Question

Cell A has an osmotic potential of −20 bars and a pressure potential of +6 bars. What will be its water potential?
A. −14 bars
B. +14 bars
C. −20 bars
D. −26 bars

Answer 1

Hint: Compared to the free energy of pure water at constant temperature and pressure, the difference of free energy present in the water of a system is called water potential. By adding solutes, the quantity of water is lowered in a solution called osmotic potential. (\[Psi \pi -\]) is the symbol of osmotic potential. It always indicates a negative value. After adding water to a cell, the amount of water potential increases called the pressure potential. (\[Psi p\]) is the symbol of pressure potential. A positive value is often seen.

Formula Used: We will use the formula,\[\Psi w=\Psi s+\Psi p\] \[where~\Psi w=\]water potential, \[\Psi s=\]osmotic potential and \[\Psi p\]=pressure potential.

Complete answer:
Water potential is defined as the difference in standard conditions between the chemical potential of water at any point in a system and that of pure water. It is indicated by the Psi symbol. In a plant cell, there are three main components of water potential.

These are: (a)the potential of the matrix, (b) the potential of the solute, and (c) the potential of strain. The osmotic potential is also known as the solute potential. It can be defined as the sum by which, in negative numbers, water potential is always expressed.

The cell wall and cell membrane reflect the pressure potential. The pressure (WP) on the cell material is exerted by the cell wall. Similarly, due to its solvent content, the cell membrane often exerts pressure on the cell wall. The potential for pressure is optimistic.
\[\Psi w=\Psi s+\Psi p\]
\[where~\Psi w=\]water potential, \[\Psi s=\]osmotic potential and \[\Psi p\]=pressure potential
\[\Psi w=(-20)+(+6)=-14~bars.\]

Hence, option A-14 bars is the correct answer.

Note: In the old terminology of turgor pressure (T.P.), osmotic pressure (O.P.), and diffusion pressure deficit (D.P.D.), the difference between water potential and osmotic potential can easily be understood by studying water relations of a cell. The pressure of the turgor can be defined as the actual pressure developed at a given time in a cell. With the passage of time, it differs.