Interactions Between Cells and the Extracellular Environment 145the release of neurotransmitters from axon terminals (chapter 7,
section 7.3) and muscle contraction (chapter 12, section 12.2).
Figure 6.18 presents a simplified model of the Ca^2 1 pump.
Notice that there is a binding site that is accessible to Ca^2 1 from
the cytoplasm, and that the pump is activated by phosphorylation,
using the P i derived from ATP. Newer studies of these pumps
have revealed the following: (1) binding of a cytoplasmic Ca^2 1 to
an amino acid site in the pump activates the ATPase, causing the
hydrolysis of ATP into ADP and P i , which are bound to the pump;
(2) both of the exits for Ca^2 1 are now momentarily blocked;
(3) the ADP is released, producing a shape change in the protein
that opens a passageway for Ca^2 1 to the extracellular fluid (or
cisterna of the endoplasmic reticulum), so that Ca^2 1 can move to
the other side of the membrane; (4) the P i group is released from
the pump, allowing the carrier to return to its initial state where
cytoplasmic Ca^2 1 once again has access to the binding site.The Sodium-Potassium Pump
A very important primary active transport carrier found in all
body cells is the Na 1 / K 1 pump. Like the Ca^2 1 pumps previ-
ously described, the Na^1 /K^1 pumps are also ATPase enzymes.
The Na^1 /K^1 pump cycle occurs as follows: (1) three Na^1 ions
in the cytoplasm move partway into the pump and bind to three
amino acid sites; (2) this activates the ATPase, hydrolyzing
ATP into ADP and P i and causing both exits to be momentarilyproton (H^1 ) pump (responsible for the acidity of the stom-
ach’s gastric juice), and the Na^1 /K^1 pump ( fig. 6.19 )—are also
ATPase enzymes, and their pumping action is controlled by the
addition and removal of phosphate groups obtained from ATP.
The Ca^2 1 Pump
Ca^2 1 pumps are located in the plasma membrane of all cells,
and in the membrane of the endoplasmic reticulum (chapter 3)
of striated muscle cells and others. Active transport by these
pumps removes Ca^2 1 from the cytoplasm by pumping it into
the extracellular fluid or the cisternae of the endoplasmic reticu-
lum. Because of the concentration gradient thus created, when
ion channels for Ca^2 1 are opened in the plasma membrane or
endoplasmic reticulum, Ca^2 1 will diffuse rapidly down its con-
centration gradient into the cytoplasm. This sudden rise in cyto-
plasmic Ca^2 1 serves as a signal for diverse processes, including
Figure 6.18 An active transport pump. This carrier
protein transports Ca^2 1 from a lower concentration inside the
cell to a higher concentration outside of the cell, and is thus
known as a Ca^2 1 pump. (1) Ca^2 1 within the cell binds to sites in
the carrier protein. (2) ATP is hydrolyzed into ADP and phosphate
(P i ), and the phosphate is added to the carrier protein; this
phosphorylation causes a hingelike motion of the carrier. (3) The
hingelike motion of the carrier protein allows Ca^2 1 to be released
into the extracellular fluid.
CytoplasmCytoplasmCa2+Low Ca2+ High Ca^2 +Carrier proteins
(active transport
pump)Binding
siteExtracellular fluidExtracellular fluidCa^2 +ADP + Pi ~AT P123Figure 6.19 The exchange of intracellular Na^1 for K^1
by the Na^1 /K^1 pump. The active transport carrier itself is an
ATPase that breaks down ATP for energy. Dashed arrows indicate
the direction of passive transport (diffusion); solid arrows indicate
the direction of active transport. Because 3 Na^1 are pumped
out for every 2 K^1 pumped in, the action of the Na^1 /K^1 (ATPase)
pumps help to produce a difference in charge, or potential
difference, across the membrane.ADP + PiAT P3 Na+2 K+Na+K+Cytoplasm Extracellular
fluidPlasma membrane