secondary active transport

Meaning: Secondary active transport moves a solute against its gradient by coupling that movement to a solute moving down its gradient through the same carrier. The favorable gradient, most often Na⁺ and sometimes H⁺, is built by primary active transport, such as the Na⁺/K⁺ ATPase using ATP. The carrier does not hydrolyze ATP. A symporter moves both solutes the same direction (the Na⁺-glucose cotransporter); an antiporter moves them in opposite directions (the Na⁺/Ca²⁺ exchanger).

Equation: ΔG = RT ln([X]in/[X]out) + zFΔψ, where ΔG is free energy per mole of X moved inward (J/mol), R is 8.314 J/(mol·K), T is temperature (K), z is charge (unitless), F is 96,485 C/mol, and Δψ is the membrane potential, inside minus outside (V).

On the MCAT: Passages add ouabain, deplete ATP, or remove extracellular Na⁺. Predict that uptake of the coupled solute, such as glucose, falls because the Na⁺ gradient dissipates, though the cotransporter uses no ATP. Contrast with GLUT facilitated diffusion, which is passive.

Common confusion: Not hydrolyzing ATP does not make it passive. The solute moves against its gradient, so the process is active; ATP is used indirectly to maintain the Na⁺ gradient. Symport and antiport describe direction, not primary versus secondary.