APK 2105C Lecture Notes - Lecture 17: Osmotic Pressure, Osmotic Concentration, Passive Transport
23 May 2018
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Chapter 4, Lecture 3
Cell Membrane Transport
• Passive transport of H2O
o Osmosis = flow of water across a membrane
▪ Always passive (down concentration gradient)
▪ Water molecules cannot cross membrane—not fat-soluble
• Have to have a transmembrane protein (aquaporins) to cross the
membrane
▪ Unaffected by Vm
• Water is uncharged so it’s not affected by membrane potential
▪ Osmotic driving forces = how water goes down its own concentration
gradient
• Higher solute conc = lower water conc
• Water will flow towards lower water conc, higher solute conc
• Osmolarity = total solute particle concentration of a soln (can be
ECF OR ICF)
o Concentration of solutes
▪ 1 mole of solute = 1 osmolar (Osm)
• Described as osmoles instead of moles
because some molecules dissociate
▪ 1 milliosmol (mOsm) = 1/1000 Osmole
• 300 mOsm is standard solute conc for most
cells
o Some solutes dissociate into multiple particles
▪ Glucose does not dissociate
• 0.1 mole of glucose has osmolarity of 0.1
osmolar
▪ NaCl dissociates into Na and Cl so 0.1 mole of
NaCl has osmolarity of 0.2 Osm
o Iso-osmotic = 2 solns with equal osmolarity
▪ Same solute conc, same H2O conc
o Hyperosmotic = a soln whose osmolarity is higher than
another
▪ Higher solute conc, lower H2O conc
o Hypo-osmotic = a soln whose osmolarity is lower than
another
▪ Lower solute conc, higher H2O conc
▪ Osmotic pressure (pi)
• Osmotic pressure = osmolarity = total solute conc of a soln
• Still looking at the total solute conc, but how able it is to draw
water towards it
• Water travels up an osmotic pressure gradient
o Tonicity = concentration of impermeant solutes relative to ICF
▪ Only talking about ECF
▪ How many solutes are getting left outside the cell—impacts how the cell
will change in size
▪ Really want to know what is the impact on the shape of the cell
▪ Have to wait for things to stop moving across the membrane to get an
accurate reading
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