Biomedical Engineering Reference
In-Depth Information
across a membrane could occur in the opposite direction, which is called counter-
transport (antiport) or in the same direction, which is called cotransport (symport).
In countertransport [Figure 2.8(b)], one solute is transferred from a high concen-
tration region to a low concentration region, which yields the energy to drive the
transport of the other solute from a low concentration region to a high concentra-
tion region. An example for countertransport is the sodium-calcium exchanger or
antiporter, which allows three sodium ions into the cell to transport one Ca
2+
out.
Many cells also posses a calcium ATPase, enzymes that can operate at low intrac-
ellular concentrations of calcium, and sets the normal or resting concentration of
this important second messenger. But the ATPase exports calcium ions more slow-
ly: only 30 per second versus 2,000 per second by the exchanger. The exchanger
comes into service when the calcium concentration rises steeply and enables rapid
recovery.
Cotransport [Figure 2.8(c)] uses the flow of one solute species from a high to
low concentration to move another molecule. An example is the glucose symporter,
which cotransports two Na
+
ions for every molecule of glucose it imports into the
cell. Also, the active transport of small organic molecules such as amino acids is
mediated by a Na
+
ion cotransport. Active transport can also transport in a group,
which is called group translocation.
Figure 2.8
Active transport of molecules: (a) primary transport, (b) secondary countertransport,
and (c) secondary cotransport.
