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Water moving both directions during osmosis?

I just did a revision quiz (for bio a-level) and one question was:

'During osmosis, water molecules only move in one direction across the bilayer: from a high water potential to a low water potential.' True or False?

The answer was false but... why?

Reply 1

leage_beagle

Water will be moving in both directions - imagine you put a net half way across your bath and filled one half with food dye; the dye would leech across, but some dyed particles would cross back again as the water flowed, until eventually it was equally distributed (in equilibrium).

Water in osmosis is the same: it will move both way, but the net (or overall) movement is from the higher to lower concentration.