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 (O2, CO2, H2O) can
– large molecules (protein, DNA) cannot
• Lipid Solubility
– YES: non-polar molecules (O2, cholesterol),
– NO: charged atoms/molecules (Na+, Cl-, HCO3-), 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
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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 1023 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