Osmotic Pressure

Osmotic pressure is the minimum pressure that must be exerted to prevent the passage of the pure solvent into the solution when the two are separated by a semi-permeable membrane.
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Osmotic pressure of a solution is a measure of its tendency to take in water by osmosis.

Relative concentration of the solutions that determines the direction and extent of diffusion is called tonicity.

• Osmotic Pressure (OP) is the pressure required to stop osmosis - i.e., the pressure needed to prevent solvent molecules from entering the cell through a semipermeable membrane. It has a positive value.
• Key equation - DPD = OP − TP (since TP = WP, also written as DPD = OP − WP). In a flaccid cell, TP = 0, so DPD = OP. In a turgid cell, DPD = 0, so TP = OP.
• Importance of Turgor Pressure (TP) - keeps cells stretched, supports non-woody tissues, is essential for cell enlargement during growth, maintains cell shape, and helps in the opening and closing of stomata.
• Importance of Osmosis - responsible for absorption of water into roots, maintains cell turgidity, facilitates cell-to-cell water movement, and offers resistance to drought and frost.
• Osmosis also causes drooping of leaflets in the "Touch Me Not" plant (Mimosa pudica) and helps in cell-to-cell movement of water in plant tissues.