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Compares external and intracellular osmolarity to classify the solution as hypotonic, isotonic or hypertonic and predict the direction of water movement.
Tonicity
Gradient = external osmolarity - intracellular osmolarity; water moves down its own concentration gradient towards the higher solute concentration
No. Tonicity counts only solutes that cannot cross the membrane, so urea raises osmolarity without raising tonicity.
Animal cells lyse; plant cells become turgid because the cell wall resists further expansion.