Membrane Transport Mechanisms · USMLE Step 1

What distinguishes primary active transport from secondary active transport (cotransport)?

  1. Only primary active transport uses carrier proteins
  2. Only secondary active transport is able to move ions
  3. Primary active transport uses ATP directly; secondary uses an ion gradient
  4. Secondary active transport directly requires ATP hydrolysis
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Correct answer: Primary active transport uses ATP directly; secondary uses an ion gradient

Primary active transport directly uses ATP hydrolysis to move solutes against their gradient. Secondary active transport (cotransport) harnesses the energy stored in an ion gradient (established by primary active transport) to move another solute against its gradient.

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