Water crosses the cell membrane easily; most dissolved particles do not. Water moves from where its water potential Ψ is higher to where it is lower. Dissolved particles lower Ψ (Ψs = −c·R·T) and pressure raises it (Ψp). Pure water at normal pressure has Ψ = 0. In a dilute (hypotonic) bath, water flows into a cell. An animal cell swells and can burst. A plant cell presses against its wall and becomes firm, or turgid; the wall pushes back (turgor pressure) until no more water enters. In a concentrated (hypertonic) bath, water leaves. An animal cell shrinks and crinkles (crenation), and the living part of a plant cell shrinks away from its wall: plasmolysis. When the outside matches the inside (isotonic), there is no net flow.