Lab 2: Osmolarity, Tonicity, and Osmosis

1. Differentiating Osmolarity and Tonicity

• Osmolarity refers to the concentration of solute particles in a solution, specifically non-penetrating solutes (those that cannot pass through the membrane). It is expressed in osmoles per liter (Osm/L).

• Tonicity refers to the effect of a solution on the volume of a cell, considering both penetrating and non-penetrating solutes. Tonicity describes how a solution will affect the shape or morphology of cells when placed in it.

  • Isotonic: No net movement of water, cell volume remains the same.

  • Hypotonic: Water moves into the cell, causing it to swell or burst.

  • Hypertonic: Water moves out of the cell, causing it to shrink.

2. Osmosis

• Osmosis is the movement of water across a semipermeable membrane from an area of lower solute concentration to an area of higher solute concentration.

• The net movement of water depends on the concentration of non-penetrating solutes (solutes that cannot pass through the membrane). Water will move towards the side with the higher concentration of non-penetrating solute particles.

3. Penetrating vs. Non-Penetrating Solutes

• Penetrating solutes: These solutes can cross the cell membrane (e.g., urea, ethanol). Their movement does not directly affect tonicity because they equilibrate across the membrane.

• Non-penetrating solutes: These solutes cannot pass through the membrane and contribute to the osmolarity of the solution, affecting water movement (e.g., NaCl, glucose).

4. Osmolarity of a Solution Relative to the ICF of Erythrocytes

• Hypo-osmotic: A solution with lower osmolarity than the intracellular fluid (ICF) of erythrocytes. Water will move into the cells, potentially causing them to swell.

• Iso-osmotic: A solution with the same osmolarity as the ICF of erythrocytes. There will be no net movement of water.

• Hyper-osmotic: A solution with higher osmolarity than the ICF of erythrocytes. Water will move out of the cells, potentially causing them to shrink.

5. Tonicity of a Solution Relative to the ICF of Erythrocytes

• Hypotonic: A solution with a lower concentration of non-penetrating solutes compared to the ICF. Water enters the cell, causing it to swell or lyse (hemolysis).

• Isotonic: A solution with the same concentration of non-penetrating solutes as the ICF. No change in cell shape or volume.

• Hypertonic: A solution with a higher concentration of non-penetrating solutes compared to the ICF. Water leaves the cell, causing it to shrink (crenation).

6. Effects of Iso-, Hypo-, and Hypertonic Solutions on Erythrocytes

• Isotonic solution: No change in morphology; the cell retains its normal shape.

• Hypotonic solution: The cell swells as water enters, potentially leading to lysis (bursting).

• Hypertonic solution: The cell shrinks as water leaves, resulting in crenation (shrinking).

7. Osmolarity Calculation (Given Molarity)

Osmolarity = Molarity × Number of particles in solution

• Example: For NaCl, which dissociates into 2 ions (Na⁺ and Cl⁻), the osmolarity of a 1 M NaCl solution = 1 M × 2 = 2 Osm/L.

8. Osmolarity or Molarity Calculation (Given Percent)

To calculate the osmolarity or molarity given a percent concentration:

• Step 1: Convert the percentage to grams of solute per liter of solution (e.g., 1% NaCl = 1 g NaCl/100 mL = 10 g NaCl/L).

• Step 2: Use the molar mass of the solute to convert grams to moles (e.g., NaCl molar mass = 58.44 g/mol).

• Step 3: If applicable, multiply by the number of particles into which the solute dissociates to get the osmolarity.

9. Hemolysis Test: Penetrating vs. Non-Penetrating Solutes

• Penetrating solutes: Will not cause hemolysis unless the solution is also hypotonic. Since they can move into the cell, water may follow, but the tonicity is determined by the non-penetrating solutes.

• Non-penetrating solutes: Will cause hemolysis in a hypotonic solution as water moves into the cells. If a solution is isotonic, there will be no hemolysis.

10. Relative Osmolarity and Tonicity of Given Solutions

• NaCl: Non-penetrating solute; it dissociates into two ions (Na⁺ and Cl⁻), making it osmotic. For a 1 M NaCl solution:

  • Osmolarity = 2 Osm/L

  • Tonicity depends on the surrounding solution—hypertonic if surrounding solution is hypotonic to cells.

• Glucose: Penetrating solute; it does not dissociate. For a 1 M glucose solution:

  • Osmolarity = 1 Osm/L

  • Tonicity depends on the surrounding solution—hypertonic to cells if surrounding solution is hypotonic.

• Urea: Penetrating solute; it does not dissociate. For a 1 M urea solution:

  • Osmolarity = 1 Osm/L

  • Tonicity depends on the surrounding solution—hypertonic to cells if surrounding solution is hypotonic.

Key Terms to Remember:

• Osmolarity: Concentration of solute particles (non-penetrating).

• Tonicity: Effect of a solution on the shape/volume of cells.

• Hypotonic: Lower osmolarity, causes swelling.

• Isotonic: Same osmolarity, no change in cell volume.

• Hypertonic: Higher osmolarity, causes shrinkage.

• Penetrating solutes: Can cross the membrane (e.g., urea, glucose).

• Non-penetrating solutes: Cannot cross the membrane (e.g., NaCl).