A type of passive transport where specific proteins help molecules cross cell membranes down their concentration gradient without energy input. These transport proteins act like selective doorways for particular substances.
From Latin 'facilis' meaning easy and 'diffundere' meaning to spread out. The term emerged in the 1950s when scientists discovered that some molecules needed protein assistance to cross membranes, even when moving naturally downhill.
Facilitated diffusion is like having a bouncer at a club who only lets certain VIP molecules through the membrane door! The transport proteins are incredibly selective - glucose transporters won't help amino acids, and ion channels are picky about which ions they allow through.
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