Osmolarity and Tonicity

Osmolarity

• Definition: Osmolarity is used to compare the relative solute concentration of two solutions.
• Measurement: Expressed in milliosmoles per liter (mOsm/L).
• Iso-osmotic: When two solutions have the same amount of solute concentration.
• Hyperosmotic and Hypoosmotic:
  • Hyperosmotic: A solution with higher osmolarity compared to another.
  • Hypoosmotic: A solution with lower osmolarity compared to another.

Example Scenarios

• Two solutions, A and C, one liter each with different osmolarities:
  • Solution A is hyperosmotic to Solution C.
  • Solution C is hypoosmotic to Solution A.

Reflection Coefficient

• Definition: Measures a solute's ability to cross a membrane.
• Range:
  • 1: Non-permeable solute.
  • 0: Freely permeable solute.

Osmolarity and Membrane Permeability

• Scenario with Non-Penetrating and Penetrating Solutes:
  • If a solute can cross the membrane, it equilibrates on both sides, affecting water movement.
  • Non-penetrating solutes cannot move, affecting osmotic balance.
  • Equal solute on both sides = no net water movement.
  • Different solute concentrations across membranes can cause osmosis (water movement).

Tonicity

• Definition: Describes a solution's effect on cell size, based on effective osmolarity.
• Focus: Effective osmolarity affects water movement in cells, important in physiology.

Tonicity and Cell Size

• Isotonic Solution: No net movement of water, cell size remains the same.
  • Example: Cell in 300 mOsm solution matches intracellular osmolarity.
• Hypotonic Solution: Causes cell to swell as water moves into the cell.
  • Example: Cell in 250 mOsm solution.
• Hypertonic Solution: Causes cell to shrink as water moves out.
  • Example: Cell in 350 mOsm solution.

Summary

• Osmolarity alone doesn't indicate water movement; solute permeability is crucial.
• Tonicity provides insight into how solutions affect cell volume and water movement.