Lecture Notes: Tonicity and Osmolarity

Introduction

• Tonicity and osmolarity are crucial concepts in understanding how cells interact with their environment.
• Focus on how cell volume changes when placed in hypotonic and hypertonic solutions.
• Start with basic definitions and concepts, then explore solutions and their effects on cell volume.

Key Definitions

• Tonicity: Ability of extracellular fluid (ECF) to cause water to move in or out of a cell due to osmosis.
• Osmolarity: Total concentration of solute in a solution.

Osmosis and Cellular Environment

• Intracellular Fluid (ICF): Fluid inside the cell.
• Extracellular Fluid (ECF): Solution surrounding the cell, consisting of solvent (often water) and solute (e.g., proteins, sodium).
• Solution: Combination of solvent and solute.

Osmolarity

• Determines the concentration of solute in the solution.
• Important for comparing the relative concentrations between ECF and ICF.

Example: Red Blood Cell and Urea

• Consider a red blood cell with four molecules of urea.
• Placed in a solution with the same concentration of urea (4 molecules) as inside the cell, termed as isotonic.
• Isotonic Solution: Same concentration of solute as ICF, leading to no net movement of water.

Membrane Permeability

• Water: Permeable to plasma membrane, allowing easy passage.
• Solute: Impermeable to plasma membrane, cannot pass through.
• Plasma membrane allows water movement but restricts solute movement.

Osmosis in Isotonic Solutions

• Water moves freely across the membrane, but no net movement occurs because solute concentrations are equal on both sides.
• Volume of Red Blood Cell: No change in volume occurs in an isotonic solution.
• Key Point: Plasma membrane's permeability to water, but not solute, dictates movement.

Conclusion

• Tonicity: Evaluation of ECF's ability to move water in or out of the cell.
• In an isotonic solution, there is no net movement of water because solute concentration is balanced.
• Understanding these principles helps in predicting cellular behavior in different solutions.