel.
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