The difference between the free energy of water molecule in pure water and the free energy in any other system (water in a solution or in a plant cell or tissues) is called, the water potential of that system.
Water potential is represented by Greek letter Psi (Ψ). Water potential is usually measured in bars. Water always moves from the area of high water potential to the area of low water potential. Thus, the movement of water molecule occurs down the energy gradient. The free energy of a solvent can be increased by increasing the temperature and pressure. The presence of solute particles reduces the free energy of water and thus, decreases the water potential. Water potential of pure water at atmospheric pressure is zero. Water potential of the solution is always less than zero or has negative values. Free energy of water in cell sap or solution is less than that of pure water, i.e., less than zero.
Water potential is equal but opposite in sign to the diffusion pressure deficit (D.P.D.). If a difference in water potential exists between two regions, spontaneous movement of water will take place and the direction of flow will be energetically downhill, i.e., from the region of higher water potential to the region lower water potential.
Components of Water Potential:
For a solution such as contents of cell, water potential is determined by three sets of internal factors like:
- Matric potential (Ψm).
- Solute potential (Ψs).or osmotic potential
- Pressure potential (Ψp).
Ψ= Ψm+ Ψs+Ψp
Matric potential: Matrix is the term used for the surface (such as soil particles, cell wall, protoplasm etc.) to which water molecules are adsorbed. Matric potential is the component of water potential which is influenced by the presence of matrix. It has got a negative value in case of plant cell and tissue, the matric potential is often disregarded because it is not significant in osmosis. The equation for water potential may be simplified as follows:
Solute potential or osmotic potential: Solute potential is also known as osmotic potential. Osmotic potential or solute potential is the amount by which the water potential is reduced as a result of the presence of solute. Solute potentials are always in negative values. A reference standard is pure water at a water potential equal to zero. A reduction from this value gives a negative number.
Pressure potential: Pressure potential is a hydrostatic pressure. This is a pressure which develops in osmotic system due to osmotic entry or exit of water from it. A positive pressure develops in a plant cell or system due to entry of water into it. Positive hydrostatic pressure is also called turgor pressure. Loss of water produces negative hydrostatic pressure. It develops in xylem due to loss of water in transpiration. Pressure potential increases the water potential in the system.
The relationships among the various potentials can be expressed as follows:
Ψ= Ψm+ Ψs+Ψp
Since matric potentials are insignificant and can be ignored, the above relationships can be simplified as:
A flaccid cell has no turgor pressure or pressure potential is zero bars and if the osmotic potential or solute potential is -10 bars, then
Water potential=osmotic pressure+pressure potential
Ψ= Ψs+Ψp=-10 bars
In a fully turgid cell, there would be no water movement. For example, if a cell has solute potential of -10 bars and pressure potential of 10 bars, the water potential would be zero.
Ψ= Ψs+Ψp=-10 bars+10 bars=0 bars
FUNCTIONS OF TURGOR PRESSURE
- The opening and closing of stomata are caused by gain and loss of turgidity by their guard cell. Hence, they are often called “turgor-operated valves.”
- Turgor pressure (pressure potential or hydrostatic pressure) keeps a check on the excess entry of water into the cell.
- It keeps the cells and their organelles stretched which is essential for the proper functioning of the cells.
- It keeps the leaves fully expanded and properly oriented to light. Flower, young stems and other softer organs are able to maintain their form due to turgidity or turgor pressure.
- In case of loss of turgidity, the shoots droop down and the leaves show wilting.
- Sleep movements (nyctinasty) of the leaves of many legumes and shock movements,(seismonasty) of sensitive plants, (for example, Mimosa pudica) are produced due to turgor changes in the cells of their pulvini.
- Autochory of some fruits is dependent upon the release of turgor pressure.
DIFFERENCE BETWEEN OSMOTIC PRESSURE AND OSMOTIC POTENTIAL
|OSMOTIC PRESSURE||OSMOTIC POTENTIAL|
|1. Osmotic pressure is the maximum amount of pressure that can be developed in a solution separated from solvent by a semipermeable membrane.||1. Osmotic potential or solute potential is the amount by which the water potential is reduced as a result of the presence of solute.|
|2. Osmotic pressure develops only in a confined system.||2. It develops in both confined as well as in unconfined system.|
|3. The value is positive, though it is numerically equal to osmotic potential.||3. The value is negative, though it is numerically equal to osmotic pressure.|
DIFFERENCE BETWEEN MATRIC POTENTIAL AND OSMOTIC POTENTIAL
|MATRIC POTENTIAL||OSMOTIC POTENTIAL|
|1. It is reduction in free energy of water due to formation of thin immobile surface layer of water molecules.||1. It is reduction in free energy of water due to decrease in number of water molecules per molal volume.|
|2. Matric potential is caused by adsorbant or colloidal particles.||2. Osmotic potential is caused by the presence of solute particles.|
|3. It is essential for seed germination, early seedling growth and growth of developing fruits.||3. It is important in water relation of plant cells.|
DIFFERENCE BETWEEN WATER POTENTIAL AND DIFFUSION PRESSURE DEFICIT
|WATER POTENTIAL||DIFFUSION PRESSURE DEFICIT|
|1 It has a negative value.||1. It has a positive value.|
|2. Water potential is the reduction in free energy of solvent in a system over its pure state.||2. D.P.D. is the reduction of in diffusion pressure of a solvent in a system over its pure state.|
|3. Water is absorbed by a system with lower water potential from another system with higher water potential.||3. Water is absorbed by a system having higher D.P.D. from another system with lower D.P.D.|