Membrane Transport Mechanisms · USMLE Step 1

Which of the following phenomena best illustrates secondary active transport (cotransport)?

  1. Na⁺/K⁺-ATPase pumping Na⁺ out and K⁺ in using ATP directly
  2. Simple diffusion of dissolved O₂ into a cell
  3. Glucose entry into gut cells driven by the Na⁺ gradient
  4. Water movement through aquaporin membrane channels
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Correct answer: Glucose entry into gut cells driven by the Na⁺ gradient

Glucose uptake that relies on Na⁺ moving down its gradient to drive glucose uptake against its gradient (via Na⁺–glucose symporter) is a classic example of secondary active transport, coupling one solute’s favorable movement to another’s unfavorable movement.

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