Essential Cell Biology 5th edition

Transporters and Their Functions
399
Figure 12−13 The high concentration of Na + outside the cell is like
water behind a high dam. The water behind the dam has potential
energy, which can be used to drive energy-requiring processes. In
the same way, an ion gradient across a membrane can be used to
drive active processes in a cell, including the active transport of other
molecules across the plasma membrane. Shown here is the Table Rock
Dam in Branson, Missouri, USA. (Gary Saxe/Shutterstock.)
transporters and ion channels in the plasma membrane. In this way,
the pump keeps the Na + concentration in the cytosol about 10–30 times
lower than that in the extracellular fluid and the K + concentration about
10–30 times higher (see Table 12–1, p. 391).
This steep concentration gradient of Na + across the plasma membrane
acts together with the membrane potential to create a large Na + electrochemical
gradient (see Figure 12–5A). This high concentration of Na +
outside the cell, on the uphill side of its electrochemical gradient, is like
a large volume of water behind a high dam: it represents a very large
store of energy (Figure 12−13). Even if one artificially halts the operation
of the Na + pump with ouabain, this stored energy is sufficient to sustain
for many minutes the various gradient-driven pumps in the plasma
membrane that are fueled by the downhill flow of Na + , which we discuss
shortly.
Ca 2+ Pumps Keep the Cytosolic Ca 2+ Concentration Low
Ca 2+ , like Na + , is also kept at a low concentration in the cytosol compared
with its concentration in the extracellular fluid. But Ca 2+ is much
less plentiful than Na + , both inside and outside cells (see Table 12–1).
The movement of this ion across cell membranes is nonetheless crucial,
because Ca 2+ can bind tightly to a variety of proteins in the cell, altering
their activities. An influx of Ca 2+ into the cytosol through Ca 2+ channels,
for example, is used by different cells as an intracellular signal to trigger
various complex processes, such as muscle contraction (discussed in
Chapter 17), fertilization (discussed in Chapters 16 and 19), and nerve cell
communication, which is discussed later.
The lower the background concentration of free Ca 2+ in the cytosol, the
more sensitive the cell is to an increase in cytosolic Ca 2+ . Thus eukaryotic
cells in general maintain a very low concentration of free Ca 2+ in their
cytosol (about 10 –4 mM) compared to the much higher concentration of
Ca 2+ outside of the cell (typically 1–2 mM). This huge concentration difference
is achieved mainly by means of ATP-driven Ca 2+ pumps in both
the plasma membrane and the endoplasmic reticulum membrane, which
actively remove Ca 2+ from the cytosol.
Ca 2+ pumps are ATPases that work in much the same way as the Na +
pump depicted in Figure 12–12. The main difference is that Ca 2+ pumps
return to their original conformation without a requirement for binding
and transporting a second ion (Figure 12−14). The Na + and Ca 2+ pumps
have similar amino acid sequences and structures, indicating that they
share a common evolutionary origin.
Gradient-driven Pumps Exploit Solute Gradients to
Mediate Active Transport
A gradient of any solute across a membrane, like the electrochemical
Na + gradient generated by the Na + pump, can be used to drive the active
transport of a second molecule. The downhill movement of the first solute
down its gradient provides the energy to power the uphill transport
of the second solute. The active transporters that work in this way are