Question

If ${ \Psi }_{ w }$ = water potential; ${ \Psi }_{ s }$ = solute potential; ${ \Psi }_{ p }$ = pressure potential, then select the correct equation showing their inter-relation.
(a) ${ \Psi }_{ w }$=${ \Psi }_{ s }$-${ \Psi }_{ p }$
(b) ${ \Psi }_{ w }$=${ \Psi }_{ s }$+${ \Psi }_{ p }$
(c) ${ \Psi }_{ s }$=${ \Psi }_{ w }$+${ \Psi }_{ p }$
(d) ${ \Psi }_{ w }$=${ \Psi }_{ s }$=${ \Psi }_{ p }$

Answer 1

Hint: Concepts like water potential are very important to understand the movement of water inside the plant cells through osmosis. Water potential is determined by two important concepts known as the solute potential and water potential.

Complete Answer:
Water potential (${ \Psi }_{ w }$): Water potential is a measure of the kinetic energy possessed by the molecules of water. If a solution has less amount of solute, more water molecules will be available to possess kinetic energy. That is a more dilute solution will have higher water potential. Hence, we can say that pure solvent will have the highest value of water potential. By convention, the highest value of water potential is taken to be 0. Hence any solution with some solute present in it will have negative water potential.
Solute Potential (${ \Psi }_{ s }$): The magnitude by which water potential decreases for a solution from a pure solvent is defined as solute potential. Hence, the value of the solute potential is always negative. The solute potential is a measure of the solute particles i.e. the more the amount of solute present, the more negative will be the value of solute potential.
Pressure potential (${ \Psi }_{ p }$): When external pressure is applied to a solution, its water potential is increased. When a cell is turgid, turgor pressure is applied on the walls of the cell and this results in the increase of water potential.
Hence, the relation is as follows:
Water potential = Solute potential + Pressure potential
So, the correct option is ‘${ \Psi }_{ w }$=${ \Psi }_{ s }$+${ \Psi }_{ p }$‘.

Note:
- Pressure potential is negative in some cases such as suction of water columns in the xylem vessels.
- For a solution with no external pressure, water potential will be equal to solute potential.
- Osmotic pressure is the negative value of the solute potential.