Membrane Transport Mechanisms · USMLE Step 1

Which transport mechanism requires no energy and utilizes a membrane-embedded channel or carrier protein to move solutes down their concentration gradient?

  1. Primary active transport
  2. Symport secondary active transport
  3. Facilitated diffusion
  4. Receptor-mediated endocytosis
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Correct answer: Facilitated diffusion

Facilitated diffusion uses specific membrane proteins (channels or carriers) to help polar or charged solutes cross the membrane down their concentration gradient without using ATP, classifying it as a passive transport mechanism.

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