Cells : Permeability and Water Potential
The entry and exit of water into and out of the plant cells is due to a phenomenon called permeability of the plasma membrane. The plasma membrane is considered to be selectively permeable because it allows the solvent, water and a few selected molecules and ions to pass through it.
The plant being a multiphase system, here the movement of water is expressed in terms of free energy. Water will flow from a region of higher free energy to a region of lower free energy. Free energy is the thermodynamic parameter which determines the direction along which physical and chemical changes should occur and may be defined as the sum of the energy of a system capable of doing work.
Based on free energy, water potential may be defined as the difference between the free energy of water molecules in pure water and the free energy of water in any other system (eg) water in a solution or water contained in the plant cell. Water potential is denoted by the Greek letter psi Y and is measured in bars. Thus, water potential is the chemical potential of water. The water potential of pure water is zero bar and water potential in a plant tissue is always less than zero bar and hence a negative number.
When a typical plant cell containing cell wall, vacuole and cytoplasm is placed in a medium containing pure water, there are a number of factors which determine the water potential of the cell sap. These are called the components of water potential and are named as
i. Matric potential, ii. Solute potential and iii. Pressure potential
The term 'matric' is used for surfaces which can absorb water such as cell walls, protoplast and soil particles. Matric potential is the component influenced by presence of a matric and possesses a negative value and denoted as Ym.
It is a component of water potential which is also called Osmotic potential which represents the amount of solute present and is denoted as Ys. Ys of pure water is zero and so Ys values are negative.
The cell wall exerts a pressure on the cellular contents inwards called wall pressure causing a hydrostatic pressure to be exerted in the vacuole called turgor pressure which is equal and opposite to wall pressure. Pressure potential which is denoted as Yp is equivalent to either the wall pressure or turgor pressure. Water potential Y = Ym + Ys + Yp.
Water potential is thus the sum of the three potentials.
Thus the plant cell acts as an osmotic system having its own regulatory control over absorption and movement of water through the concerted effect of phenomena such as imbibition, diffusion and osmosis.