2. 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).
Condition Necessary for Imbibition
1. A water potential gradient should occur between imbibant and liquid imbibe.
2. There should be some force of attraction between imbibant and imbibed liquid.
3. Increase in temperature brings about an increased imbibition.
3. Characteristics of Imbibition
1. Adsorption: Imbibant substance is an adsorbent. Imbibant holds imbibate (water) by adsorption due to the great force of attraction
between the two.
2. Water Potential: Imbibants have a very high negative water potential. It is called matric potential. Water has (the highest) maximum
water potential (maximum being zero).
3. Water Potential Gradient: When dry imbibant (with high negative water potential) comes in contact with water (maximum water
potential), a steep gradient of water potential is created and water diffuses rapidly from its higher potential into the imbibant.
4. Heat of Wetting: Energy in the form of heat is released during imbibition. It is called heat of wetting.
5. Increase in Volume: Volume of the imbibant increases during imbibition e.g. swelling of soaked seeds, swelling of wooden frames during rainy
season, etc.
Imbibing Capacity: Imbibing capacity differs/varies in different imbibants for example, proteins being
hydrophilic colloids have maximum imbibing capacity. As compared, starch has lesser and cellulose has the least
capacity. As a result, when soaked in water, protein containing seeds e.g. pea swell more than starch storing
grains of wheat, maize etc.
Imbibition Pressure: When the imbibing substance is kept in a confined space, pressure is developed due to
the increase in the volume of the imbibant. This is called imbibition pressure. It develops due to the matric
potential of the imbibant, hence called matric potential and is denoted as Ѱm (= psi) Ѱm measured in bars or
mega pascals (MPa). Now a days, the term imbibition pressure is replaced by the new term matric potential.
5. Importance of Imbibition
Imbibition is the first step in the absorption of water by the roots and cells,
Imbibition of water by cell walls helps to keep the cells moist, and
Imbibition pressure is helpful in seed germination, growth of seedling through
the soil, ascent of sap in plants, etc.
6. Turgor Pressure
The osmotic entry of water in an osmometer or a plant or an organ will result in
the development of a pressure in the confined solution. It is spoken as turgor
pressure. Turgor pressure is defined as the pressure which develops in the confined
part of an osmotic system due to the osmotic entry of water into it. It is also
called hydrostatic pressure or pressure potential.
The cells having full turgor are termed as turgid. Due to turgor pressure the
protoplast of a plant cell will press the cell wall to the outside. The cell walls are
generally elastic to some extent. They press the protoplast with an equal and as
wall pressure. Normally wall pressure is equal and opposite to the turgor
pressure. It prevents bursting of plant cells and stops entry of water beyond a
certain limit. At the point where further entry of water is stopped, WP or TP
becomes equal to osmotic pressure or solute potential of the cell contents. Animal
cells and isolated organelles will burst in water due to absence of wall. They can
be maintained only in a solution of particular strength.
7. Importance
TP maintains the special spatial arrangement required by the cell organelles
( eg. Mitochondria, plastids).
It maintains the proper shape and stretching of softer oragans (eg. Fruits,
Flowers, Young shoots).
The growth of cells because of plastic stretching is partially dependent upon
turgor pressure.
It prevents excessive entry of water in the plant cells.
Phloem transport is based on development and gradient of turgor pressure.
8. Wall Pressure
It is the pressure exerted by wall
over the enclosed contents.
WP opposes the excessive entry of
water into the system and prevents
its bursting.
WP is caused by turgor or
hydrostatic pressure.
WP is equal to and opposite to TP
except in a plasmolysed cell and
during growth.
WP does not decreased below zero
except when it is pulled inwardly
by plasmodesmal strands of a
plasmolysed cell.
Turgor Pressure
TP is hydrostatic or swelling
pressure developed in a system due
to osmotic entry of water into it.
It develops due to entry of water
into the system & acts against the
wall.
TP is due to osmotic potential of
the system.
It is equal & opposite to WP except
when it becomes negative or
during cell growth.
TP becomes zero at the time of
limiting plasmolysis and below zero
during incipient and evident
plasmolysis.
9. PLASMOLYSIS
Plasmolysis is the shrinkage of the protoplast of a cell from its cell wall under
the influence of a hypertonic solution.
Osmosis is responsible for the occurrence of plasmolysis. Osmosis is a special
type of diffusion that occurs when water flows into or out of a membrane
such as a cell’s plasma membrane. It occurs based on the type of solution that
a cell is in. A solution is a mixture that contains a fluid, or solvent (usually
water), and a solute that is dissolved in the solvent.
When a cell is placed into a hypertonic solution, there is a higher
concentration of solutes outside the cell, so water flows out of the cell to
balance the concentration on both sides of the membrane. Since plasmolysis
is the loss of water from a cell, it occurs when a cell is in a hypertonic
solution.
10. when a cell is placed into a hypotonic solution, there is a lower solute
concentration outside the cell than inside, and water rushes into the cell. In
an isotonic solution, solute concentrations are the same on both sides, so there
is no net gain or loss of wate
Plant cells fare best in hypotonic solutions. This is because when plant cells are
full of water, they push against each other to form the basic support structure
for the plant and allow it to stand upright. Plant calls full of water are known
as turgid cells; they exert turgor pressure on each other. The cells’ rigid cell
wall keeps them from bursting. Unlike plant cells, animal cells do not have a
cell wall in addition to their cell membrane. When animal cells are placed in a
hypotonic solution and too much water rushes in, they will lyse, or burst. They
fare best in isotonic solutions instead.
