Lecture on Water Movement and Osmosis

Overview of Water as a Solvent

• Importance: Water is a critical solvent in the body.
• Focus: Understanding how water moves across membranes.
• Diffusion: Includes simple diffusion and facilitated diffusion, primarily concerning solute molecules.

Osmosis

• Definition: The passive movement of water across membranes.
• Cause: Driven by differences in solute concentration on either side of the membrane.

Water and Cell Membranes

• Hydrophilic Nature: Water is highly hydrophilic (water-loving).
• Movement: Generally unrestricted across most cell membranes despite being a small molecule.
• Phospholipid Bilayer: Typically lipophilic, but water is an exception.
• Aquaporins: Special water channels required in some tissues where direct movement is restricted.

Osmotic Pressure

• Concept: Dictates the direction of water movement.
• Calculation: Related to solute concentration differences.

Van't Hoff Equation

• Denoted by the symbol π.
• Variables:
  • R (Gas Constant) & T (Absolute Temperature): Generally stable in the body.
  • C (Concentration of Solute): Can vary.
  • Σ (Reflection Coefficient): Ranges from 0 to 1.

Reflection Coefficient

• Definition: A unitless number indicating a solute's permeability.
• Range: From 0 (freely penetrating) to 1 (completely non-penetrating).

Reflection Coefficient Examples

• Coefficient of 0:
  • Solute equilibrates across the membrane, no solute concentration difference.
  • No osmosis occurs.
• Coefficient of 1:
  • Solute cannot cross the membrane, causing a concentration gradient.
  • Water moves toward the side with higher solute concentration, causing osmosis.

Osmotic Pressure in Practice

• Preventing Osmosis: Pressure can be applied to counteract water movement.
• Magnitude of Pressure (Osmotic Pressure): Reflects the solution's tendency to cause osmosis.
• Non-Penetrating Solutes: Higher concentration increases osmotic pressure.

Summary

• Reflection Coefficient 0 or 1:
  • 0: No osmosis due to equal solute distribution.
  • 1: Osmosis occurs due to solute gradient.
• Real-World Application: Most substances have a reflection coefficient between 0 and 1, affecting water movement in nuanced ways.