Download 10. Plasmolysis and the effect of Osmosis on Cells

Plasmolysis and the effect of Osmosis on Plant Cells
Introduction:
No study of osmosis is complete without covering the concept of plasmolysis in plant cells.
Osmosis is the movement of water across a semi-permeable membrane from an area in which
water has a higher concentration (because it has fewer solutes) to an area where it has a lower
concentration (because it has more solutes).
In cells the semi-permeable membrane is the cell or plasma membrane. In plant cells this
membrane is normally not seen because it is:
 very thin and
 pressed up on the inside of the cell wall.
It is only when the cell wall and cell membrane separate that the cell membrane becomes visible
as in the photograph below.
Cell wall
Position of cell
membrane
The cell wall is made of cellulose fibrils bundled together into fibres. These are held by a variety of
materials that act as a glue but the cell wall is essentially porous to water and dissolved
substances.
Endosmosis – water movement into a Plant Cell
If the cell is an environment in which the surrounding water has a lower solute concentration (a
higher water concentration) than the cytoplasm, water will tend to enter the cell. The cell will
expand and the cell membrane will be pressed against the inside of the cell wall. In this state, cells
are said to be turgid and the pressure exerted by the cytoplasm on the cell wall is referred to as
turgor pressure.
If the plant is herbaceous (with thin cell walls) the turgor pressure will help support the plant and
hold it upright.
Exosmosis – water movement out of a Plant Cell
If the cell is in an environment in which the surrounding water has a higher solute concentration (a
lower water concentration) than the cytoplasm, water will tend to leave the cell. The cell will shrink
and the cell membrane will exert no turgor pressure. The cell will be flaccid.
As water continues to be lost the cytoplasm will shrink within the cell wall. In this state, cells are
said to be plasmolysed and the pressure exerted by the cytoplasm on the cell wall will be zero.
If the plant is herbaceous, the lack of turgor pressure will mean the plant loses some support and it
will become wilted.
How to plasmolyse cells
This is very straightforward. Simply mount your slides using a drop of 10% salt (NaCl) solution as
the mounting medium. Add a cover slip as normal. The water movement and plasmolysis will take
less than a minute.
The concentration of the salt solution can be altered to vary the rate and degree of plasmolysis that
will occur.
Cells to use:
It is best to use any cell where there is colour involved (chlorophyll in green cells) or choose a cell
with a pigment in its cytoplasm (Red onion or Rhubarb petiole epidermis).
Elodea leaves:
Above: Normal (left) and plasmolysed leaves (right). Low Power (x40)
Below: Focussed on cells within the leaf, normal (left) and plasmolysed cells (right). Because the
leaf is several cell layers thick, other unfocused cells layers are present. High Power (x400)
Red Onion Epidermal skin cells
Normal cells (left) and plasmolysed cells (below
left and right). Medium Power (x100)
Rhubarb petiole epidermal tear
It is quite difficult to get a tear that is only one cell thick but do persevere, it takes some practice.
Even if you do get a tear several layers thick the red pigmented epidermal layer will be easy to
identify. Other layers will blur the image however.
Above: Normal (left) and plasmolysed cells (right). Medium Power (x100)
Below: Normal (left) and plasmolysed cells (right). High Power (x400)
Filamentous Green algae and Spirogyra
Above: Normal (left) and plasmolysed cells (right) Filamentous Green Algae. (High Power x400)
Below: Plasmolysed Spirogyra Low Power (x40) (left) and High Power (x400) (right).
Nitella
Nitella is a genus of macro-algae with huge multinucleated cells. Individual cells can be 1 mm in
diameter and up to 3 cm long.
Normal cells above and plasmolysed cells below. (both images Low Power x40)