Osmolarity

Introduction to Osmolarity

• Focus on sodium (Na) and potassium (P) effects on body fluids

  • ECF (extracellular fluid) is impacted by various factors:

  • Diet (what I eat)

  • Hydration (what I drink)

  • Physical activity (how much I exercise)

  • Urination (how much I pee)

  • Changes in ECF influence ICF (intracellular fluid) within cells

Definition of Osmolarity

• Osmolarity defined as:

  • The number of solute particles per liter of solvent.

  • Serves as a quantitative descriptor (a numerical value)

• Differences between osmolarity and tonicity:

  • Tonicity refers to effects of a solution on cells

  • Osmolarity provides a measurable quantity

Units of Osmolarity

• Units for osmolarity:

  • Measured in osmoles per liter

  • Commonly expressed in milliosmoles per liter (mOsm/L)

• Normal osmolarity for sheep red blood cells and most human cells:

  • 300 milliosmoles (mOsm)

• Solute particles contributing to osmolarity include:

  • Potassium ions (K+)

  • Chloride ions (Cl-)

  • Some phosphate molecules

Isotonic Solutions

• Isotonic solution defined as:

  • A solution with an osmolarity equal to the cell's osmolarity

  • For instance, 300 mOsm outside the cell

• Effects of isotonic solutions:

  • ECF osmolarity = 300 mOsm

  • No net movement of water across cell membranes

Hypertonic Solutions

• Hypertonic definition:

  • A solution with higher osmolarity than the cell (e.g., 315 mOsm)

• Consequence of hypertonic solutions:

  • Creates a concentration gradient for water

  • Non-penetrating solute particles remain outside the cell and cannot diffuse into it

  • Water is drawn out of the cell to balance solute concentration

  • Cells will shrink as a result

Hypotonic Solutions

• Hypotonic definition:

  • A solution with lower osmolarity than the cell

• Consequence of hypotonic solutions:

  • Penetrating solute particles enter the cell

  • Water is drawn into the cell along with the solute

  • Cells can swell and eventually burst

Determining Penetration

• Factors that determine if a solute is penetrating:

  • Lipid solubility

  • Molecular size

  • Charge of the molecule

• Examples of molecules under consideration:

  • Sucrose

  • Glycerol

  • Urea

Experimental Design

• We will conduct tests on three solutions, all with the same osmolarity

• Control solutions:

  • Pure water

  • Isotonic saline solution (0.9% saline)

• Test solutions:

  • Sucrose

  • Glycerol

  • Urea

• All test solutions set at 500 mOsm

• Note about concentrations:

  • Test solutions have low concentrations of solute inside the cells (approximately 0 moles)

Predicting Cell Behavior

• If solutes are penetrating:

  • Substances move from high concentration (outside) to low concentration (inside)

  • Cells expand and may burst due to influx of solute and accompanying water

• If solutes are non-penetrating:

  • Water will be drawn out of the cell because of the high external concentration

  • Cells will shrink but remain intact

Observations

• Cloudiness of the solution indicates non-penetrating solute presence:

  • Intact cell membranes cause a cloudy appearance

  • E.g., cloudiness in 0.9% saline will maintain turbidity

• Clarity of the solution indicates penetration of solutes:

  • If cells burst, the solution becomes clear with a pink tint from released hemoglobin

• Importance of this observation:

  • Helps determine whether solutes are penetrating or non-penetrating

Conclusion and Preparation for Experiment

• Ensure readiness for testing:

  • Wear gloves and glasses for safety

• This experiment will help us learn more about the behavior of cells in different osmotic environments