Water potential

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Water potential of negative water solution

Water potential is the tendency of water to move from one place to another. It is typically measured in units of atmospheric pressure: pascals or pounds force per square inch or bars or dynes per square centimeter. It is a measure of the ability of a solution to absorb water by osmosis.

Pure water can absorb no more water - it has a defined water potential of zero. Solutions, however, can absorb more water, because all solutions have negative water potentials. The stronger the solution, the more negative its water potential. It is possible for the water potential to be positive or negative depending on the size of $Ψ p$ or $Ψ π$

1 Relation to free energy
2 Relation to water content
3 Simple Systems
- 3.1 Pressure potential
- 3.2 Solute potential
4 Complex Systems

[edit] Relation to free energy

Water potential ( $Ψ$ ) is related to Gibbs free energy by the following equation (where $V w$ is the molar volume of water):

$\Psi = \frac{G}{V_w}$

[edit] Relation to water content

A water retention curve depicts matric water potential ( $Ψ m$ ) as it relates to water content (θ). Different wetting and drying curves may be distinguished due to hysteresis.

[edit] Simple Systems

Many different potentials affect the total water potential. In a simple system, the primary 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 (sometimes called turgor pressure) is increased as water enters a plant 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 cell is usually positive. The opposite situation occurs when the water is pulled through an open sysem such as garden hose pipe or a plant vessel. In that the pressure potential 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 - as more solute is added, the solute potential becomes negative. The relationship of solute concentration (in molality) to solute potential is given by the Van't Hoff Equation:

Ψ π = - m i R T

where $m$ is the concentration in molality 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 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 cytoplasm to pull away from the cell wall, leading to plasmolysis.

[edit] Complex Systems

There are other contributors to water potential, and their contribution is given by the following equation:

Ψ = Ψ 0 + Ψ π + Ψ p + Ψ g + Ψ v + Ψ m

where $Ψ g$ is the gravimetric component, $Ψ v$ is the potential due to humidity, and $Ψ m$ is the potential due to matrix effects (eg, fluid cohesion and surface tension.)

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