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Water Potential (qp)
The chemical potential of water is called water potential. The total kinetic energy of the molecules is called water potential. The chemical potential of water depends on the number of water molecules. Thus it is expressed as energy per mole of water molecule. The chemical potential of pure water is taken as zero.
The kinetic energy of the molecules increases with the increase of temperature. More molecules are converted into vapours. It increases the free energy. Thus it also increases the water potential. Water molecules moves from the region of higher water potential to the region of lower water potential.
The water potential is expressed in bars. One bar = 0.987 atm. There are following three components of water potential. These are metric potential, solute potential and pressure
(a) Matric potential kW
The decrease in the water potential due to affinity of water molecules with the colloidal substances (protoplasm and cell wall) is called matric potential. It is represented by tvm. The water molecules are tightly bottnu on the colloidal surface. It reduces the movement of water molecules. Therefore, water potential decreases. Metric potential is very high in dry seed. It is -100 to -200 bars in it. It is also very high in clay soil particles. Thus The chemical potential of the seeds and soil depend upon metric potential.
(b) Solute potential (sp.)
The decrease in water potential due US dissolution of solute molecules or ions in water is called solute potential. The solute molecules start colliding with the water molecules. Therefore, water potential is reduced. It is also called osmotic potential or osmotic pressure (OP). It is expressed as bars with negative sign.
(c) Pressure potential (wp)
The turgor pressure (TP) exerted by the vacuole is called pressure potential. Its value is positive. Thus pressure potential increases the water potential. It varies from +4 to +5 bars during day time. But it is +10 to +15 bars at night. The young cells are with out vacuoles. Therefore, it is zero in these cells.
Interrelationships of components of water potential In cell
I. Water present in cell is never pure. It always contains some dissolved minerals, sugars and organic acids. Therefore, it always has some solute potential.
- Similarly, water is present in the vacuole. It always exerts some pressure potential.
- Cells always have some colloidal substances. They exert some matric potential.
Thus the water potential of the cell is the sum of different component of potentials as follows.
Water potential = Solute potential + Pressure potential + Matric potential
tP (water) = + ‘Pp +
But matric potential is negligible in cell. Therefore,
‘P (water) = + `Pp