Erythrocytes placed in 0.30 M urea initially look unchanged yet swell and lyse within minutes, while cells in 0.30 M sucrose remain stable. Both solutions register the same osmolarity. Which account fits?

Core Concept

Tonicity counts only nonpenetrating solutes. Sucrose stays outside and sustains a lasting balancing pull on water. Urea, being membrane permeant, diffuses in, so the effective concentration outside falls while intracellular particles accumulate; water follows inward down the widening potential difference and the cells burst. Identical measured osmolarity conceals opposite tonicities.