Movement of Molecules Across Cell Membranes
115
substances can undergo a net movement from a low concen-
tration on one side of an epithelium to a higher concentra-
tion on the other side, or in other words, can undergo active
transport across the overall epithelial layer. Examples include
the absorption of material from the gastrointestinal tract into
the blood, the movement of substances between the kidney
tubules and the blood during urine formation, and the secre-
tion of salts and fl uid by glands.
Figures 4–22
and
4–23
illustrate two examples of active
transport across an epithelium. Sodium is actively transported
across most epithelia from lumen to blood side in absorptive
processes, and from blood side to lumen during secretion. In
our example, the movement of sodium from the lumen into
the epithelial cell occurs by diffusion through sodium chan-
nels in the luminal membrane (see Figure 4–22). Sodium dif-
fuses into the cell because the intracellular concentration of
sodium is kept low by the active transport of sodium back out
of the cell across the basolateral membrane on the opposite
side, where all of the Na
+
/K
+
-ATPase pumps are located. In
other words, sodium moves downhill into the cell and then
uphill out of it. The net result is that sodium can be moved
from lower to higher concentration across the epithelium.
Figure 4–23 illustrates the active absorption of organic
molecules across an epithelium. In this case, the entry of an
organic molecule X across the luminal plasma membrane
occurs via a secondary active transporter linked to the down-
hill movement of sodium into the cell. In the process, X moves
from a lower concentration in the luminal fl
uid to a higher con-
centration in the cell. The substance exits across the basolateral
membrane by facilitated diffusion, which moves the material
from its higher concentration in the cell to a lower concentra-
tion in the extracellular fl uid on the blood side. The concentra-
tion of the substance may be considerably higher on the blood
side than in the lumen because the blood-side concentration
can approach equilibrium with the high intracellular concen-
tration created by the luminal membrane entry step.
Although water is not actively transported across cell
membranes, net movement of water across an epithelium can
occur by osmosis as a result of the active transport of solutes,
especially sodium, across the epithelium. The active transport
of sodium, as previously described, results in a decrease in the
sodium concentration on one side of an epithelial layer (the
luminal side in our example) and an increase on the other.
These changes in solute concentration are accompanied by
Figure 4–23
The transepithelial transport of most organic solutes (X) involves
their movement into a cell through a secondary active transport
driven by the downhill fl ow of sodium. The organic substance
then moves out of the cell at the blood side down a concentration
gradient by means of facilitated diffusion. Shown below the cell
is the concentration profi le of the transported solute across the
epithelium.
3Na
+
2K
+
ADP
ATP
Na
+
Na
+
Sodium
channel
Blood
vessel
Na
+
/K
+
-
ATPase
pump
Diffusion
Active transport
Blood concentration
Sodium concentration
Lumen
concentration
Intracellular
concentration
Lumen side
Epithelial cell
Blood side
Figure 4–22
Active transport of sodium across an epithelial cell. The
transepithelial transport of sodium always involves primary active
transport out of the cell across one of the plasma membranes,
typically via a Na
+
/K
+
-ATPase pump as shown here. The movement
of sodium into the cell across the plasma membrane on the opposite
side is always downhill. Sometimes, as in this example, it is by
diffusion through sodium channels, whereas in other epithelia this
downhill movement occurs through a secondary active transporter.
Shown below the cell is the concentration profi le of the transported
solute across the epithelium.
Figure 4–22
physiological
inquiry
What would happen in this situation if the cell’s ATP supply was
to decrease signifi cantly?
Answer can be found at end of chapter.
Blood
vessel
X
Facilitated
diffusion
X
Na
+
Na
+
Active
transport
X concentration
X
Secondary
active
transport
Facilitated
diffusion
X
Lumen side
Epithelial cell
Blood side
Lumen
concentration
Intracellular
concentration
Blood
concentration
3Na
+
2K
+
ADP
ATP
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