A technician drops human red blood cells into distilled water and watches them swell rapidly and then burst. Relative to the cytosol, the distilled water is best described as:

Core Concept

Water diffuses toward the side holding more nonpenetrating solute. Distilled water has essentially none, so it draws water inward across the membrane until the cell swells beyond what the membrane tolerates and lyses. Calling the solution hypotonic captures precisely this solute imbalance and predicts the observed swelling.