Q.2. Discuss About Water Potential: Components and Osmotic Relations of Cells.

Water potential
term was coined by Slatyer and Taylor (1960). It is modern term which is used
in place of DPD, The movement of water in plants cannot be accurately explained
in teams of difference in concentration or in other linear expression.

The best way to
express spontaneous movement of water from one region to another its in terms
of the difference of free energy of water betveen two regions (from higher free
energy level to lower free energy level).

According to
principles of thermodynamics, every components of system is having definite
amount of free energy which is measure of potential work which the system can
do. Water Potenti al is the difference in the free energy or chemical potential
per unit molar volume of water in systern and that of pure water at the same
temperature and pressure.

It is represented
by Greek letter or the value of is measured in bars, pascals or atmospheres.
Water always moves from the area of high water potentiai to the area of low
water potential. Water potential of pure water at normal temperature and
pressure is zero. This value is considered to be the highest. The presence of
solid particles reduces the free energy of water and decreases the water
potential. Therefore, water potential of a solution is always less than zero or
it has negative value.

Components of Water Potential:

A typical plant
cell consists of a çell wall, a vacuole filled with an aqueous solution and a
laver of cytoplasm between vacuole and cell wall. When such a cell is subjected
to the movemen of water then many factors begin to operate which ultimately
determine the water potential of cell sap.

For solution such
as contents of cells, water potential is detemined by 3 major sets of internal
factors:

(a) Matrix potential

(b) Solute potential or osmotic
potential

(c) Pressure potential

Water potentialin a
plant cell or tissue can be written as the sum of matrix potential (due to
Linding of water to cell and cytoplasm) the solute potential (due to
concentration of dissolve solutes which by its effect on the entropy components
reduces the water potentia) and pressere potential (due to hy drostatie
pressure, which by its effect on energy components increases tlie water
potential).

In case of plant
cell, m is usually disregarded and it is not significant in osmosis. Hence, the
above given equation is written as follows.

Solute Potential:

It is defined as
the amount by which the water potential is reduced as the result of the
presence of the solute, s are always in negative values and it is expressed in
bars with a negative sign.

Pressure Potential:

Plant cell wall is plastic
and it exerts a pressure on the cellular contents. As a result the inward wall
pressure, hydrostatic pressure is developed in the vacuole it is termed as torpor
pressure. The pressure potential is usually positive and operates in plant
cells as wall pressure and turgor pressure, Its magnitude varies between +5
bars (during day) and +15 bars (during night).

Important Aspects of Water Potential:

(1) Pure water has the maximum water
potential whicl. by definition is zero. (2) Water always moves from a region of
higher to one lower.

(3) All solutions have lower w than
pure water.

(4) Osmosis in terms of water
potential occurs region of higher water potential to a region of lower water
potential through a semi permeable membrane.

Osmotic Relations of Cells According
to Water Potential:

In case of fully turgid cell:

The net movement of
water into the cell is stopped. The cell is in equilibrium with the water
outside. Consequently the water potential in this case becomes zero. Water
potential is equal to osmotic potential + pressure potential.

In case of flaccid
cell: The turgor becomes zero. A cell at zero turgor has an osmotic potential
equal to its water potential.

In case of plasmolysed cell:

When the vacuolated
parenchymatous cells are placed in solution of sufficient strength, the
protoplast decreases in volume to such an extent that they shrink away from the
cell wall and the cells are plasmolysed. Such cells are negative value of
pressure potential (negative torpor pressure).

Numerical Problems:

1.
Suppose there are two cells A and B, cell A has osmotic potential -16 bars,
pressure potential - 6 bars and cell B as osmotic potential --10 bars and
pressure potential- 2 bars, What is the direction of movement of water?

Water potential of
cell A v, +y,"- 16 + 6--10 bars y of cell B=-10 +2-8 bars.

As movement of
water is from higher water potential (lower DPD) to lower water potent is (
higher DPD), hence the movement of water is from cell B to cell A.

2. If
osmotic potential of a cell is - 14 bars and its pressure potential is 7 bars.
What would be its water potential?

We know w. v,+ v,

Given, osmotic potential (y.) is-14
bars.

Pressure potentials (y) is 7 bars

Therefore, Water potential = (-14) +
5= -9 bars.

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