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35 Water Relations in Plant Cells
Key Idea: Plant cells in a hypertonic solution lose water and undergo plasmolysis. In a hypotonic solution, they gain water creating turgor pressure. Osmosis across the partially permeable cell membrane is the main way by which water enters and leaves the cell. When
the external concentration of free water molecules is the same as that of the cytoplasm there is no net movement of water. Changing the tonicity of the external environment will cause a net movement of water into or out of the cell as water moves down its concentration gradient.
PR E O V N IE LY W
When the watery contents of a plant cell push against the cell wall they create turgor (tightness) which helps to provide support for the plant body. When cells lose water, there is a loss of cell turgor and the plant will wilt. Complete loss of turgor from a cell is called plasmolysis and is irreversible. Two systems (cell and environment) with the same effective osmotic pressure are termed isotonic and there is no net movement of water molecules. However, when there is an osmotic gradient between the cell and environment there will be a net movement of water molecules down their concentration gradient. The diagram below shows two different situations: when a plant cell is in a hypertonic solution and when it is in a hypotonic solution.
Cell in hypertonic salt solution
Wilted plant (cells have lost turgor) Plant cells are turgid
Cell in pure water (hypotonic)
Cell wall bulges outward but prevents cell rupture
Cell wall is freely permeable to water.
Water
The cytoplasm has a lower solute concentration than outside. Water leaves the cell.
Cell contents more dilute than the external environment
Water
Water
Cell contents less dilute than the external environment
Cytoplasm
Plasma membrane
MCC UW
Osmosis and tonicity
The cytoplasm has a higher solute concentration than outside. Water enters the cell, putting pressure on the cell wall.
Water
Water
Water
Plasmolysis in a plant cell
Turgor in a plant cell
Tonicity is a measure of the osmotic pressure of a solution. In a hypertonic solution, the external free water concentration is lower than the free water concentration of the cell. Water leaves the cell and, because the cell wall is rigid, the cell membrane shrinks away from the cell wall. This is called plasmolysis and the cell becomes flaccid.
In a hypotonic solution, the external free water concentration is higher than the cell cytoplasm. Water enters the cell, causing it to swell tight. A wall (turgor) pressure is generated when the cell contents press against the cell wall. Turgor pressure increases until no more water enters the cell (the cell is turgid).
1. Identify the outcome of the following situations:
(a) A plant cell is placed in a hypertonic solution:
(b) A plant cell is placed in a hypotonic solution:
(c) A plant cell in an isotonic solution:
(b) Discuss the role of cell turgor in plants:
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N AS OT SR F OO OR M US E
2. (a) Explain the role of cell wall pressure in generating cell turgor in plants: