Osmotic Pressure, Turgor Pressure, Wall pressure, Diffusion Pressure Deficient & Water Potential

Osmotic Pressure –

Osmotic pressure is a solution’s pressure which stops solvent’s movement through semipermeable membrane when it is applied against the direction of solvent’s movement.

OP of the pure solvent is also 0. OP is a character of solute.

Atriplex contertifolia has maximum OP. (It’s OP is about 202.5 atm.)

Significance of Osmotic pressure –

I. Water absorption by roots in soil.

II. Important role in opening and closing of stomata.

III. Movement in Mimosa pudica.

IV. Important for Turgor pressure.

Turgor Pressure (TP) & Wall Pressure (WP) –

When a plant cell is drowned in water, molecules of water is started to enter into the cell because of the high OP of vacuole sap. In this process the protoplasm started to push cell wall from inside to outside, this pressure is called Turgor Pressure (TP).

We know that the cell wall is also rigid, so it pushes back against Turgor Pressure, it is called Wall Pressure (WP). So we can say that the opposite TP is WP. TP is also called static pressure.

                                         TP = WP

 TP of a loose cell = 0

TP = WP = OP = max. for a turgid cell.

Limit of TP in plants – 0 to OP.

Diffusion Pressure Deficient (DPD) –

‘Diffusion pressure Deficient’ word is given by B.S. Meyer.

‘Sucker Pressure’ word is given by Renner.

It is also called sucker pressure (SP). Difference between the solution’s DP and pure solvent’s DP on a constant temperature, is called DPD. Solution’s DP is also less than solvent’s DP. DPD is also called “Need of Water.”

Generally, in a cell OP is greater than TP, and difference between OP & TP is called DPD.

DPD = OP – TP

DPD is proportional to concentration of solution.

DPD’s value –

for a normal cell – DPD = OP – TP

for fully turgid cell – DPD = 0, because for fully turgid cell OP = TP.

for loose cell – DPD = OP, because TP = 0.

for a plasmolised cell – DPD = OP + TP, because in this case TP is –ve.

Water Potential (ψ_w) –

Concept of water potential is given by Slatyer & Taylor.

Difference between free energy of pure water and water in solution, is the water potential of the system. Pure water’s water potential is highest / maximum. And its value is 0.

ψ_{w =} ψ_{s +} ψ_p

ψ_w = water potential

ψ_s = solute potential / osmotic potential

ψ_p = pressure potential

for a solution always - ψ_w = -ve, ψ_s = -ve, ψ_p = +ve.

Values of ψ_w –

for a normal cell – ψ_w = ψ_s + ψ_p

for fully turgid cell – ψ_w = 0, because OP = TP & ψ_s (-ve) = ψ_p (+ve).

for loose cell – ψ_w = ψ_s + ψ_p

                           ψ_w = ψ_s = -ve, because TP = 0 & ψ_p = 0.

for a plasmolised cell - ψ_w = ψ_s + ψ_p

                                         ψ_w = more negative, because TP = -ve & ψ_p = -ve)


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