Movement of Molecules Across Cell Membranes
105
plasma membranes by the Na
+
/K
+
-ATPase pump. This trans-
porter, which is present in all cells, moves sodium ions from
intracellular to extracellular fl
uid, and potassium ions in the
opposite direction. In both cases, the movements of the ions are
against their respective concentration gradients.
Figure 4–11
illustrates the sequence the Na
+
/K
+
-ATPase pump is believed
to use to transport these two ions in opposite directions.
(1) Initially the transporter, with an associated molecule of
ATP, binds three sodium ions at high-affi nity sites on the intra-
cellular surface of the protein. Two binding sites also exist for
K
+
, but at this stage they are in a low-affi nity state and thus do
not bind intracellular K
+
. (2) Binding of Na
+
results in activation
of an inherent ATPase activity of the transporter protein, caus-
ing phosphorylation of the cytosolic surface of the transporter
and releasing a molecule of ADP. (3) Phosphorylation results
in a conformational change of the transporter, exposing the
bound sodium ions to the extracellular fl
uid, and at the same
time reducing the affi nity of the binding sites for sodium. The
sodium ions are released from their binding sites. (4) The new
conformation of the transporter results in an increased affi nity
of the two binding sites for K
+
, allowing two molecules of K
+
to
bind to the transporter on the extracellular surface. (5) Binding
of K
+
results in dephosphorylation of the transporter. This
returns the transporter to its original conformation, resulting
in reduced affi nity of the K
+
binding sites and increased affi nity
of the Na
+
binding sites. K
+
is therefore released into the intra-
cellular fl uid, allowing new molecules of Na
+
(and ATP) to be
bound at the intracellular surface.
The pumping activity of the Na
+
/K
+
-ATPase primary
active transporter establishes and maintains the character-
istic distribution of high intracellular potassium and low
intracellular sodium relative to their respective extracellular
concentrations (
Figure 4–12
). For each molecule of ATP
hydrolyzed, this transporter moves three sodium ions out
of a cell, and two potassium ions into a cell. This results in
a net transfer of positive charge to the outside of the cell,
and thus this transport process is not electrically neutral, a
point that will be described in detail in Chapter 6 when we
consider the electrical charge across plasma membranes of
nerve cells.
Figure 4–11
Active transport of Na
+
and K
+
mediated by the Na
+
/K
+
-ATPase pump. See text for the numbered sequence of events occurring during transport.
Extracellular fluid
Intracellular fluid
High Na
+
Low Na
+
3 Na
+
Low K
+
High K
+
1
ATP
ADP
2
P
+
3
P
3 Na
+
4
P
2 K
+
5
2 K
+
ATP
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