BIOLOGY Volume 2 - A Guide to General Biology - 2004

13. PLANT TRANSPORT

13.1. Plant Water Relations

13.1.3. Water Potential (ψ)

Water potential is a fundamental indicator of the thermodynamic state of water. Water molecules possess kinetic energy because they are in rapid, random motion in both the liquid state and as a gas (water vapor). The higher the concentration of these molecules in a system, the greater their total kinetic energy and the higher The water potential. Thus, water potential is at its maximum in pure water. When two water-containing systems (such as the soil and the atmosphere, or a Cell and its surrounding solution) come into contact, the random motion of water molecules results in a net movement toward the region where they are less abundant—from an area of higher water potential (higher energy) to one of lower water potential (lower energy)—until their concentrations in both systems are balanced.

In biology, water potential is typically measured in units of pressure (such as Pascals) ¹. It is at its maximum in pure water, which is conventionally assigned a value of zero.

Keep the following key points in mind:

1) the WATER POTENTIAL OF pure water is at its maximum and, by definition, is equal to zero (under "standard" conditions, i.e., normal atmospheric pressure and 25 °C);

2) water always moves down a ψ gradient: from where it is higher to where it is lower;

3) all solutions have a lower water potential than pure water, meaning that under standard conditions, their water potential is expressed as a negative value;

4) osmosis can be defined as the Movement of water molecules across a selectively permeable membrane along a water potential gradient.

The Advantage of Using METABOLISM/2.html">THE CONCEPT OF "Water Potential"

Water potential reflects the tendency of water molecules to move in a specific direction. The higher the water potential (the less negative it is), the greater the tendency of the molecules to leave a given system. If two systems (not necessarily separated by a membrane) are in contact with each other, water will move from the system with the higher water potential to the system with the lower water potential.

Thus, the concept of "water potential" makes it possible to determine the direction of water movement between any two systems—not only from one plant cell to another, but also, for example, from the soil to a ROOT, or from a leaf to the atmosphere. It can be said that water moves through the plant along a water potential gradient from the soil to the air. The steeper this gradient, the faster the flow of water.

¹ Formerly, pressure was measured in atmospheres (atm), but with the adoption of the SI system, Pascals (Pa) are now used:

1 Pa = 1 N/m² (N = Newton)

1 bar = 0.987 atm = 10⁵ Pa = 100 kPa

1 atm = 1.0132 bar = 1.0132 x 10⁵ Pa

1000 kPa = 1 MPa