Extracellular fluid
Pump protein binds ATP; releases K
+
to
the inside, and Na
+
sites are ready to bind
Na
+
again. The cycle repeats.
Na
+
binding promotes hydrolysis of ATP.
The energy released during this reaction
phosphorylates the pump.
Three cytoplasmic Na
+
bind to pump
protein.
Na
+
Na
+
–K
+
pump
K
+
released
ATP-binding site
Na
+
bound
Cytoplasm
ADP
ATP
ATP
P
K
+
K
+
binding triggers release of the
phosphate. The dephosphorylated pump
resumes its original conformation.
Phosphorylation causes the pump to
change shape, expelling Na
+
to the outside.
Two extracellular K
+
bind to pump.
Na
+
released
K
+
bound
P
K
+
P
P
i
6
5
4
3
2
1
Available at www.masteringaandp.com
Figure 3.10
Primary active transport is the process in which solutes are moved
across cell membranes against electrochemical gradients using energy supplied
directly by ATP. The action of the Na
+
–K
+
pump is an important example of
primary active transport.
FOCUS
Primary Active Transport: The Na
+
–K
+
Pump
74
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