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Let us make in-depth study of the components of water potential and osmotic relations of cells according to water potential.
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 terms of difference in concentration or in other linear expression.
The best way to express spontaneous movement of water from one region to another is in terms of the difference of free energy of water between two regions (from higher free energy level to lower free energy level).
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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 Potential is the difference in the free energy or chemical potential per unit molar volume of water in system 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 potential 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 cell wall, a vacuole filled with an aqueous solution and a layer of cytoplasm between vacuole and cell wall. When such a cell is subjected to the movement 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 determined by 3 major sets of internal factors:
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(a) Matrix potential (Ψm)
(b) Solute potential or osmotic potential (Ψs)
(c) Pressure potential (Ψp)
Water potential in a plant cell or tissue can be written as the sum of matrix potential (due to binding 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 potential) and pressure potential (due to hydrostatic pressure, which by its effect on energy components increases the water potential).
Ψw = Ψs + Ψp + Ψm
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.
Ψw = Ψs + Ψp
Solute Potential (Ψs):
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.
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Pressure Potential (Ψp):
Plant cell wall is elastic 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 turgor 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 (Ψw):
(1) Pure water has the maximum water potential which by definition is zero.
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(2) Water always moves from a region of higher Ψw to one of lower Ψw.
(3) All solutions have lower w than pure water.
(4) Osmosis in terms of water potential occurs as a movement of water molecules from the 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:
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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.
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In case of plasmolysed cell:
When the vacuolated parenchymatous cells are placed in solutions 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 turgor 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 = Ψs +Ψp = – 16 + 6 = – 10 bars
Ψ of cell B = -10 + 2 = -8 bars.
As movement of water is from higher water potential (lower DPD) to lower water potential (higher DPD), hence the movement of water is from cell B to cell A.
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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 Ψw = Ψs + Ψp
Given, osmotic potential (Ψs) is – 14 bars.
Pressure potentials (Ψp) is 7 bars
Therefore,
Water potential = (-14) + 5 = – 9 bars.
