Overview
This lecture covers the principles of osmosis, osmotic pressure, and tonicity, detailing how solute concentration and membrane permeability affect water movement and cell volume.
Osmosis and Key Concepts
- Osmosis is the flow of water across a semi-permeable membrane due to a solute concentration difference.
- A solute is the substance dissolved (e.g., sugar); the solvent is the liquid that does the dissolving (e.g., coffee).
- A semi-permeable membrane only allows water to move, not solutes.
- Osmosis moves water from low solute concentration (high water concentration) to high solute concentration (low water concentration).
- The driving force for osmosis is the pressure difference (osmotic pressure), not just water concentration.
Osmotic Pressure
- Osmotic pressure: the pressure needed to stop water movement across a membrane due to solute concentration.
- For each mOsm (milliosmole) of impermeable solute, osmotic pressure increases by 19.3 mm Hg.
- Human intracellular osmolarity is about 280-300 mOsm/L; pure water exposure would generate over 5400 mm Hg pressure.
- It's essential to maintain correct electrolyte levels for proper osmotic pressure.
Osmolarity and Osmolality
- Osmotic pressure depends on the number of solute particles per unit volume, not their mass.
- Osmolarity = g × c, where g is the number of particles per mole and c is the concentration.
- Glucose (g=1), NaCl (g=2), CaClâ‚‚ (g=3); thus, 50 mmol/L NaCl has double the osmolarity of 50 mmol/L glucose.
- Osmolality is similar but uses kg of solvent instead of L of solution.
Tonicity and Effects on Cells
- Tonicity refers to how a solution affects cell volume based on membrane permeability to solutes.
- Isotonic solution: osmolarity equal to the cell's (no net water movement, no volume change).
- Hypertonic solution: higher osmolarity than the cell (water leaves cell, cell shrinks).
- Hypotonic solution: lower osmolarity than the cell (water enters cell, cell swells).
- Tonicity differs from osmolarity—depends on whether solute can cross the membrane (e.g., urea vs. sucrose).
- Urea can cross membranes, so even if osmolarity is equal, urea solutions can be hypotonic, causing cell swelling and lysis.
Effects of Tonicity on Red Blood Cells
- Isotonic—normal cell shape.
- Hypertonic—cell shrinks and spikes.
- Hypotonic—cell swells, may burst (lysis).
Key Terms & Definitions
- Osmosis — water movement across a semi-permeable membrane due to solute differences.
- Solute — substance dissolved in solvent.
- Solvent — liquid in which solute is dissolved.
- Osmotic Pressure — pressure created by solute concentration difference across a membrane.
- Osmolarity — concentration of osmotically active particles per liter of solution.
- Osmolality — concentration of osmotic particles per kilogram of solvent.
- Tonicity — ability of a solution to change cell volume depending on solute permeability.
- Isotonic — solution with same osmolarity as cell; no volume change.
- Hypertonic — higher osmolarity; cell shrinks.
- Hypotonic — lower osmolarity; cell swells.
Action Items / Next Steps
- Review the definitions and differences between osmolarity, osmolality, and tonicity.
- Practice identifying the effect of different solutions on cell volume.
- Prepare for questions about scenarios involving membrane permeability and solution effects on cells.