Osmosis in Plant Cells

Plant cell in a more dilute solution

A dilute surrounding solution has a relatively high water potential. If that water potential is higher than the water potential inside the cell, the sequence runs like this:

• water enters the cell by osmosis;
• the volume of the cell contents increases;
• pressure builds as those contents press outwards against the cell wall;
• the cell becomes firm.

A cell in this state is described as turgid. The pressure of the water-filled contents pressing outwards against the wall is turgor pressure. Because the cell wall is rigid and resists further expansion, a normal plant cell does not simply keep swelling indefinitely the way an animal cell placed in the same conditions might — the wall is what stops it.

Turgor and support in plants

Turgid cells are not just a curiosity of what happens under a microscope — they are how many non-woody parts of a plant hold themselves up. When enough cells in a tissue contain sufficient water, their contents press outwards against their cell walls throughout the tissue, and the combined effect of all that pressure is what keeps the tissue firm.

Cambridge Core candidates are specifically required to know that plants are supported by the pressure of water inside their cells pressing outwards on the cell wall — which is exactly why a wilting plant, one whose cells have lost that pressure, droops rather than standing upright. Water inside a plant cell is not simply “present”; the pressure it generates is doing mechanical work.

Plant cell in a more concentrated solution — Supplement

A concentrated surrounding solution has a lower water potential. If that water potential is lower than the water potential inside the cell, the whole sequence above runs in reverse:

• water moves out of the cell through the cell membrane by osmosis;
• the amount of water inside the cell decreases;
• turgor pressure decreases;
• the cell becomes less firm.

A cell in this state is flaccid — it has lost the turgor pressure that made it firm, but its membrane has not yet pulled away from the wall. If enough water continues to leave, the cell contents shrink further, and eventually the cell membrane pulls away from the cell wall entirely. That further stage is called plasmolysis.

These four terms describe related but genuinely distinct stages, and Cambridge expects you to use them precisely rather than interchangeably:

• turgid — the cell contains enough water to generate strong turgor pressure;
• turgor pressure — the pressure produced as the water-filled cell contents press against the wall;
• flaccid — the cell has lost turgor pressure and is no longer firm, but the membrane still touches the wall;
• plasmolysis — water loss has gone far enough that the cell membrane pulls away from the cell wall.

Why water potential and osmosis matter — Supplement

A cell cannot control its own water balance independently of what surrounds it. Wherever a partially permeable membrane separates two regions with different water potentials, there can be net movement of water across it, and that movement is not neutral — it changes real, measurable things about the cell or tissue involved:

• the amount of water inside the cell;
• the cell’s volume and internal pressure;
• how firm a plant tissue is overall;
• the organism’s overall water balance.

Osmosis is therefore the explanation for both uptake and loss of water in an organism, which is exactly what Cambridge expects Supplement candidates to be able to reason through, including in situations you have not seen described before. The method is always the same four steps: identify the two regions separated by the membrane, work out which has the higher water potential and which has the lower, move water from higher to lower, then predict what that does to the cell or tissue in front of you.

For the full definition of osmosis and how water potential itself works, see osmosis. For the practical investigations that generate the mass-change data behind these predictions, see osmosis experiments.