Active vs passive transport
MCAT glossary · Biological & Biochemical Foundations of Living Systems
Definition
Active uses ATP to move against gradient (Na⁺/K⁺ pump); passive includes simple diffusion, facilitated diffusion, osmosis.
Active vs passive transport in plain English
Cells constantly move molecules across their outer wall, and there are two ways to do it — the cheap way and the effortful way. Passive transport is the cheap way: stuff drifts naturally from where it's crowded to where it's sparse, needing no energy (like a smell spreading across a room on its own); this includes simple diffusion, helper-protein 'facilitated' diffusion, and osmosis (water moving this way). Active transport is the effortful way: the cell spends energy (ATP, its fuel currency) to push molecules the 'wrong' way — from sparse to crowded — like pumping water uphill. A classic example is the sodium-potassium pump, which burns ATP to force ions against their natural flow.
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