Water moves in response to the difference in water potential between two systems (the left and right sides of the tube)
Scientists use this term to explain the tendency of water to leave one place in favor of another. Water always moves from an area of higher water potential to an area of lower water potential. Water potential is usually affected by two factors: pressure and the amount of solute. For example, imagine a red blood cell dropped into distilled water. Water will move into the red blood cell and cause the cell to expand, stretching the flexible membrane. At some point, the pressure of the incoming water will cause the cell to pop, just like an over-filled balloon. Similarly, If a plant cell is placed in distilled water, water will enter the cell and the cell contents will expand. However, the elastic cell wall exerts a back pressure, which will limit the net gain of water.
The best way to express spontaneous movement of water from one region to another is in terms of the difference of free energy of water between two regions (from higher free energy level to lower free energy level). Water potential is the potential energy of water per unit volume relative to pure water in reference conditions. Water potential quantifies the tendency of water to move from one area to another due to osmosis, gravity, mechanical pressure, or matrix effects such as capillary action (which is caused by surface tension). To comprehend plant-water relations, an understanding of certain standard terms is necessary.
- Water potential (Ψw) is a concept fundamental to understanding water movement.
- Solute potential(Ψs) and pressure potential (Ψp) are the two main components that determine water potential.
Water molecules possess kinetic energy (energy which a body possesses by virtue of being in motion). In liquid and gaseous form they are in random motion that is both rapid and constant. The greater the concentration of water in a system, the greater is its kinetic energy or ‘water potential’. Hence, it is obvious that pure water will have the greatest water potential. If two systems containing water are in contact, random movement of water molecules will result in net movement of water molecules from the system with higher energy to the one with lower energy.
Thus water will move from the system containing water at higher water potential to the one having low water potential. This process of movement of substances down a gradient of free energy is called diffusion. Water potential is denoted by the Greek symbol Psi or Ψ and is expressed in pressure units such as pascals (Pa). By convention, the water potential of pure water at standard temperatures, which is not under any pressure, is taken to be zero.
If some solute is dissolved in pure water, the solution has fewer free water and the concentration of water decreases, reducing its water potential. Hence, all solutions have a lower water potential than pure water; the magnitude of this lowering due to dissolution of a solute is called solute potential or Ψs. Ψs is always negative. The more the solute molecules, the lower (more negative) is the Ψs . For a solution at atmospheric pressure (water potential) Ψw = (solute potential) Ψs.
If a pressure greater than atmospheric pressure is applied to pure water or a solution, its water potential increases. It is equivalent to pumping water from one place to another. Can you think of any system in our body where pressure is built up? Pressure can build up in a plant system when water enters a plant cell due to diffusion causing a pressure built up against the cell wall, it makes the cell turgid ; this increases the pressure potential. Pressure potential is usually positive, though in plants negative potential or tension in the water column in the xylem plays a major role in water transport up a stem. Pressure potential is denoted as Ψp. Water potential of a cell is affected by both solute and pressure potential. The relationship between them is as follows:
Ψw = Ψs + Ψp