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

602 Chapter 11: Membrane Transport of Small Molecules and the Electrical Properties of Membranes
lipid
bilayer
LIGHT
electrochemical
gradient
Figure 11–7 Three ways of driving
active transport. The actively transported
molecule is shown in orange, and the
energy source is shown in red. Redox
driven active transport is discussed in
Chapter 14 (see Figures 14–18 and 14–19).
P
ADP
COUPLED
TRANSPORTER
ATP
ATP-DRIVEN
PUMP
LIGHT-DRIVEN
PUMP
K m ; they are called uniporters. Others function as coupled transporters, in which
the transfer of one solute strictly depends on the transport of a second. Coupled
transport involves either the simultaneous transfer of a second solute in the same
direction, performed by symporters (also called co-transporters), or the transfer
of a second solute in the opposite MBoC6 direction, m11.07/11.07 performed by antiporters (also called
exchangers) (Figure 11–8).
The tight coupling between the transfer of two solutes allows the coupled
transporters to harvest the energy stored in the electrochemical gradient of one
solute, typically an inorganic ion, to transport the other. In this way, the free
energy released during the movement of an inorganic ion down an electrochemical
gradient is used as the driving force to pump other solutes uphill, against
their electrochemical gradient. This strategy can work in either direction; some
coupled transporters function as symporters, others as antiporters. In the plasma
membrane of animal cells, Na + is the usual co-transported ion because its electrochemical
gradient provides a large driving force for the active transport of a
second molecule. The Na + that enters the cell during coupled transport is subsequently
pumped out by an ATP-driven Na + -K + pump in the plasma membrane
(as we discuss later), which, by maintaining the Na + gradient, indirectly drives
the coupled transport. Such ion-driven coupled transporters as just described
are said to mediate secondary active transport. In contrast, ATP-driven pumps are
said to mediate primary active transport because in these the free energy of ATP
hydrolysis is used to directly drive the transport of a solute against its concentration
gradient.
Intestinal and kidney epithelial cells contain a variety of symporters that are
driven by the Na + gradient across the plasma membrane. Each Na + -driven symporter
is specific for importing a small group of related sugars or amino acids
into the cell. Because the Na + tends to move into the cell down its electrochemical
gradient, the sugar or amino acid is, in a sense, “dragged” into the cell with
it. The greater the electrochemical gradient for Na + , the more solute is pumped
transported molecule
co-transported ion
lipid
bilayer
UNIPORT SYMPORT ANTIPORT
coupled transport
Figure 11–8 This schematic diagram
shows transporters functioning as
uniporters, symporters, and antiporters
(Movie 11.1).