106
Chapter 4
In addition to the Na
+
/K
+
-ATPase transporter, the major
primary active-transport proteins found in most cells are
(1) Ca
2+
-ATPase; (2) H
+
-ATPase; and (3) H
+
/K
+
-ATPase.
Ca
2+
-ATPase is found in the plasma membrane and several
organelle membranes, including the membranes of the endo-
plasmic reticulum. In the plasma membrane, the direction of
active calcium transport is from cytosol to extracellular fl
uid.
In organelle membranes, it is from cytosol into the organ-
elle lumen. Thus, active transport of Ca
2+
out of the cytosol,
via Ca
2+
-ATPase, is one reason that the cytosol of most cells
has a very low Ca
2+
concentration, about 10
–7
mol/L, com-
pared with an extracellular Ca
2+
concentration of 10
–3
mol/L,
10,000 times greater.
H
+
-ATPase is in the plasma membrane and several
organelle membranes, including the inner mitochondrial and
lysosomal membranes. In the plasma membrane, the H
+
-
ATPase moves hydrogen ions out of cells, and in this way helps
maintain cellular pH.
H
+
/K
+
-ATPase is in the plasma membranes of the acid-
secreting cells in the stomach and kidneys, where it pumps one
hydrogen ion out of the cell and moves one potassium in for
each molecule of ATP hydrolyzed.
Secondary Active Transport
Secondary active transport is distinguished from primary active
transport by its use of an electrochemical gradient across a
plasma membrane as its energy source, rather than phosphory-
lation of a transport molecule by ATP. In secondary active
transport, the movement of an ion down its electrochemical
gradient
is coupled to the transport of another molecule, such
as a nutrient like glucose or an amino acid.
Thus, transporters that mediate secondary active trans-
port have two binding sites, one for an ion—typically but not
always sodium—and another for the cotransported molecule.
An example of such transport is shown in
Figure 4–13
. In this
example, the electrochemical gradient for sodium is directed
into the cell because of the higher concentration of sodium in
the extracellular fl uid and the excess negative charges inside
the cell. The solute to be transported, however, must move
against
its concentration gradient, uphill into the cell. High-
affi nity binding sites for sodium exist on the extracellular sur-
face of the transporter. Binding of sodium increases the affi
nity
of the binding site for the transported solute. The transporter
Na
+
15 mM
3Na
+
K
+
150 mM
Na
+
145 mM
K
+
5 mM
2K
+
ADP
ATP
Intracellular fluid
Extracellular fluid
Na
+
/K
+
–ATPase
Figure 4–12
The primary active transport of sodium and potassium ions in
opposite directions by the Na
+
/K
+
-ATPase in plasma membranes
is responsible for the low sodium and high potassium intracellular
concentrations. For each ATP hydrolyzed, three sodium ions move
out of a cell, and two potassium ions move in.
Figure 4–13
Secondary active transport model. In this example, the binding of a sodium ion to the transporter produces an allosteric increase in the affi nity
of the solute binding site at the extracellular surface of the membrane. Binding of Na
+
and solute causes a conformational change in the
transporter that exposes the binding sites to the intracellular fl
uid. Na
+
diffuses down its electrochemical gradient into the cell, which returns
the solute-binding site to a low-affi nity state.
Intracellular fluid
Extracellular fluid
Low Na
+
/ High solute
High Na
+
/ Low solute
–––––
–––––
Na
+
Intracellular fluid
Extracellular fluid
Low Na
+
/ High solute
Na
+
––
High Na
+
/ Low solute
Transporter
protein
Solute to be
cotransported
Excess
negative
charge
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