Molecular Biology of the Cell by Bruce Alberts, Alexander Johnson, Julian Lewis, David Morgan, Martin Raff, Keith Roberts, Peter Walter by by Bruce Alberts, Alexander Johnson, Julian Lewis, David Morg

612 Chapter 11: Membrane Transport of Small Molecules and the Electrical Properties of Membranes
they can pass up to 100 million ions through one open channel each second—a
rate 10 5 times greater than the fastest rate of transport mediated by any known
transporter. As discussed earlier, however, channels cannot be coupled to an
energy source to perform active transport, so the transport they mediate is always
passive (downhill). Thus, the function of ion channels is to allow specific inorganic
ions—primarily Na + , K + , Ca 2+ , or Cl – —to diffuse rapidly down their electrochemical
gradients across the lipid bilayer. In this section, we will see that the ability
to control ion fluxes through these channels is essential for many cell functions.
Nerve cells (neurons), in particular, have made a specialty of using ion channels,
and we will consider how they use many different ion channels to receive, conduct,
and transmit signals. Before we discuss ion channels, however, we briefly
consider the aquaporin water channels that we mentioned earlier.
Aquaporins Are Permeable to Water But Impermeable to Ions
Because cells are mostly water (typically ~70% by weight), water movement across
cell membranes is fundamentally important for life. Cells also contain a high concentration
of solutes, including numerous negatively charged organic molecules
that are confined inside the cell (the so-called fixed anions) and their accompanying
cations that are required for charge balance. This creates an osmotic gradient,
which mostly is balanced by an opposite osmotic gradient due to a high
concentration of inorganic ions—chiefly Na + and Cl – —in the extracellular fluid.
The small remaining osmotic force tends to “pull” water into the cell, causing it to
swell until the forces are balanced. Because all biological membranes are moderately
permeable to water (see Figure 11–2), cell volume equilibrates in minutes or
less in response to an osmotic gradient. For most animal cells, however, osmosis
has only a minor role in regulating cell volume. This is because most of the cytoplasm
is in a gel-like state and resists large changes in its volume in response to
changes in osmolarity.
In addition to the direct diffusion of water across the lipid bilayer, some prokaryotic
and eukaryotic cells have water channels, or aquaporins, embedded in
their plasma membrane to allow water to move more rapidly. Aquaporins are particularly
abundant in animal cells that must transport water at high rates, such as
the epithelial cells of the kidney or exocrine cells that must transport or secrete
large volumes of fluids, respectively (Figure 11–19).
Aquaporins must solve a problem that is opposite to that facing ion channels.
To avoid disrupting ion gradients across membranes, they have to allow the rapid
passage of water molecules while completely blocking the passage of ions. The
three-dimensional structure of an aquaporin reveals how it achieves this remarkable
selectivity. The channels have a narrow pore that allows water molecules to
traverse the membrane in single file, following the path of carbonyl oxygens that
line one side of the pore (Figure 11–20A and B). Hydrophobic amino acids line
the other side of the pore. The pore is too narrow for any hydrated ion to enter, and
the energy cost of dehydrating an ion would be enormous because the hydrophobic
wall of the pore cannot interact with a dehydrated ion to compensate for the
loss of water. This design readily explains why the aquaporins cannot conduct K + ,
aquaporins
water
ions
ion pumps
and channels
duct
apical membrane
basolateral membrane
fluid
Figure 11–19 The role of aquaporins in
fluid secretion. Cells lining the ducts of
exocrine glands (as found, for example, in
the pancreas and liver, and in mammary,
sweat, and salivary glands) secrete large
volumes of body fluids. These cells are
organized into epithelial sheets in which
their apical plasma membrane faces the
lumen of the duct. Ion pumps and channels
situated in the basolateral and apical
plasma membrane move ions (mostly
Na + and Cl – ) into the ductal lumen,
creating an osmotic gradient between the
surrounding tissue and the duct. Water
molecules rapidly follow the osmotic
gradient through aquaporins that are
present in high concentrations in both the
apical and basolateral membranes.