Osmosis
Exam-ready definition
The net movement of water molecules from a region of higher water potential to a region of lower water potential, through a partially permeable membrane.
How this is marked
This is a 2-3 mark definition that appears constantly in Papers 2, 4 and 6. The three mark points are: water molecules (not 'water particles'), higher to lower water potential (not 'concentration'), and partially permeable membrane. Writing 'high concentration to low concentration' without saying water potential usually drops a mark at Extended level.
In an exam answer
If a question asks why a plant cell becomes turgid in pure water, write: 'Water moves into the cell by osmosis, from the higher water potential outside to the lower water potential of the cell sap, through the partially permeable cell membrane.'
Full topic guide: Movement into & out of cells
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