Essential Cell Biology 5th edition

408 CHAPTER 12 Transport Across Cell Membranes
Figure 12–27 Different types of gated
ion channels respond to different types
of stimuli. Depending on the type of
channel, the probability of gate opening is
controlled by (A) a change in the voltage
difference across the membrane,
(B) the binding of a chemical ligand to
the extracellular face of a channel,
(C) ligand binding to the intracellular face
of a channel, or (D) mechanical stress. In
the case of the voltage-gated channels,
positively charged amino acids (white
plus signs) in the channel’s voltage sensor
domains become attracted to negative
charges on the extracellular surface of the
depolarized plasma membrane, pulling
the channel into its open conformation.
CLOSED
resting
plasma
membrane
OPEN
depolarized
plasma
membrane
+ ++ ++ +
+ +
–
–
CYTOSOL
–– –
– – –
+ +
+ ++ ++ +
CYTOSOL
(extracellular
ligand)
(intracellular
ligand)
(A) voltagegated
(B) ligand-gated (C) ligand-gated (D)
mechanicallygated
indicates that the channel has moving parts and is snapping back and
forth between open and closed conformations as the channel is knocked
from one conformation to the other by the random thermal movements
ECB5 e12.26/12.27
of the molecules in its environment. Patch-clamp recording was the
first technique that could detect such conformational changes, and the
picture it paints—of a jerky piece of machinery subjected to constant
external buffeting—is now known to apply also to other proteins with
moving parts.
The activity of each ion channel is very much “all-or-none”: when an ion
channel is open, it is fully open; when it is closed, it is fully closed. That
raises a fundamental question: If ion channels randomly snap between
open and closed conformations even when conditions on each side of
the membrane are held constant, how can their state be regulated by
conditions inside or outside the cell? The answer is that when the appropriate
conditions change, the random behavior continues but with a
greatly changed bias: if the altered conditions tend to open the channel,
for example, the channel will now spend a much greater proportion of its
time in the open conformation, although it will not remain open continuously
(see Figure 12–26).
Different Types of Stimuli Influence the Opening and
Closing of Ion Channels
There are more than a hundred types of ion channels, and even simple
organisms can possess many different types. The human genome contains
80 genes that encode different but related K + channels alone. Ion
channels differ from one another primarily with respect to their ion selectivity—the
type of ions they allow to pass—and their gating—the conditions
that influence their opening and closing. For a voltage-gated channel,
the probability of being open is controlled by the membrane potential
(Figure 12–27A). For a ligand-gated channel, opening is controlled by
the binding of some molecule (a ligand) to the channel (Figure 12–27B
and C). For a mechanically-gated channel, opening is controlled by a
mechanical force applied to the channel (Figure 12–27D).
The auditory hair cells in the ear are an important example of cells that
depend on mechanically-gated channels. Sound vibrations pull the channels
open, causing ions to flow into the hair cells; this ion flow sets up
an electrical signal that is transmitted from the hair cell to the auditory
nerve, which then conveys the signal to the brain (Figure 12–28).