Water potential

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Water potential is the potential energy of water relative to pure water in reference conditions. It quantifies the tendency of water to move from one area to another due to osmosis, gravity, mechanical pressure, or capillary action. Water potential is measured in units of pressure and is commonly represented by the greek letter Ψ. This concept has proved especially useful in understanding water transport within plants, animals, and soil.

Typically, pure water in a reference condition is defined as having a water potential of 0. Then, water at a higher elevation relative to the reference will have a positive water potential; water lower than the reference will have a negative water potential. If it is allowed to flow, water will move from the higher water potential pool to the region with lower (or more negative) water potential.

One very common example is water that contains a dissolved salt, like sea water. These solutions typically have negative water potentials, relative to the pure water reference. If there is no restriction on flow, water molecules will proceed from the pool of pure water to the more negative water potential of the solution.

[edit] Simple Systems

Many different potentials affect the total water potential, and these effects are additive. In a simple system, two components are the pressure potential (Ψ_p) and the solute potential (Ψ_π sometimes also Ψ_s). In this simple system, the water potential is given by the following formula:

Ψ = Ψ_p + Ψ_π

[edit] Pressure potential

Pressure potential is increased as water enters a cell. As water passes through the cell wall and cell membrane, it increases the total amount of water present inside the cell, which exerts a pressure on the cell wall that is retained by the structural rigidity of the cell.

The pressure potential in a cell is usually positive. The opposite situation occurs when the water is pulled through an open system such as garden hose pipe or a plant xylem vessel. In that, the pressure potential (also called tension) is negative and usually pulls the walls of the hose pipe or the vessel inwards. In plasmolysed cells, pressure potential is almost zero.

[edit] Solute potential

Pure water has a solute potential (Ψ_π) of zero. Solute potential can never be positive relative to pure water. The relationship of solute concentration (in molarity) to solute potential is given by the Van't Hoff Equation:

Ψ_π = − miRT

where m is the concentration in molarity of the solute, i is the Van't Hoff factor, the ionization constant of the solute (1 for glucose, 2 for NaCl, etc.) R is the ideal gas constant, and T is the temperature.

For example, when a solute is dissolved in water, the water molecules are less likely to diffuse away via osmosis than when there is no solute. Assuming atmospheric pressure to be constant, a solution will have a lower and hence more negative water potential than pure water. The more solute molecules present, the lower (and more negative) the solute potential is. The more concentrated a solution is, the more negative its water potential will be. Because water will spontaneously attain the lowest energy level possible, water will move from a higher potential to a lower potential. Thus, a cell with a lower solute concentration than the surrounding environment will have a higher water potential than the surrounding environment, and will lose water to the surrounding environment. In the case of a plant cell, this will eventually cause the plasma membrane to pull away from the cell wall, leading to plasmolysis.