Editing Active transport (section)

===Antiporter===
[[Image:Porters.PNG|right|thumb|Function of [[symporter]]s and [[antiporter]]s.]]
In an antiporter two species of ions or other solutes are pumped in opposite directions across a membrane. One of these species is allowed to flow from high to low concentration, which yields the [[entropy|entropic energy]] to drive the transport of the other solute from a low concentration region to a high one.

An example is the [[sodium-calcium exchanger]] or [[antiporter]], which allows three sodium ions into the cell to transport one calcium out.<ref>{{cite journal|last1=Yu|first1=SP|last2=Choi|first2=DW|title=Na<sup>+</sup>-Ca<sup>2+</sup> exchange currents in cortical neurons: concomitant forward and reverse operation and effect of glutamate.|journal=The European Journal of Neuroscience|date=June 1997|volume=9|issue=6|pages=1273–81|pmid=9215711|doi=10.1111/j.1460-9568.1997.tb01482.x|s2cid=23146698}}</ref> This antiporter mechanism is important within the membranes of cardiac muscle cells in order to keep the calcium concentration in the cytoplasm low.<ref name="ncbi.nlm.nih.gov"/> Many cells also possess [[calcium ATPase]]s, which can operate at lower intracellular concentrations of calcium and sets the normal or resting concentration of this important [[second messenger]].<ref>{{cite journal|last1=Strehler|first1=EE|last2=Zacharias|first2=DA|s2cid=9062253|title=Role of alternative splicing in generating isoform diversity among plasma membrane calcium pumps.|journal=Physiological Reviews|date=January 2001|volume=81|issue=1|pages=21–50|pmid=11152753|doi=10.1152/physrev.2001.81.1.21}}</ref> But the ATPase exports calcium ions more slowly: only 30 per second versus 2000 per second by the exchanger. The exchanger comes into service when the calcium concentration rises steeply or "spikes" and enables rapid recovery.<ref>{{cite journal|last1=Patterson|first1=M|last2=Sneyd|first2=J|last3=Friel|first3=DD|title=Depolarization-induced calcium responses in sympathetic neurons: relative contributions from Ca<sup>2+</sup> entry, extrusion, ER/mitochondrial Ca<sup>2+</sup> uptake and release, and Ca<sup>2+</sup> buffering.|journal=The Journal of General Physiology|date=January 2007|volume=129|issue=1|pages=29–56|pmid=17190902|doi=10.1085/jgp.200609660|pmc=2151609}}</ref> This shows that a single type of ion can be transported by several enzymes, which need not be active all the time (constitutively), but may exist to meet specific, intermittent needs.