STPM Biology Biological Molecules Part 20 Osmotic, Turgor, Wall Pressure and Water Potential

Osmotic Pressure

• When a solution is separated from pure water by semi-permeable membrane, there will be net water moving across into the solution.
• The minimum pressure that has to be exerted by the solution to prevent water from moving in is called the osmotic pressure of the solution.

Osmotic pressure examples

Turgor Pressure

• Turgor pressure = the pressure of cytoplasm exerted against the walls of a turgid cell.
• This pressure is counteracted by the wall pressure.

Wall Pressure (Ψ p)

• Wall pressure = the pressure of the cell wall exerted against the cytoplasm of the plant cell.
• The wall pressure is also known as pressure potential (Ψ p) for plant cells.
• Pressure potential usually has a positive value.

The relationship between turgor pressure and wall pressure

Water Potential (Ψ)

• Water potential = the potential of water to move out of a solution by osmosis.
• The water potential of a cell is the potential of water to move out of a cell through osmosis.
• Symbol = (Ψ) ; Unit = kPa (kiloPascal; 1kPa = 1000Pa) or MPa (MegaPascal), 1MPa = 100,000Pa)
• Pure water has the highest water potential. The water potential of pure water is 0 kPa at atmosphere pressure (101325 kPa).
• The water potential of a plant cell (Ψ) = solute potential (Ψ s) + pressure potential (Ψ p)

The water movement from a dilute to a concentrated solution

Solute Potential (Ψ s)

• Solute potential = the potential of a solution to take in water by osmosis due to the presence of solute materials.
• Solute potential is also known as osmotic potential.

Water Potential for Solution (Ψ sol)

• (Ψ sol) = the potential of water to move out of a solution by osmosis.
• The water potential of a solution is negative in value.
• This is because water potential for pure water is 0 kPa and pure water has the highest water potential.
• Solution with larger negative water potential value have low water potential.
• For example, cell A with water potential of -0.5 kPa has higher water potential than cell B with water potential value of -0.9 kPa.  Thus, water will flow from A to B.

The movement of water through different types of solution