1. Diffusion
If
a small bottle filled with some gas or vapours is opened at a certain place in
the room, very soon its molecules become evenly distributed throughout the
available space in that room.
Similarly, if a solute is placed in its
solvent, it is dissolved and its particles move so that they are evenly
distributed throughout the container.
This movement of particles or molecules from a
region of higher concentration to a region of lower concentration is called as
diffusion.
The rate of diffusion of gases is faster than
liquids or solutes.
The
diffusing particles have a certain pressure called as the diffusion pressure
which is directly proportional to the number or concentration of the diffusing
particles.
Therefore,
the diffusion takes place always from a region of higher diffusion pressure to
a region of lower diffusion pressure i.e., along a diffusion pressure gradient.
The
rate of diffusion increases if:
(i)
The diffusion pressure gradient is steeper.
(ii)
The temperature is increased.
(iii)
The density of the diffusing particles is lesser.
(iv)
The medium through which diffusion occurs is less concentrated.
Diffusion
of more than one substance at the same time and place may be at different rates
and in different directions but is independent of each other. A very common
example of this is the gaseous exchange in plants.
Role
of Diffusion in the life of the plants:
i.
It is an essential step in the exchange of gases during respiration and
photosynthesis.
ii.
During passive salt uptake, the ions are absorbed by simple process of
diffusion.
iii.
Last step in stomatal transpiration is the diffusion of water vapours from the
intercellular spaces into the outer atmosphere through open stomata.
2. Osmosis
If
a solution and its pure solvent are separated by a semi permeable membrane
(which allows only solvent and not the solute to pass through it) the solvent
molecules diffuse into the solution. This diffusion of solvent molecules into
the solution through a semi permeable membrane (SPM) is called as osmosis.
In
case, there are two solutions of different concentrations separated by the
semi-permeable membrane, the diffusion of solvent will take place from the less
concentrated solution into the more concentrated solution till both the
solutions attain equal concentration.
The
phenomenon of osmosis can be demonstrated by the following simple experiment:
Mouth
of a thistle funnel is tied with goat bladder (it acts as semi-permeable
membrane) and concentrated sugar solution is filled in it whose level is marked
on its narrow neck. It is now placed in a beaker of water. After sometime the
level of the sugar solution in the thistle funnel rises.
Osmotic
Pressure:
As
a result of the separation of solution from its solvent or the two solutions by
the semi-permeable membrane, a pressure is developed in solution due to the
presence of dissolved solutes in it. This is called as osmotic pressure (O.P.).
Osmotic
pressure is measured in terms of atmospheres.
Osmotic
pressure is directly proportional to the concentration of dissolved solutes in
the solution. More conc. solution has higher osmotic pressure.
Osmotic
pressure of solution is always higher than its pure solvent.
Osmotic
pressure does not increase by the addition of insoluble solute in the solution.
Thus,
during osmosis the movement of solvent molecules takes place from the solution
whose osmotic pressure is lower (i.e., less concentrated or hypotonic) into the
solution whose osmotic pressure is higher (i.e., more concentrated or
hypertonic).
Osmotic
diffusion of solvent molecules will not take place if the two solutions
separated by the semi-permeable membrane are of equal concentrations having
equal osmotic pressures (i.e., they are isotonic).
Plant
Cells as Osmotic Systems:
Living
cells in plants form osmotic systems due to the presence of semi-permeable
plasma membrane and the cell sap having a certain osmotic pressure.
Plasma-membrane actually is not truly semi-permeable as it allows certain
solutes to pass through it and hence, it is known as selectively permeable or
differentially permeable membrane. The tonoplast or the vacuolar membrane also
possesses the same nature. The solvent in case of plants is always water. The
cell wall is permeable.
If
a living plant cell or tissue is placed in water or hypotonic solution (whose
O.P. is lower than that of cell sap) water enters into the cell sap by osmosis.
This process is called as end- osmosis. As a result of entry of the water into
the cell sap, a pressure is developed which presses the protoplasm against the
cell wall and the cell becomes turgid. This pressure is called as turgor
pressure. Consequence of the turgor pressure is the wall pressure which is
exerted by the elastic cell wall against the expanding protoplasm. At a given
time turgor pressure (T.P.) equals the wall pressure (W.P.).
T.P.
= W.P.
If
on the other hand, the plant cell or the tissue is placed in hypertonic
solution (whose O.P. is higher than that of cell sap) the water comes out of
the cell sap into the outer solution and the cell becomes flaccid. This process
is known as ex-osmosis. Cell or tissue will remain as such in isotonic
solution.
Significance
of Osmosis in Plants:
(1)
Large quantities of water are absorbed by roots from the soil by osmosis.
(2)
Cell to cell movement of water and other substances dissolved in it involves
this process.
(3)
Opening and closing of stomata depend upon the turgor pressure of the guard
cells.
(4)
Due to osmosis the turgidity of the cells and hence the shape or form of their
organs is maintained.
(5)
The resistance of plants to drought and frost increases with increase in
osmotic pressure of their cells.
(6)
Turgidity of the cells of the young seedlings allows them to come out of the
soil.
3. Plasmolysis
In
normal condition the protoplasm is tightly pressed against the cell wall. If
this plant cell or tissue is placed in a hypertonic solution, water comes out
from the cell sap into the outer solution due to ex-osmosis and the protoplasm
begins to contract from the cell wall. This is called as incipient plasmolysis.
If
the outer hypertonic solution is very much concentrated in comparison to the
cell sap, the process of ex-osmosis and contraction or shrinkage of protoplasm
continues and ultimately the protoplasm separates from the cell wall and
assumes a spherical form. This phenomenon is called as plasmolysis and the cell
or the tissue is said to be plasmolysed. Because of the permeable cell wall
the space in between the cell wall and plasma-membrane in plasmolysed cells is
filled with outer hypertonic solution.
If
a plasmolysed cell or tissue is placed in water, process of end-osmosis takes
place. Water enters into the cell sap, the cell becomes turgid, and the
protoplasm again assumes its normal shape and position. This phenomenon is
called deplasmolysis.
Advantages
of Plasmolysis:
1.
It indicates the semi-permeable nature of the plasma-membrane.
2.
This phenomenon is utilized in salting of meat and fishes and addition of
concentrated sugar solution to jams and jellies to check the growth of fungi
and bacteria which become plasmolysed in conc. solution.
3.
It is also used in determining the O.P. of the cell sap.
4. Imbibition
The
term imbibition means the absorption of water by the solid particles of an
adsorbent without forming a solution.
The
absorption of water by the solid particles of an adsorbent without forming a
solution is called Imbibition.
In
other words the adsorption of water by hydrophilic colloids is known as Imbibition.
Solid substance or adsorbent which take part
in imbibition are called Imbibants.
The
liquid which is imbibed is known as Imbibate.
Imbibition
of water of increases the volume of the imbibant due to which pressure is
created known as Imbibitional Pressure (IP).
Role
of Imbibition In Plants:
(i)
Imbibition is the first step in the absorption of water by the roots and cells,
(ii)
Imbibition of water by cell walls helps to keep the cells moist, and
(iii)
Imbibition pressure is helpful in seed germination, growth of seedling through
the soil, ascent of sap in plants, etc.
Significance
of Imbibition:
1.
It is the dominant and first step of water absorption.
2.
Imbibition is the first step of seed germination.
3.
Seedling is able to come out of soil due to development of imbibitional
pressure.
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