<P> In an antiporter 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 entropic energy to drive the transport of the other solute from a low concentration region to a high one . </P> <P> An example is the sodium - calcium exchanger or antiporter, which allows three sodium ions into the cell to transport one calcium out . This antiporter mechanism is important within the membranes of cardiac muscle cells in order to keep the calcium concentration in the cytoplasm low . Many cells also possess calcium ATPases, 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 . </P> <P> A symporter uses the downhill movement of one solute species from high to low concentration to move another molecule uphill from low concentration to high concentration (against its concentration gradient). Both molecules are transported in the same direction . </P> <P> An example is the glucose symporter SGLT1, which co-transports 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, heart, and brain . It is also located in the S3 segment of the proximal tubule in each nephron in the kidneys . Its mechanism is exploited in glucose rehydration therapy This mechanism uses the absorption of sugar through the walls of the intestine to pull water in along with it . Defects in SGLT2 prevent effective reabsorption of glucose, causing familial renal glucosuria . </P>

Which of the following would not be classified as active transport