In the examples in Figure 3-4, 0.25 M sucrose and 0.25 M urea had the same osmolarity: 0.25 Osm. But the two solutions had dramatically different effects on cell volume. In 0.25 M sucrose, cell volume didn't change, while in 0.25 M urea the cell exploded. To take into account the differing biological effects of solutions of the same osmolarity, we will use the concept of tonicity. An isotonic solution has no final effect on cell volume; a solution that causes cells to swell at equilibrium is called a hypotonic solution; and a solution that causes cells to shrink at equilibrium is called a hypertonic solution. Thus, the 0.25 M sucrose solution was isotonic, and the 0.25 M urea solution was hypotonic. Note that an isotonic solution must have the same osmolarity as the fluid inside the cell, but that having the same osmolarity as the ICF does not guarantee that an external fluid is isotonic.

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