D2.3 Water Potential and Movement of Water

Properties of Water

• Excellent Solvent:
  • Water's polarity allows it to dissolve solutes effectively.
  • Ionic compounds:
    • Water forms shells around ions to prevent rejoining.
    • Hydrogens in water are attracted to negative ions; oxygens to positive ions.
  • Polar molecules:
    • Forms hydrogen bonds with compounds like glucose.

Osmosis

• Definition:
  • Passive movement of water across a semipermeable membrane towards higher solute concentration.
  • Water follows higher solute concentration.
• Types of Solutions:
  • Isotonic: Equal solute concentration as the comparative solution.
  • Hypotonic: Lower solute concentration compared to another solution.
  • Hypertonic: Higher solute concentration than the comparative solution.
• Water Movement:
  • In hypotonic solutions, water enters the cell.
  • In hypertonic solutions, water exits the cell.
  • In isotonic solutions, no net movement of water.

Osmolarity

• Definition: Total solute concentration in a cell.
• Determining Osmolarity:
  • Place plant tissue in solutions of various concentrations.
  • Measure percent change in mass to find isotonic point.

Cell Membranes and Water Movement

• Permeability:
  • Water is permeable; solutes are not.
  • Aquaporins increase water permeability.
• Controlling Osmosis:
  • Cells adjust solute concentration and membrane permeability.

Plant Cells

• Cell Wall:
  • Prevents excess water entry and bursting.
  • Maintains structural integrity.
• Turgidity vs. Plasmolysis:
  • Turgid: High internal pressure, occurs in hypotonic solutions.
  • Flaccid: Occurs in isotonic solutions.
  • Plasmolysis: Cell membrane shrinks away from cell wall in hypertonic solutions.

Animal Cells

• No Cell Wall:
  • Risk of bursting in hypotonic solutions.
  • Risk of shrinking in hypertonic solutions.
• Isotonic Environment:
  • Animal cells prefer isotonic solutions to maintain balance.
  • Use of saline solutions for rehydration and organ preservation.

Practical Applications

• Saline Solutions:
  • Used in IVs to rehydrate and maintain isotonic conditions for cells.
• Organ Transplantation:
  • Organs are kept in isotonic solutions to prevent damage during transfer.