Secondary Active Transport

Secondary active transport is a form of active transport in which one substance is moved against its concentration gradient by coupling it to the downhill movement of a second substance (often an ion) along its gradient. In other words, it uses the energy stored in an electrochemical gradient (established by primary active transport) to drive the uphill transport of another molecule, rather than using ATP directly at that transporter.

• Secondary active transport typically involves carrier proteins known as <u>cotransporters</u>. If both substances move in the same direction across the membrane, it's called a <u>symporter</u>; if they move in opposite directions, it's an <u>antiporter</u> (exchanger). These carriers rely on the gradient of one ion (often Na+) to power the movement of another substance.
• A classic example: the <u>sodium-glucose cotransporter</u> in intestinal cells. As Na+ ions flow passively into the cell (down their concentration gradient established by the Na+/K+ pump), glucose is dragged in with them, against glucose's gradient. This allows cells to absorb glucose efficiently even when it's more concentrated inside than outside.
• Another example: the <u>sodium-calcium exchanger</u> (an antiporter in cardiac muscle cells). It uses the inward flow of Na+ (down its gradient) to expel Ca2+ from the cell against the Ca2+ gradient, helping regulate intracellular calcium levels. Importantly, if the Na+ gradient dissipates (e.g., if Na+/K+ ATPase is poisoned), secondary active transport will stop because its energy source is gone.

• If an exam question describes a transport protein that does not directly use ATP but instead uses "the sodium gradient" or another ion gradient to move a different molecule, that is secondary active transport. Key phrases include "cotransport" or "coupled transport".
• For instance, a question might ask how glucose is absorbed from the gut lumen into intestinal cells against a concentration gradient: the answer is via Na+-glucose symport (a secondary active transport mechanism that uses the Na+ gradient).
• Distinguish from primary active transport: if the question highlights a pump breaking down ATP (like Na+/K+ ATPase), that's primary. If it highlights one solute's gradient driving another's uptake or export, that's secondary. An example scenario: calcium ions being removed from a cell by swapping with incoming sodium ions - this is secondary active transport (the Na+ gradient, set up by ATP elsewhere, drives Ca2+ out).