PLANT BIOPHYSICS - Y. I. Posudin - 2004

II. TRANSPORT PROCESSES IN THE SOIL-PLANT-ATMOSPHERE SYSTEM

6. MASS TRANSPORT

6.3. WATER POTENTIAL

The movement of a solvent such as Water across a membrane is called osmosis. The membrane allows small solvent molecules to pass through while restricting the passage of large solute molecules. Similar to molecular diffusion and bulk flow, osmosis occurs spontaneously driven by specific forces.

While the driving force for diffusion is the concentration gradient and for bulk flow it is the pressure gradient, osmosis is governed by both gradients. The direction and rate of water flow across a membrane depend on two driving forces: the concentration gradient and the pressure gradient. This necessitates introducing METABOLISM/2.html">THE CONCEPT OF the Free energy gradient of water, or, per mole, the chemical potential gradient.

In plant biophysics, this gradient is referred to as the water potential gradient. It is The water potential that drives water movement within the soil-plant-atmosphere continuum. Water potential is denoted by the Greek letter ψ_w, with the typical unit of measurement being MPa (1 MPa = 10⁶ Pa). Qualitatively, this parameter characterizes the tendency of water to move from regions of higher water potential to lower water potential; quantitatively, it corresponds to the difference in free energy between water in a given system and pure water at atmospheric pressure. Pure water at the Earth's surface and air with a relative humidity of 100% have a water potential equal to zero.

Example. Suppose a xylem vessel is in contact with three Cells A, B, and C. The water potential values are 0, -0.1, -0.2, and -0.3 MPa for the vessel and cells A, B, and C, respectively. In which direction does the water move?

Solution. Water potential characterizes the ability of water to move from higher to lower water potential values, meaning that water will flow from the xylem vessel toward Cell C.