Water is a good solvent. The water molecules possess free energy. The free energy as per the laws of thermodynamic is the energy of a system capable of doing works. Thus it is the energy that determines the direction in which physical and chemical changes would occur.
The chemical potential is the free energy per mole of any substance i.e., per gram molecular weight. Chemical potential of water is of great importance in explaining movement of water from soil to plant and within plant and is called water potential. Water potential may be defined as the difference in free energy per unit volume, between pure water and osmotic ally constrained metrically bound or pressurized water (e.g., solution) at the same temperature. So, water potential of any system is equivalent to the chemical potential of water in that system. Water potential is denoted by the symbol of (Psi, a Greek letter) and is measured in terms of pressure e.g., bars or atmospheres.
The water potential of pure water is arbitrarily set at zero (o). When a solute is added to pure free water, the water potential becomes negative as the free energy gets decreased. In other words of a solution becomes less than zero i.e., a negative value.
Thus the water potential of all solutions is always less than zeros (a negative value). When pure free water is compressed or heated the water potential becomes positive as the free energy increases.
A difference in water potential between two regions determines the movement of water. Water moves from higher potential to lower potential. For example, if two regions (A & B) in an aqueous system have water potentials \j/A and \j/B respectively, the difference in water potential will be the two systems having different water potentials are separated by a semi permeable membrane, the movement of water molecules always takes place from the system having higher water potential (dilute solution) towards the system having lower water potential (concentrated solution).
The movement of water will continue till the water potentials of the two systems become equal and a stage of equilibrium is reached. At this stage, the net movement of water molecules will cease.
(a) The components of water potential:
The water potential of a living cell is determined by three major components (potentials) such as osmotic or solute potential metric potential and pressure potential (\|/p). The water potential is actually the sum of all the above three potentials.
(b) Solute potential
It is a component of water potential. This is due to the presence of solutes in a solution. Solute potential is also otherwise known as osmotic potential.
The presence of solute in water reduces the value of water potential. Reduction value of water potential is directly proportional to the amount of solute particles (molecules or ions) present in water. The term solute potential is a new term for osmotic pressure.
The value of solute potential of a solution is same as that of osmotic pressure but with a negative sign and is expressed in bars. For example if OP is atmosphere of the same solution.
The term pressure potential is the Hydrostatic pressure that a cell exerts from time to time due to osmotic entry of water into the cell. It is positive in sign. It increases the free energy of water and this raises water potential (y). It is equivalent to turger pressure.
The term matric is used for such surfaces which can absorb water molecules, e.g., cell walls, protoplasm and soil particles etc. Matric potential is the component of water potential influenced by the presence of a matric and possesses negative value. The \j/m in case of plant tissues and cells is often neglected because it is insignificant in osmosis. Thus the equation is written in simplified form as follows.
i.e., the water potential of plant cells is the sum of solute potential and pressure potential.
When a cell is fully turgid, its water potential is highest (=zcro). So p=s but both are opposite in sign. When a cell is flaccid, the pressure potential is zero, so y=ys + yp, both have equal sign.