Movement of Molecules Across Cell Membranes
107
then undergoes a conformational change, which exposes the
binding sites to the intracellular side of the membrane. When
the transporter changes conformation, sodium moves into the
intracellular fl uid by simple diffusion down its electrochemical
gradient. At the same time, the affi nity of the solute binding
site decreases, which releases the solute into the intracellular
uid. Once the transporter releases both molecules, the pro-
tein assumes its original conformation. The most important
distinction, therefore, between primary and secondary active
transport is that secondary active transport uses the stored
energy of an electrochemical gradient to move both an ion
and a second solute across a plasma membrane. The creation
and maintenance of the electrochemical gradient, however,
depends on the action of primary active transporters.
To summarize, the creation of a sodium concentration
gradient across the plasma membrane by the primary active
transport of sodium is a means of indirectly “storing” energy
that can then be used to drive secondary active-transport
pumps linked to sodium. Ultimately, however, the energy for
secondary active transport is derived from metabolism in the
form of the ATP that is used by the Na
+
/K
+
-ATPase to cre-
ate the sodium concentration gradient. If the production of
ATP were inhibited, the primary active transport of sodium
would cease, and the cell would no longer be able to maintain
a sodium concentration gradient across the membrane. This, in
turn, would lead to a failure of the secondary active-transport
systems that depend on the sodium gradient for their source
of energy. Between 10 and 40 percent of the ATP a cell pro-
duces under resting conditions is used by the Na
+
/K
+
-ATPase
to maintain the sodium gradient, which in turn drives a mul-
titude of secondary active-transport systems.
As noted earlier, the net movement of sodium by a sec-
ondary active-transport protein is always from high extracel-
lular concentration into the cell, where the concentration of
sodium is lower. Thus, in secondary active transport, the move-
ment of sodium is always
downhill,
while the net movement of
the actively transported solute on the same transport protein is
uphill,
moving from lower to higher concentration. The move-
ment of the actively transported solute can be either into the
cell (in the same direction as sodium), in which case it is known
as
cotransport,
or out of the cell (opposite the direction of
sodium movement), which is called
countertransport
(
Figure
4–14
). The terms
symport
and
antiport
are also used to refer to
the processes of cotransport and countertransport, respectively.
In summary, the distribution of substances between the
intracellular and extracellular fl uid is often unequal (
Table 4–1
)
due to the presence in the plasma membrane of primary and
secondary active transporters, ion channels, and the membrane
High Na
+
Plasma
membrane
Cotransport
Low X
Countertransport
Extracellular fluid
High Na
+
High X
Extracellular fluid
Figure 4–14
Cotransport and countertransport during secondary active
transport driven by sodium. Sodium ions always move
down
their
concentration gradient into a cell, and the transported solute always
moves
up
its gradient. Both sodium and the transported solute X
move in the same direction during cotransport, but in opposite
directions during countertransport.
Table 4–1
Composition of Extracellular and
Intracellular Fluids
Extracellular
Concentration,
mM
Intracellular
Concentration,*
mM
Na
+
145
15
K
+
5
1
5
0
Ca
2+
1
0.0001
Mg
2+
1.5
12
Cl
100
7
HCO
3
24
10
P
i
24
0
Amino acids
2
8
Glucose
5.6
1
ATP
0
4
Protein
0.2
4
*The intracellular concentrations differ slightly from one tissue to another, depending on the
expression of plasma membrane ion channels and transporters. The intracellular concentrations
shown above are typical of most cells. For Ca
2+
, values represent free concentrations. Total
calcium levels, including the portion sequestered by proteins or in organelles, approach 2.5 mM
(extracellular) and 1.5 mM (intracellular).
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