Essential Cell Biology 5th edition

402 CHAPTER 12 Transport Across Cell Membranes
Figure 12−17 Two types of glucose
transporters enable gut epithelial cells
to transfer glucose across the epithelial
lining of the gut. Na + that enters the
cell via the Na + -driven glucose symport
is subsequently pumped out by Na +
pumps in the basal and lateral plasma
membranes, keeping the concentration
of Na + in the cytosol low—and the Na +
electrochemical gradient steep. The diet
provides ample Na + in the gut lumen to
drive the Na + gradient-driven glucose
symport. The process is shown in
Movie 12.5.
Na + -driven
glucose symport
lateral domain
of plasma
membrane
glucose
GLUCOSE IS ACTIVELY
TAKEN UP FROM GUT
glucose
Na +
Na +
GUT LUMEN
apical domain
of plasma
membrane
covering a
microvillus
tight
junctions
intestinal
epithelium
low
glucose
concentration
high
glucose
concentration
passive glucose
uniport
K +
GLUCOSE IS PASSIVELY
RELEASED FOR USE BY
OTHER TISSUES
glucose
basal
domain
Na +
Na + pump
low
glucose
concentration
EXTRACELLULAR FLUID
QUESTION 12–2
A rise in the intracellular Ca 2+
concentration causes muscle cells
to contract. In addition to an ATPdriven
Ca 2+ pump, muscle cells
that contract quickly and regularly,
such as those of the heart, have an
additional type of Ca 2+ pump—an
antiport that exchanges Ca 2+ for
extracellular Na + across the plasma
membrane. The majority of the
Ca 2+ ions that have entered the
cell during contraction are rapidly
pumped back out of the cell by
this antiport, thus allowing the
cell to relax. Ouabain and digitalis
are used for treating patients with
heart disease because they make
heart muscle cells contract more
strongly. Both drugs function by
partially inhibiting the Na + pump in
the plasma membrane of these cells.
Can you propose an explanation
for the effects of the drugs in the
patients? What will happen if too
much of either drug is taken?
Electrochemical H + Gradients Drive the Transport of
Solutes in Plants, Fungi, and Bacteria
Plant cells, bacteria, and fungi (including yeasts) do not have Na + pumps
in their plasma membrane. ECB5 Instead e12.16/12.17 of an electrochemical Na + gradient,
they rely mainly on an electrochemical gradient of H + to import solutes
into the cell. The gradient is created by H + pumps in the plasma membrane
that pump H + out of the cell, thus setting up an electrochemical
proton gradient across this membrane and creating an acid pH in the
medium surrounding the cell. The import of many sugars and amino
acids into bacterial cells is then mediated by H + symports, which use the
electrochemical H + gradient in much the same way that animal cells use
the electrochemical Na + gradient to import these nutrients.
In some photosynthetic bacteria, the H + gradient is created by the activity
of light-driven H + pumps such as bacteriorhodopsin (see Figure 11–28).
In other bacteria, fungi, and plants, the H + gradient is generated by H +
pumps in the plasma membrane that use the energy of ATP hydrolysis
to pump H + out of the cell; these H + pumps resemble the Na + pumps and
Ca 2+ pumps of animal cells discussed earlier.
A different type of ATP-dependent H + pump is found in the membranes
of some intracellular organelles, such as the lysosomes of animal cells
and the central vacuole of plant and fungal cells. These pumps—which
resemble the turbine-like enzyme that synthesizes ATP in mitochondria
and chloroplasts (discussed in Chapter 14)—actively transport H + out of
the cytosol into the organelle, thereby helping to keep the pH of the cytosol
neutral and the pH of the interior of the organelle acidic. An acid
environment is crucial to the function of many organelles, as we discuss
in Chapter 15.
Some of the transmembrane pumps considered in this chapter are shown
in Figure 12−18 and are listed in Table 12–2.