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Transport:
5. Examples
of osmotic action in individual animal or plant cell types - blood cells and
plant cells
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INDEX
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(5)
Some details of examples of osmotic action in individual animal or plant cell types
(i)
The effect or pure water and salt
(sodium chloride) solution on red blood cells.
If cells are placed in pure water
(distilled or deionised) OR in a salt solution, the movement of water
through the partially permeable cell membrane by osmosis can have some
pretty devastating effects - illustrated by the diagram above concerning
those rather vital red blood cells, but it can happen to most cells!
On the left: The cells are in
a less concentrated solute solution compared to the cytoplasm - or in
just water.
If red blood cells are put in
pure water, the greater external water potential (more concentrated)
of the less dilute solution, means that water will pass into the
cell's cytoplasm by osmosis.
The diffusion gradient is into
the cells - which have a greater solute concentration than pure
water.
The result is the cells swell up,
burst open and die.
On the right: The cells are in
a more concentrated solution compared to the concentration of solutes in
the cytoplasm.
If red blood cells are put in a
salt solution, the greater internal water potential of the more
dilute solution of the cell's cytoplasm, means that water will pass
into the cell by osmosis
The result is the cells shrink
and shrivel up and die.
(ii)
The formation of a
plasmolysed plant cell
1. Turgid plant cell
When a plant has sufficient water,
the water passes into the cells by osmosis and the vacuole fills and
swells up.
The vacuole pushes against the cell
wall making the cell turgid.
This gives the plant structural
support so it doesn't droop/wilt - tall trees are an impressive example
of this!
2. Flaccid plant cell
If water passes out of the cells by
osmosis, the vacuole shrinks and the plant cell becomes flaccid.
The cytoplasm can begin to move away
from the cell wall.
3. Plasmolysed plant cell
If a plant cell loses a lot of water
by osmosis, cytoplasm of the cell
peels away from the cell wall, leaving gaps between the cell wall and the
membrane and making the plant cell shrink and crumple - wilt and droop.
Plasmolysis is the shrinking of the
cytoplasm of a plant cell in response to diffusion of water out of the cell
and into a high salt concentration solution by osmosis. During plasmolysis, the cell
membrane pulls away from the cell wall, but this does not happen in low salt
concentration because of the rigid plant cell wall.
Not surprisingly, plasmolysis can
happen in very dry conditions, but on watering (rain or us), most paler
wilted plants recover to the fully 'green' upright plant.
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