Which transport mechanism allows a human epithelial cell to transport sodium ions out of the cytoplasm against their steep electrochemical gradient?

Answer and explanation

Correct answer: A. Primary active transport via Na⁺/K⁺ ATPase pumps

  1. Step 1: Moving solutes against an electrochemical gradient requires direct metabolic energy expenditure (ATP hydrolysis).
  2. Step 2: The Na⁺/K⁺ ATPase actively pumps 3 Na⁺ ions out and 2 K⁺ ions in against their respective concentration gradients, defining primary active transport.
Answer
Primary active transport via Na⁺/K⁺ ATPase pumps.
Why not the others: Facilitated and simple diffusion are passive down-gradient processes; aquaporins exclusively transport water molecules.

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