11. Stages in Plasmolysis
Limiting plasmolysis: It is the first stage of the Plasmolysis. In this stage,
water starts flowing out of the cell; initially, the cell shrinks in volume
and cell wall become detectable.
Inicipient plasmolysis: It is the second stage of the Plasmolysis. In this stage,
the cell wall has reached its limit of contraction and cytoplasm gets detached
from the cell wall attaining the spherical shape.
Evident plasmolysis: It is the third and the final stage of the Plasmolysis. In
this stage, the cytoplasm will be completely free from the cell wall and
remains in the centre of the cell.
12. Types of Plasmolysis
Concave Plasmolysis: During the concave plasmolysis, both the cell membrane and
protoplasm shrink away and begins to detach from the cell wall, which is caused
due to the loss of water. Concave plasmolysis is a reversible process and it can be
revised by placing the cell in a hypotonic solution, which helps calls to regain the
water back into the cell.
Convex Plasmolysis: During the convex plasmolysis, both the cell membrane and
protoplasm lose so much water that they completely get detach from the cell
wall. Later, the cell wall collapses and results in the destruction of the cell.
Alike concave plasmolysis, convex plasmolysis cannot be reversed, and this
happens when a plant wilts and dies from lack of water. This type of plasmolysis is
more complex compared to convex plasmolysis.
Deplasmolysis: When the plasmolysed cell is placed in a hypotonic solution, (the
solution in which solute concentration is less than the cell sap), the water travels
into the cell due to the higher concentration of water outside the cell. Then the
cell swells and becomes turgid. This is known as deplasmolysis.
When the living cells are placed in isotonic solution(both solutions have an equal
amount of solute particles), the water does not flow within or outside. Here, the
water passes in and out of the cell and in an equilibrium state, and Therefore, the
cells are called as flaccid.
13. Importance of Plasmolysis
Plasmolysis shows that the cell membranes are semi-permeable.
Plasmolysis can be shown by only living cells. It can therefore, determine
whether a cell is living or dead.
It shows that the cell wall is elastic and permeable.
If a cell is allowed to remain for a sufficient time in a highly hypertonic
solution, it becomes permanently plasmolysed and gets killed. Weeds from
clay tennis courts and other playing grounds are killed by salting or
application of excessive salts.
Plants are not allowed to grow in the cracks of walls by a similar method of
salting.
Salting of pickles, meat or fish and sweetening of the jams or jellies kill the
spores of bacteria and fungi which would otherwise spoil them.
Excessive concentration of chemical fertilizers at one place on the soil should
be avoided, otherwise plants will die down.
14. Water Potential
Water Potential is the term used by plants physiologists for chemical potential
of water. It is the difference in the chemical potential or free energy per unit
molal volume of water in a system and that of pure water at the same temp.
Since WP is related to the difference in the vapour pressures of water in the
system and pure water state it is equivalent to diffusion pressure deficit or
DPD.
WP is designated by the symbol of Ψ. It is equal to the algebraic sum of three
potentials- Ψs stands for solute potential, Ψp for pressure potential,and
Ψm for the matric potential.
A fourth value called gravity potential(Ψg )becomes operational when water
or solution is at a height as it has a value of 0.1 bar/m.
The formula used to calculate Ψ is the following: Ψ = ΨS + ΨP + ΨG + ΨM
15. Matric potential :Is the force of adsorption with which some water is held
over the surface of colloidal particles present in the cytoplasm and cell wall.
The value of matric potential is taken as negative. In mature cells of
mesophytic plants, the value of Ψm is only about -0.1 atm . Therefore, its
contribution to total water potential is negligible and is often neglected i.e.
Ψ = ΨS + ΨP
Solute Potential: Of a solution is a decrease in the water potential or free
energy of water caused by the presence of ionic or non-ionic solute
particles.it is equivalent to the maximum osmotic pressure which can be
developed under ideal conditions but written with negative sign.
Pressure Potential: is the hydrostatic pressure which develops in a plant cell
due to inward flow of water it is also called turgor pressure.the swollen
protoplast exert a pressure on the cell wall which is elastic and develops an
opposite pressure called wall pressure. Turgor pressure or pressure potential
is usually positive. Pressure potential shows diurnal rhythm.
16. IMPORTANCE OF WATER POTENTIAL
Water potential is an important force which determines the water status ofa
plant organ or cell. A cell will lose water to nearby cells if its water potential
is high (less negative) and absorbs the same if the potential is low (more
negative). The cells suffering from water deficit or water stress are
therefore, able to avoid injury by obtaining water from other cells.
Leaves growing higher up on a tree have lower water potential than the ones
near the grounds. They therefore can exert a higher force to pull the water
column towards them.
A negative WP develops in xylem. It is imp. For moving water over long
distances.
Because of lower WP root hairs are able to absorb water from soil solution.
Xerophytes and halophytes have very low water potential in order to hold
water.
