Water potential

File:Solute Potential.svg

Diagram illustrating solute potential.

Water potential is a measure of the potential energy in water, specifically, the potential energy difference between a given water sample and pure water, under the same conditions. It is a key concept in understanding the movement of water in biological systems, particularly in plants.

Water potential is denoted by the Greek letter _ (Psi) and is measured in units of pressure, typically megapascals (MPa). The water potential of pure water at standard conditions is defined as zero.

Components of Water Potential

Water potential is composed of several components:

Solute Potential (__s)

Solute potential, also known as osmotic potential, is the component of water potential that is due to the presence of solute molecules. It is always negative or zero, as the addition of solutes lowers the water potential. The more solute present, the more negative the solute potential.

Pressure Potential (__p)

Pressure potential is the component of water potential that results from the exertion of pressure on a solution. In plant cells, this is often referred to as turgor pressure. Pressure potential can be positive or negative, depending on whether the pressure is applied or released.

Matric Potential (__m)

Matric potential is the component of water potential that results from the adhesion of water molecules to non-dissolved structures, such as cell walls or soil particles. It is typically negative and is significant in dry soils.

Importance in Plants

Water potential is crucial for understanding how water moves through plants. Water moves from regions of higher water potential to regions of lower water potential. This movement is essential for processes such as:

• Transpiration - the evaporation of water from plant leaves, which creates a negative pressure that pulls water upward from the roots.
• Osmosis - the movement of water across cell membranes, which is driven by differences in solute potential.
• Turgor pressure - the pressure of the cell contents against the cell wall, which is important for maintaining cell structure and function.

Related Pages

• Osmosis
• Transpiration
• Turgor pressure
• Plant physiology