Essential Cell Biology 5th edition

Transporters and Their Functions
397
cell
membrane
LIGHT
Figure 12–10 Pumps carry out active
transport in three main ways. The actively
transported solute is shown in gold, and the
energy source is shown in red.
electrochemical
gradient
P
ATP
GRADIENT-DRIVEN
PUMP
ADP
ATP-DRIVEN
PUMP
LIGHT-DRIVEN
PUMP
ionic composition and for importing solutes that are at a lower concentration
outside the cell than inside. For these purposes, cells depend on
transmembrane pumps, which can carry out active transport in three
main ways (Figure 12–10): (i) gradient-driven pumps link the uphill
transport of one solute ECB5 across e12.10/12.10
a membrane to the downhill transport of
another; (ii) ATP-driven pumps use the energy released by the hydrolysis
of ATP to drive uphill transport; and (iii) light-driven pumps, which are
found mainly in bacterial cells, use energy derived from sunlight to drive
uphill transport, as discussed in Chapter 11 for bacteriorhodopsin (see
Figure 11–28).
These different forms of active transport are often linked. Thus, in the
plasma membrane of an animal cell, an ATP-driven Na + pump transports
Na + out of the cell against its electrochemical gradient; this Na + can then
flow back into the cell, down its electrochemical gradient, through various
Na + gradient-driven pumps. The influx of Na + through these gradientdriven
pumps provides the energy for the active transport of many other
substances into the cell against their electrochemical gradients. If the
ATP-driven Na + pump ceased operating, the Na + gradient would soon run
down, and transport through Na + gradient-driven pumps would come to
a halt. For this reason, the ATP-driven Na + pump has a central role in the
active transport of small molecules across the plasma membrane of animal
cells. Plant cells, fungi, and many bacteria use ATP-driven H + pumps
in an analogous way: in pumping H + out of the cell, these proteins create
an electrochemical gradient of H + across the plasma membrane that is
subsequently harnessed for solute transport, as we discuss later.
The Na + Pump in Animal Cells Uses Energy Supplied by
ATP to Expel Na + and Bring in K +
The ATP-driven Na + pump plays such a central part in the energy economy
of animal cells that it typically accounts for 30% or more of their
total ATP consumption. This pump uses the energy derived from ATP
hydrolysis to transport Na + out of the cell as it carries K + in. The pump is
therefore sometimes called the Na + -K + ATPase or the Na + -K + pump.
During the pumping process, the energy from ATP hydrolysis fuels a stepwise
series of protein conformational changes that drives the exchange
of Na + and K + ions. As part of the process, the phosphate group removed
from ATP gets transferred to the pump itself (Figure 12−11).
The transport of Na + ions out, and K + ions in, takes place in a cycle in
which each step depends on the one before (Figure 12−12). If any of the
individual steps is prevented from occurring, the entire cycle halts. The
toxin ouabain, for example, inhibits the Na + pump by preventing the binding
of extracellular K + , arresting the cycle.