Osmotic Pressure and Osmotic Potential
Osmotic pressure varies according to the amount of dissolved material
in a solution that is according to the strength of a solution. The stronger
the solution, the greater is the osmotic pressure. Water flows across
a semi permeable membrane from a solution with a weak osmotic pressure
to one with a stronger osmotic pressure until both are equal. However,
there are circumstances in which strong solutions are prevented from taking
in water by osmosis. Under these conditions it is more accurate to describe
the solution as having an osmotic potential rather than an osmotic pressure.
Osmotic pressure is measured in kilo Pascals
Osmosis in plants cells
All living cells whether from plants or animals are surrounded by a semi-permeable
membrane This is the cell membrane. In plants, however the wall of cellulose
surrounds the cell membrane. The cellulose has no osmotic properties.
It is freely permeable to all liquid and dissolved substances but the
fact that it stretches very little limits the size to which plants cells
can swell when they take in water by osmosis. if you have a solution A
which is stronger than B that means A is hypertonic to B, B is hypotonic
to A and if they are equal in strength they are isotonic.
What happens to one plant cell under various osmotic conditions?
In A, the liquid inside the cell called cell sap has higher osmotic pressure
than the surrounding fluid (hypertonic to the surrounding fluid). The
cell sap has a lower osmotic pressure than the surrounding fluid (is hypotonic
to the surrounding fluid). The cell sap has a lower osmotic pressure than
the surrounding fluid (is hypotonic to the surrounding fluid). The cell
sap osmotic pressure is equal to the osmotic pressure of the surrounding
The cell wall is freely permeable and just slightly elastic and the cell
membrane is semi-permeable
In A the surrounding fluid which has a weaker osmotic pressure will
pass through the semi permeable membrane of the cell to the cell sap with
a higher osmotic pressure by osmosis. This and will result into formation
of a uniform mixture with the solution level in the cell being greater
than that in the surrounding fluid. The vacuole expands.
The water from the cell sap will pass through the semi permeable membrane
of the cell to the surrounding fluid by osmosis and will form a uniform
mixture with the solution level in the cell being less than in the surrounding
fluid. The vacuole will shrink.
The water molecules will be in constant motion but since the cell sap
is isotonic to the surrounding fluid, osmosis will not take place.
Aim: To demonstrate osmosis in living tissues;
Apparatus and materials; Knife, potatoes, Bunsen burner, water, petri
-Cut three half potatoes peel them thoroughly and cut cavities in them.
-Immerse one of then in boiling water for about ten minutes to kill the
-Place each of them in a petri dish of water and place some salt (sodium
chloride) in the cavities of the boiled potato and also in one of the
In A water in the petri dish reduced and a #solution# was formed in the
sodium chloride. In B it remains the same and so does in C
Osmosis takes place only in living tissues.
Experiment B; to demonstrate rigidity and plasmoylsis;
Apparatus; A large potato, cork borer
-Prepare two cylinders of potato tissue by pushing a cork borer into a
large potato and the core of tissue extracted by pushing with the flat
end of the pencil and the end trimmed with a razor blade or scapal.
-Cut the cylinders into two equal pieces of four centimetres each. Place
one cylinder in a test tube of solution A and the other in a test tube
of solution B. After forty minutes remove the cylinders and measure again.
Expected observation; Solution A was less concentrated than the cell sap.
Osmosis took place and water was taken into the cylinder. The cell became
Solution B was more concentrated than the cell sap Osmosis took place
and water was taken from the potato cylinder. The cell became deflated
and thus flaccid (plasmolysis)
As the cell absorbs more and more water by osmosis the vacuole enlarges
because it takes up most of the water and it pushes the contents against
the cell wall. That force by which the vacuole pushes the cell contents
against the cell wall is called turgor pressure and it goes on increasing
as more and more water comes in until when the cell wall cannot stretch
anymore. When the cell becomes inflated due to turgor pressure it is said
to be turgid. And when the cell wall is fully stretched then it said to
be fully turgid
This is the inward force, which is exerted by the cell wall, and which
is opposed to the outward turgor pressure and when the two forces are
equal then the cell stops inflating.
When a cell is fully turgid, it does not necessary mean that the cell
sap is equal to the surrounding in concentration
Turgidity is important in plants especially in young plants or plants
with herbaceous stems. It provides support that is when the cells are
turgid the stem is strong.
Water uptake by Plants:
Osmotic pressure in the cell sap is higher than the osmotic pressure in
surrounding water. The water enters the root hair by osmosis and the cell
sap in the root hair is diluted. The cell sap in the epidermis has a higher
osmotic pressure than that in the root hair by osmosis and become diluted.
The first cortical cell has a higher osmotic pressure than the epidermal
cell water from the epidermal cell will enter the first cortical cell
by osmosis making it dilute. And this goes on up to the last cell nearest
to the xylem.
Xylem is a dead tissue. Water does not move from the cortex to the xylem
by osmosis. It moves from the last cortical cell to the xylem by simple
diffusion. There are some forces through which water moves through xylem
up to the leaves