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Osmosis and Water Potential Explained

Aug 17, 2024

Osmosis and Water Potential

Introduction to Osmosis

  • Osmosis: type of passive transport.
  • Defined as the net movement of water molecules from higher to lower water potential across a partially permeable membrane.
  • Exclusively involves water molecules.
  • Requires a selectively permeable membrane, usually the cell membrane.

Understanding Water Potential

  • Water Potential: Tendency of water molecules to move from one area to another.
  • Not to be confused with the amount of water.
  • High water potential: tendency to move out of an area.
  • Low water potential: tendency to remain in an area.

Factors Affecting Water Potential

  1. Solute Concentration

    • Solutes (e.g., glucose, sucrose, ions) lower water potential.
    • Higher solute concentration = lower water potential.

  2. Pressure

    • Applied pressure can increase water potential.

Baseline for Measuring Water Potential

  • Distilled water under atmospheric pressure has a water potential of 0 kPa.
  • No solutes in distilled water.

Impact of Solute Concentration

  • Adding solutes decreases water potential.
    • Example:
      • Distilled water: 0 kPa
      • Dilute salt solution: -50 kPa
      • Concentrated salt solution: -200 kPa
  • More solutes = more negative water potential.

Arranging Water Potentials

  • Order from highest to lowest potential:
    • 0 kPa > -100 kPa > -300 kPa > -400 kPa > -650 kPa.
  • Less negative values represent higher water potential.

Osmosis in Action

  • Example: Two solutions separated by a partially permeable membrane.
    • Solution A: -100 kPa
    • Solution B: -300 kPa
  • Net movement of water from higher to lower water potential (A to B).
  • Movement continues until equilibrium is reached.

Role of Pressure in Water Potential

  • Additional pressure increases water potential positively.
    • Example: Distilled water under pressure becomes +50 kPa.

Key Takeaways

  • Osmosis involves water movement based on potential gradients.
  • Solutes and pressure significantly affect water potential.
  • Understanding of water potential is crucial for predicting osmosis behavior.

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