4.2.6—Water movement between cells

Water moves by osmosis across a partially permeable membrane from higher water potential to lower water potential. The direction of water movement comes first; the cell outcome then depends on whether the cell has a supporting cell wall.

• External solution has higher water potential than the cell: Water enters the cell. A plant cell becomes turgid as the wall resists further expansion; an animal cell swells and may burst because it has no cell wall.
• External solution has approximately equal water potential to the cell: There is no net water movement. A plant cell is flaccid rather than strongly supported by turgor; an animal cell remains near its normal volume. Individual water molecules still cross in both directions.
• External solution has lower water potential than the cell: Water leaves the cell. A plant cell loses turgor and may plasmolyse as the cell surface membrane pulls away from the wall; an animal cell loses volume and may crenate.
• Why the outcomes differ: The cellulose cell wall resists expansion and helps convert water entry into turgor pressure, but it does not stop water loss or prevent plasmolysis. Animal cells have no wall, so swelling and shrinking change the whole-cell shape more directly.

Compare the two water potentials to predict the direction of water movement, then apply the structural boundary. More dilute external conditions usually have higher water potential and drive entry; more concentrated conditions usually have lower water potential and drive exit. These concentration descriptions are relative conditions, not fixed numerical thresholds.

Do not say that water always moves into dilute cells or out of concentrated cells without comparing water potentials on both sides. No net change means equal opposing water flows, not no movement, and plant-cell walls prevent bursting but do not make plant cells immune to water loss.