Define and explain water potential.

Explanation

Water potential `(Psi_w)` is a relative term to understand water movement. It represents the difference between chemical potential of water in a system and that of a pure water. Water molecules possess kinetic energy. The greater the concentration of water in a system, the greater is its kinetic energy or water potential. Hence, pure water will have greatest water potential. The water potential of pure water at standard temperatures which are not under any pressure, is taken to be zero. Solute potential `(Psi_s)` and pressure potential `(Psi_p)` are the two main components that determine water potential.
Solute potential: 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 denoted by `Psi_s` It is always negative. The more the solute molecules the lower is the `Psi_s`. For a solution at atmospheric pressure water potential `Psi_w` = solute potential `Psi_s`.
Pressure potential : The pressure which developed due to entry of water in a cell is called pressure potential. It is always positive, it is denoted as `Psi_p`.
Water potential `Psi_w` of a cell is affected by both solute potential and pressure potential.
`Psi_w = Psi_w = Psi_p`