Tonicity and Cell Volume

Osmosis is just water diffusing across a membrane, moving from where it is more concentrated to where it is less concentrated. Since solute and water share the same space, more solute means less water by proportion. So water moves toward the side with the higher solute concentration, the side with relatively less water. Tonicity is the name for this comparison between two solutions: hypertonic means more solute than the thing it's being compared to, hypotonic means less solute, and isotonic means equal solute concentrations.

Picture an animal cell, which has no rigid wall to hold its shape. Drop it into pure water, the most hypotonic bath possible: there is no solute outside at all, so water rushes in, the cell swells, and if it keeps swelling it can burst, a process called lysis. Drop that same cell into heavy brine, a strongly hypertonic solution: water rushes out, and the cell shrivels, called crenation. In an isotonic solution, water still moves both directions across the membrane, but at equal rates, so there is no net change in cell volume.

This connects directly to why a raisin plumps up in water: the raisin's cells are packed with solute and almost no water, so dropping it into a hypotonic bath pulls water in until it swells back toward a grape. The same logic explains why cells need mechanisms to regulate internal solute concentration, since uncontrolled osmotic swelling or shrinking can damage or kill a cell.