Lab #5 Osmosis, Tonicity, and Measurements of Concentration

Diffusion and Concentration Solute particles diffuse from regions of high concentration to regions of low concentration –Proceeds until equilibrium reached

Diffusion and Membrane Transport Lipid bilayer determines what substances can readily pass through a cell membrane –if bilayer is permeable, simple diffusion –if bilayer is impermeable, no simple diffusion

Membrane Permeability Size –the smaller the particle, the more permeable –small molecules (O 2, CO 2, H 2 O) can –large molecules (protein, DNA) cannot Lipid Solubility –YES: non-polar molecules (O 2, cholesterol), –NO: charged atoms/molecules (Na +, Cl -, HCO 3 - ), large polar molecules (glucose)

Osmosis Net diffusion of water across a semi-permeable membrane –diffusion of the solvent, not the solute

Osmosis For osmosis to occur: 1.the membrane must be permeable to water and impermeable to at least one of the solutes in the solution 2.there must be a difference in solute concentration between the two sides of the membrane

Lab Exercise: Osmometer Obtain thistle tube assembly Remove stem Fill bell with sucrose solution Replace stem Place in water Mark meniscus

Measures of Concentration g/dL or % –convenient, but does not indicate # molecules/volume solution Molarity –moles solute/L solution Molality –moles solute/kg solvent Osmolality –moles of osm. active solutes/kg water Tonicity –effect of differences in osmotic pressure on net movement of water

Moles Mole = 6.02 x particles Mass of one mole of a substance = the atomic/molecular weight in grams # moles = mass (g) / m.w. (g/mole)

Molarity # moles solute / L solution Units = Molar (M) 1 M = 1 mole/L solution Calculation STEP 1: Determine # of moles solute (g/mw) STEP 2: Divide # moles by solution vol. in liters

Molality (m) # moles solute / kg solvent –for water, 1 kg = 1 L Units = molal (m) –1 m = 1 mole/kg solvent Calculated similar to molarity

Osmotic Concentration: Osmolality (Osm) total # of moles of solute particles dissolved in a given volume of water Units = Osmolal (Osm) 1 Osm = 1 mole / kg water osmolality depends on the number of solute particles, not the specific type of solute particles Calculation –Determine total moles solutes (g/mw for each individual solute) –Divide by kg water

Osmolality and Dissociation Ionic compounds dissociate in water –ionic bonds are broken Increases the # of solute particles in the solution –e.g. NaCl  Na + + Cl - –1 m NaCl solution has an osmotic concentration of 2 Osm Calculation –Determine # moles of each solute –Multiply # moles of ionic compounds by the number of particles created by dissociation –Add up total moles of particles –Divide by kg water

Tonicity Comparison of differences in osmotic pressure between two solutions separated by a semi-permeable membrane –e.g., intracellular fluid and extracellular fluid Effect of differences in osmotic pressure on osmotic movement of water

Tonicity if [osm] ECF = [osm] ICF –osmosis will not occur –extracellular fluid is isotonic if [osm] ECF > [osm] ICF –water will flow out of the ICF into the ECF –extracellular fluid is hypertonic if [osm] ECF < [osm] ICF –water will flow out of the ECF into the ICF –extracellular fluid is hypotonic

Exercise: Tonicity of Erythrocytes Observe cells in saline solutions of different concentrations –Hypotonic –Isotonic –Hypertonic