water potential(formula, diagram)

water potential

Water potential is a measure of the free energy of water in a system. It is denoted by the Greek letter psi (ψ) and is expressed in units of pressure, typically in megapascals (MPa) in the field of plant physiology. Water potential is a crucial concept in understanding the movement of water in plants, soil, and other biological systems.

The water potential of a solution is influenced by various factors, and the total water potential (ψ_total) can be broken down into components:

  1. Pressure Potential (ψ_p): This component is associated with the physical pressure on the water, such as turgor pressure in plant cells or pressure in a closed container. When pressure is applied to a solution, the water potential becomes more positive.

  2. Solute Potential (ψ_s): Also known as osmotic potential, this component is related to the concentration of solutes in a solution. As solute concentration increases, the water potential becomes more negative.

The formula for calculating total water potential is:

In practical terms, water will move from regions of higher water potential to regions of lower water potential. This movement can occur through processes like osmosis in plant cells or water uptake by plant roots in soil.

Understanding water potential is essential in fields such as plant physiology, ecology, and soil science, as it helps explain water movement in plants and ecosystems. It provides insights into processes like water uptake, transpiration, and overall water balance in biological systems.

water potential formula

The total water potential (total) is the sum of the pressure potential (p) and the solute potential (s):total=s+p

The formula for solute potential (s), also known as osmotic potential, is given by:


  • is the ionization constant (the number of particles into which a solute dissociates, e.g., 1 for glucose, 2 for NaCl),
  • is the ideal gas constant (approximately 0.0831 liter bar per mole per kelvin),
  • is the absolute temperature in kelvin.

The pressure potential (p) has two main components:

  1. Turgor pressure (): This is the pressure exerted by the cell contents against the cell wall in plant cells.

  2. Pressure in a closed system (p0): This is any physical pressure applied to a solution in a closed system.

The overall pressure potential (p) is given by:

So, when you have the values for solute potential, turgor pressure, and pressure in a closed system, you can calculate the total water potential using the first formula mentioned (total=s+p). Keep in mind that water will move from areas of higher water potential to areas of lower water potential.

water potential diagram

A water potential diagram typically illustrates the different components of water potential in a biological system, such as a plant cell. It helps in visualizing how water potential changes across various parts of the system. Below is a simple diagram to represent water potential in a plant cell:

│ ψ_total │
│ │
│ │
│ │

▼ ▼
┌───────────────────────┐ ┌───────────────────────┐
│ ψ_s (osmotic) │ │ ψ_p (pressure) │
│ │ │ │
│ │ │ │
│ │ │ │
└───────────────────────┘ └───────────────────────┘
│ ┌───────┐
▼ ▼ ▼
┌───────────────────────┐ ┌───────────┐ ┌───────────┐
│ ψ_solute │ │ P (turgor)│ │ ψ_p0 (pressure)│
│ │ │ │ │ │
│ │ │ │ │ │
│ │ │ │ │ │
└───────────────────────┘ └─────────────┘ └───────────────┘

In this diagram:

  • total represents the total water potential.
  • s is the solute potential (osmotic potential).
  • p is the pressure potential, which has two components: for turgor pressure and p0 for pressure in a closed system.

The diagram shows how the total water potential is influenced by solute potential and pressure potential. Water will move from regions of higher water potential to regions of lower water potential. In plant cells, turgor pressure is a significant contributor to the overall pressure potential, maintaining cell turgidity.

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