Examine individual changes

'[[Image:Scheme secundary active transport-en.svg|thumb|400px|secondary active transport]] In '''secondary active transport''', in contrast to [[primary active transport]], there is no direct coupling of [[Adenosine triphosphate|ATP]]; instead, the [[electrochemical potential|electrochemical potential difference]] created by pumping ions out of the cell is used. <ref>{{GeorgiaPhysiology|7/7ch05/7ch05p12}}</ref> The two main forms of this are [[antiport]] and [[symport]]. === Antiport === In antiport two species of ion 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. Many cells also possess a [[calcium ATPase]], which can operate at lower intracellular concentrations of calcium and sets the normal or resting concentration of this important [[second messenger]]. 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. 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. === Symport === Symport uses the downhill movement of one [[solute|solute species]] from high to low concentration to move another molecule uphill from low concentration to high concentration (against its [[electrochemical gradient]]). An example is the glucose symporter [[Sodium-glucose transport proteins|SGLT1]], which [[co-transport]]s one [[glucose]] (or [[galactose]]) molecule into the cell for every two sodium ions it imports into the cell. This [[symporter]] is located in the small intestines, trachea, heart, brain, testis, and prostate. It is also located in the S3 segment of the [[proximal tubule]] in each [[nephron]] in the [[kidney]]s <ref>Wright EM. (2001) "Renal Na⁺-glucose cotransporters", ''Am J Physiol Renal Physiol,'' 280:F10–F18</ref>. Its mechanism is exploited in [[Oral rehydration therapy|glucose rehydration therapy]] and defects in SGLT1 prevent effective reabsorption of glucose, causing [[Glucosuria|familial renal glucosuria]]<ref> Wright EM, Hirayama BA, Loo DF. (2007) "Active sugar transport in health and disease", ''J Intern Med,'' 261:32–43</ref>. ==References== <references/> {{Membrane transport}} [[Category:Physiology]] [[Category:Membrane biology]] [[fr:Transport actif secondaire]]'