IGCSE Biology glossary
Water potential
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Definition
A measure of the tendency of water molecules to move from one region to another. Pure water has the highest water potential. Adding solute to water lowers its water potential. Water always moves by osmosis from higher water potential to lower water potential.
What to include in an exam answer
At Extended level, use 'water potential' instead of 'concentration' in osmosis definitions. Say water moves from higher to lower water potential, not from 'dilute to concentrated'.
Example in context
Explain in terms of water potential why a red blood cell bursts in pure water (3 marks): Pure water has a higher water potential than the cytoplasm of the red blood cell. Water enters the cell by osmosis through the partially permeable membrane, from the higher water potential outside to the lower water potential inside. The cell has no cell wall to resist expansion, so it swells and bursts (lyses).
Related syllabus topic
Movement into & out of cells