Osmosis and Tonicity

Definitions

• Osmosis: Diffusion of water across a semi-permeable membrane.
• Tonicity: Relative solute concentration of two environments separated by a semi-permeable membrane.
  • Determines the direction of osmosis.

Effects of Tonicity on Cells

Example with Red Blood Cells

• Hypotonic Environment (e.g., pure water):

  • Solute concentration greater inside the cell.
  • Cells are hypertonic relative to the beaker's hypotonic contents.
  • Water enters cells, potentially causing lysis (bursting of cells).

• Hypertonic Environment (e.g., salt solution):

  • Beaker's contents are hypertonic relative to the cell's interior.
  • Water diffuses out of cells, causing crenation (shrinking of cells).

• Isotonic Environment:

  • No net water movement.
  • Beaker and cells have the same tonicity.

Key Points

• Osmotic Pressure:
  • Drives water from a hypotonic to a hypertonic environment.
  • Water moves toward high solute (salt or sugar) concentrations.

Physiological Relevance

• Osmosis is crucial for kidney function.
• Explains physiological consequences of diseases like diabetes.