Facilitated diffusion moves substances across a membrane via specific transport proteins. Is facilitated diffusion an active or passive transport process?

• A. Active, because it always uses ATP to move molecules across the membrane
• B. Active, because it moves molecules against their concentration gradient using a carrier protein
• C. Passive, because it moves molecules down their concentration gradient without requiring ATP
• D. Neither passive nor active, because it occurs only through the phospholipid bilayer without proteins

Answer: (C)

Verified step by step guidance

1. Recall the definitions of active and passive transport: Active transport requires energy (usually ATP) to move substances against their concentration gradient, while passive transport moves substances down their concentration gradient without energy input.
2. Understand that facilitated diffusion involves specific transport proteins that help molecules cross the membrane, but it does not require ATP or energy input.
3. Recognize that facilitated diffusion moves molecules from an area of higher concentration to an area of lower concentration, which means it follows the concentration gradient.
4. Compare the options given: Active transport involves energy use and movement against the gradient, which does not apply here; facilitated diffusion uses proteins but no energy and moves substances down the gradient.
5. Conclude that facilitated diffusion is a passive transport process because it moves molecules down their concentration gradient without requiring ATP, even though it uses transport proteins.