symport

Meaning: A symporter binds two solutes and carries both across the membrane in the same direction, for example both into the cell. In secondary active transport, a primary pump such as the Na⁺/K⁺ ATPase uses ATP to build a Na⁺ gradient, with Na⁺ high outside the cell. Na⁺ flowing back down its gradient releases free energy (ΔG < 0), and the symporter couples this to moving a second solute against its own gradient. The symporter itself does not hydrolyze ATP.

On the MCAT: The classic example is the Na⁺/glucose symporter (SGLT) in intestinal and kidney epithelial cells. It takes up glucose against its concentration gradient using the Na⁺ gradient. Passages often ask what happens if the Na⁺/K⁺ ATPase is inhibited (for example by ouabain). The Na⁺ gradient collapses, so symport stalls and glucose uptake falls, even though the symporter does not use ATP directly. Another common question is to classify a transporter from a description: same direction means symport, opposite directions means antiport (exchanger), and one solute only means uniport.

Common confusion: Students call symport "passive" because ATP is not used at the transporter, or call it "primary active" because it moves a solute uphill. It is secondary active transport. It depends indirectly on ATP through the gradient that the primary pump made. Also, "same direction" refers to the two solutes relative to each other, not to the direction relative to the cell. Both may enter or both may exit.