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A Handbook of Biology

PLÅSMØLÝSÏS

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If an external solution balances the osmotic pressure of the cytoplasm,

it is called isotonic. When a cell (or tissue) is placed in isotonic solution,

there is no net flow of water towards inside or outside (water flow is in

equilibrium). Such cells are said to be flaccid.

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If the external solution is more dilute (higher water potential) than the

cytoplasm, it is hypotonic. Cells swell (turgid) in hypotonic solution.

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If the external solution is more concentrated (more solutes) than the

cytoplasm, it is hypertonic.

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When a cell is placed in a hypertonic solution, water moves from the cell

(area of high water potential) across the membrane to outside (area of

lower water potential) and the cell shrinks. It is called plasmolysis. Water

is first lost from the cytoplasm and then from the vacuole.

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During plasmolysis, the cell membrane and protoplast of a plant cell

shrinks away from its cell wall. Such cells are said to be plasmolysed.

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Plasmolysis is usually reversible. When the cells are placed in a hypotonic

solution, water diffuses into the cell. As a result, the cytoplasm builds

up a pressure against the wall. It is called turgor pressure. The pressure

exerted by the protoplasts due to entry of water against the rigid walls

is called pressure potential (Ψp) The cell does not rupture due to the

rigidity of cell wall. Turgor pressure causes enlargement and extension

growth of cells.