Lecture Notes: Water Potential and Osmosis

Introduction

• Covered D2.3 standard level content related to water movement.
• Water acts as an excellent solvent due to its polarity.

Water as a Solvent

• Dissolving Ionic Compounds:
  • Water forms shells around ions to prevent rejoining.
  • Positive hydrogens surround negative ions; negative oxygens surround positive ions.
• Dissolving Polar Compounds:
  • Water forms hydrogen bonds with polar molecules like glucose.

Osmosis

• Definition: Passive movement of water across a semi-permeable membrane towards higher solute concentration.
• Conditions: Occurs when two solutions of different concentrations are separated by a semi-permeable membrane.
• Isotonic Solutions:
  • Same concentration compared to another solution.
• Hypotonic Solutions:
  • Lower concentration compared to another solution (more water into the cell).
• Hypertonic Solutions:
  • Higher concentration compared to another solution (more water out of the cell).

Water Movement in Different Solutions

• Isotonic: No net movement of water.
• Hypotonic: Water enters the cell.
• Hypertonic: Water exits the cell.

Semi-Permeable Membranes

• Allow water, not solutes, to pass through, facilitating osmosis.
• Aquaporins:
  • Channel proteins increasing membrane permeability to water.

Measuring Osmolarity

• Osmolarity: Total solute concentration.
• Determined by placing plant tissue in solutions of varying solute concentrations and measuring mass change.
• Osmolarity Determination:
  • Hypotonic solution: tissue gains mass.
  • Hypertonic solution: tissue loses mass.
  • Isotonic point: no net mass change.

Controlled Variables in Osmolarity Experiments

• Same plant tissue, surface area to volume ratio, time, and temperature.

Plant Cells vs. Animal Cells

• Plant Cells:
  • Rigid cell wall prevents bursting in hypotonic solutions.
  • Turgid: High internal pressure, upright structure.
  • Flaccid: Loss of pressure, wilting.
  • Plasmolysis: Cell membrane shrinks from cell wall in hypertonic solutions.
• Animal Cells:
  • Lack cell wall, risk bursting in hypotonic solutions or shrinking in hypertonic solutions.
  • Require isotonic environments for stability.

Applications

• IV Solutions:
  • Saline solution used for rehydration, isotonic to human cells.
• Organ Transplant Preparation:
  • Organs bathed in isotonic solutions to prevent cell damage.