Active transport across the plasma membrane
Amoves molecules down their concentration gradient using ATP
Bonly occurs for water molecules
Cneeds no energy
Dmoves molecules against their concentration gradient using ATP
Answer & Solution
Correct answer: D. moves molecules against their concentration gradient using ATP
Active transport pushes molecules from low to high concentration — against the gradient — and burns ATP to do it. The Na⁺/K⁺ pump is the canonical example. Diffusion and osmosis move things down the gradient and need no energy.
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