Osmosis, Osmotic Pressure, DPD and Water Potential

Osmosis

Osmosis is a special type of diffusion which involves the movement of solvent (only) from a region of its high concentration to a region of its low concentration across a semi-permeable or selectively permeable membrane.

Types of osmosis

Osmosis can be divided into two types on the basis of the direction of movement of solvent:

Endosmosis – when water enters the cell.
Exosmosis – when water moves out of the cell.

Osmosis can be defined in several ways using different parameters.

Osmosis is the movement of solvent from:

a region of its high concentration to a region of its low concentration across a semi-permeable membrane.
a region of low solute concentration to a region of high solute concentration across a semi-permeable membrane.
a dilute solution to a concentrated solution across a semi-permeable membrane.
a hypotonic solution to a hypertonic solution across a semi-permeable membrane.
high diffusion pressure (DP) of solvent to low diffusion pressure of solvent across a semi-permeable membrane.
low osmotic pressure (OP) to high osmotic pressure across a semi-permeable membrane.
low diffusion pressure deficit (DPD) to high diffusion pressure deficit across a semi-permeable membrane.
high solute potential to low solute potential across a semi-permeable membrane.
high water potential to low water potential across a semi-permeable membrane.

Explanations of the terms used

Types of solution

Hypotonic solution – a solution having relatively less solute concentration is called a hypotonic solution.
Isotonic solution – solutions having exactly the same solute concentration are called isotonic solutions.
Hypertonic solution – a solution having relatively high solute concentration is called a hypertonic solution.

Diffusion pressure

The pressure exerted by the diffusing particles due to their kinetic energy is called diffusion pressure.
The maximum value of diffusion pressure is zero, i.e. for a pure solvent.
On adding some solute to the pure solvent, the solvent molecules start to interact with the solutes. As a result, the number of free solvent molecules decreases, resulting in a decrease in the kinetic energy of the diffusing solvent. This decrease in the kinetic energy is called diffusion pressure. Thus, for a solution, the value of DP will always be negative.
When a solution (low DP) is separated from its pure solvent (high DP) by a semi-permeable membrane, the solution side has relatively fewer free solvent molecules than the pure solvent side. As a result, solvent will flow from the pure solvent side to the solution side to attain equilibrium. Thus, osmosis occurs from high DP to low DP.

Osmotic pressure

The pressure which develops in a solution when it is separated from its pure solvent by a semi-permeable membrane is called osmotic pressure (OP).
It is the external pressure required to be applied on the solution side to stop the flow of solvent when the solution is separated from its pure solvent by a semi-permeable membrane.
OP is measured in atmosphere or bar units.
For a pure solvent, the value of OP is zero (minimum value).
For a solution, the value of OP is always positive.
Osmotic pressure is directly proportional to the concentration of the solution.
When a solution (high OP) is separated from its pure solvent (low OP) by a semi-permeable membrane, the solvent moves from the pure solvent to the solution side until equilibrium is reached. Thus, osmosis occurs from low OP to high OP.

Turgor pressure

When a plant cell (or any cell with a cell wall) is placed in a hypotonic solution, water enters the cell (endosmosis) due to its OP. As a result, the volume of the protoplast increases and it exerts a pressure on the cell wall. This pressure is called turgor pressure (TP).
A cell is said to be turgid when no more water can enter the cell (OP = TP).
When a plant cell (or any cell with a cell wall) is placed in a hypertonic solution, the cell starts to lose water (first from the cytoplasm, then from the vacuole) due to exosmosis. As a result, the volume of the protoplast decreases, resulting in a decrease in TP. When the protoplast exerts no pressure on the cell wall (TP = 0), the cell is called a flaccid cell.
When a flaccid cell loses more water, the protoplast starts to detach from the cell wall. This is called plasmolysis.
Turgor pressure acts against osmotic pressure.

In a plasmolysed cell, the space between the cell wall and the shrunken protoplast is filled with the external solution, because the cell wall is freely permeable.

Diffusion pressure deficit/ suction pressure

The difference between the diffusion pressure of the pure solvent and the diffusion pressure of the solution is called diffusion pressure deficit (DPD).
It is calculated by the formula: DPD = OP − TP
For a pure solvent, DPD = 0 (minimum value).
For a solution, DPD is always positive.
Osmosis occurs from low DPD to high DPD.

Solute potential

When solute is added to a pure solvent, the solvent particles start to interact with the solute. As a result, the free energy of the solution decreases because there are fewer free solvent molecules. This decrease in the free energy of the solvent on the addition of solutes is called solute potential.
For a pure solvent, solute potential is zero (maximum value).
For a solution, solute potential is always negative.
The magnitude of solute potential is equal to osmotic pressure, but with a negative sign (solute potential = −osmotic pressure).
Osmosis occurs from high solute potential to low solute potential.

Pressure potential

The effect of pressure on the free energy of a solution is called pressure potential.
For a solution at atmospheric pressure, the value of pressure potential is zero.
The magnitude of pressure potential is equal to turgor pressure, with the same sign.

Water potential

The free energy of a solution depends on several factors, including solute potential, pressure potential, etc. The effect of all these factors on the free energy is called the water potential of the solution.
For a pure solvent, water potential is zero (maximum value).
For a solution, water potential is always negative.
The magnitude of water potential is equal to DPD, but with a negative sign (water potential = −DPD).
Osmosis occurs from high water potential to low water potential.

Ψ_w = Ψ_s + Ψ_p. For pure water at atmospheric pressure, Ψ_w = 0. Water always moves from higher (less negative) to lower (more negative) water potential.

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