146 Chapter 6
ion is obtained from the “downhill” transport of Na^1 into the
cell. Hydrolysis of ATP by the action of the Na^1 /K^1 pumps
is required indirectly, in order to maintain low intracellular
Na^1 concentrations. The diffusion of Na^1 down its concen-
tration gradient into the cell can then power the movement of
a different ion or molecule against its concentration gradient.
If the other molecule or ion is moved in the same direction
as Na^1 (that is, into the cell), the coupled transport is called
either cotransport or symport. If the other molecule or ion is
moved in the opposite direction (out of the cell), the process
is called either countertransport or antiport.
An example in the body is the cotransport of Na^1 and glu-
cose from the extracellular fluid in the lumen of the intestine
and kidney tubules across the epithelial cell’s plasma membrane.
Here, the downhill transport of Na^1 (from higher to lower con-
centrations) into the cell furnishes the energy for the uphill trans-
port of glucose ( fig. 6.19 ). The first step in this process is the
binding of extracellular Na^1 to its negatively charged binding
site on the carrier protein. This allows extracellular glucose to
bind with a high affinity to its binding site on the carrier. For one
form of the cotransport carrier, common in the kidney, there is
a ratio of 1 Na^1 to 1 glucose; for a different form, found in the
small intestine, the ratio is 2 Na^1 to 1 glucose. The carrier then
undergoes a conformational (shape) change that transports the
Na^1 and glucose to the inside of the cell ( fig. 6.20 ). After the
Na^1 and glucose are released, the carrier returns to its original
conformation.
An example of countertransport is the uphill extrusion of
Ca^2 1 from a cell by a type of pump that is coupled to the pas-
sive diffusion of Na^1 into the cell. Cellular energy, obtained
from ATP, is not used to move Ca^2 1 directly out of the cell in
this case, but energy is constantly required to maintain the steep
Na^1 gradient.
An easy way to understand why examples of secondary
active transport are classified as “active” is to imagine whatblocked; (3) the ADP is released, producing a shape change in
the carrier that opens a passageway for the three Na^1 ions to
exit into the extracellular fluid; (4) two K^1 ions in the extra-
cellular fluid now bind to the carrier, causing P i to be released;
(5) the release of P i allows the pump to return to its initial state
and permits the two K^1 ions to move into the cytoplasm.
In summary, the Na^1 /K^1 pumps transport three Na^1 out
of the cell cytoplasm for every two K^1 that they transport into
the cytoplasm ( fig. 6.19 ). This is active transport for both ions
because both are moved against their concentration gradients:
Na^1 is more highly concentrated in the extracellular fluid than
in the cytoplasm, whereas K^1 is more highly concentrated in
the cytoplasm than in the extracellular fluid.
Most cells have numerous Na^1 /K^1 pumps that are con-
stantly active. For example, there are about 200 Na^1 /K^1 pumps
per red blood cell, about 35,000 per white blood cell, and sev-
eral million per cell in a part of the tubules within the kidney.
This represents an enormous expenditure of energy used to
maintain a steep gradient of Na^1 and K^1 across the plasma
membrane. This steep gradient serves three functions:
- The steep Na^1 gradient is used to provide energy for the
“coupled transport” of other molecules. - The gradients for Na^1 and K^1 concentrations across the
plasma membranes of nerve and muscle cells are used to
produce electrochemical impulses needed for functions of
the nerve and muscles, including the heart muscle. - The active extrusion of Na^1 is important for osmotic reasons; if
the pumps stop, the increased Na^1 concentrations within cells
promote the osmotic inflow of water, damaging the cells.
Secondary Active Transport
(Coupled Transport)
In secondary active transport, or coupled transport, the
energy needed for the “uphill” movement of a molecule or
Figure 6.20 The cotransport of Na^1 and glucose. This carrier protein transports Na^1 and glucose at the same time, moving
them from the lumen of the intestine and kidney tubules into the lining epithelial cells. This cotransport requires a lower intracellular
concentration of Na^1 , which is dependent on the action of other carriers, the Na^1 /K^1 (ATPase) pumps. Because ATP is needed
to power the Na^1 /K^1 (ATPase) pumps, the cotransport of Na^1 and glucose depends indirectly on ATP, and so can be considered
secondary active transport. The cotransport carrier shown here transports 1 Na^1 to 1 glucose, as most commonly occurs in the kidney;
the carrier in the small intestine transports 2 Na^1 for 1 glucose (not shown).
Extracellular
fluid Glucose
Na+CytoplasmGlucose concentration
is higher on this sideGlucose moves up its
concentration gradientNa+ concentration is higher
on this sideNa+ moves down its
concentration gradient