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one with the higher energy to one with the lower energy.
Thus, water will move from the system containing water with higher potential to the
one having lower potential (down the gradient). Hence, this movement of water is
called diffusion.
Note:
* The term water potential was first used by Slatyer and Taylor.
* By convention, the water potential of pure water at standard temperatures, which is
not under any pressure, is taken to be zero.
* Water potential is always regarded as the tendency of water to leave a system.
Solute Potential
The magnitude by which water potential is reduced due , to the presence of a solute
in pure water is known as solute potential. When solute or some substance is
dissolved in pure water, the concentration of water decreases and solution will have
fewer free water. It infers that the presence of solute particles in water reduces the
free energy of water due to which water potential also decreases.
Solute potential is denoted by \J/S and is always negative (or has value less than
zero). Hence,
More number of solute molecules = Lower solute potential (more — ve).
At atmospheric pressure for a solution

Pressure Potential
If a pressure more than atmospheric pressure is applied to pure water or a solution
(containing solute), the water potential increases. This is equal to pumping water
from one place to another such as our heart buildup pressure for the circulation of
blood in the body.
When water enters a plant cell through diffusion, it becomes turgid due to building up
of pressure against the cell wall in a plant system. This leads to increase in the
pressure potential. It is usually positive known a turgor pressure and is denoted by
fp. Loss of water during transpiration produces a negative hydrostatic pressure or
tension in the xylem. This is very important in transport (Asent of sap) over long
distances in plants.
Water potential is affected by both solute and pressure potential.

Osmosis
A plant cell is surrounded by both cell wall and cell membrane. The cell wall is freely
permeable, i.e., it does not act like a barrier to the movement of water and other
substances. The cell membrane and membrane of vacuole, the tonoplast are
responsible for determination of water or other molecules in or out of the cell.
The large central vacuole and its contents (vacuolar sap) contributes to the solute
potential of the cell.
Osmosis may be defined as the net movement of solvent molecules (water
molecules) across a differentially or semipermeable membrane.
Osmosis takes place very spontaneously in response to a driving force.
The movement of solvent, in osmosis occurs due to the differences of water