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15-‐01-‐26 S.A. McFarland©2012 1
Amphiphilic compounds in aqueous solu=on
Spheroidal aggregate
Extended planar aggregate
15-‐01-‐26 S.A. McFarland©2012 2
Amphiphilic compounds in aqueous solu=on
• Twenty molecules of the detergent octyl glucoside
• Note: actual micelles and bilayers are irregular and rapidly fluctua+ng (unlike the idealized cartoon drawings)
Space-‐filling model of a micelle
Colliga=ve proper=es
• Solutes cause freezing point depression and boiling point eleva=on, making it more difficult for water molecules to crystallize as ice or to escape from solu=on into the gas phase
• Solute concentra=on also affects osmo=c pressure
• Water moves by osmosis, and solutes move by diffusion
15-‐01-‐26 S.A. McFarland©2014 3
Physical proper.es that depend on the concentra.on of dissolved substances rather than on their chemical features
Colliga=ve proper=es
15-‐01-‐26 S.A. McFarland©2014 4
Fluid inside cells and surrounding cells in mul=cellular organisms is full of dissolved substances ranging from small inorganic ions to huge molecular aggregates
hVp://2012books.lardbucket.org/books/principles-‐of-‐general-‐chemistry-‐v1.0/s17-‐05-‐colliga=ve-‐proper=es-‐of-‐solu.html
THINK OF CELLS AS SEMI-‐PERMEABLE SACS
Colliga=ve proper=es
15-‐01-‐26 S.A. McFarland©2014 5
OSMOSIS: movement of solvent across membrane from a region of high concentra.on (here pure water) to a region of lower concentra.on (water containing dissolved solute)
OSMOTIC PRESSURE Concentra=on in terms of solvent
Osmo=c pressure • Pressure that must be applied to the solu=on to prevent the inward flow of water; it is propor=onal
to the concentra=on of solute • For a 1M solu=on, the osmo=c pressure is 22.4 atm • Tonicity: measure of osmo=c pressure gradient • Animal cells surrounded by solu=on of similar osmo=c pressure (so there is no net flow of water) • Plants/bacteria enclose cell with rigid cell wall that can withstand osmo=c pressure generated
within
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Cells in high solute concentra=on solu=on
Dialysis: diffusion of solutes
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Membrane permeable to both water and solutes: basis of laboratory technique called DIALYSIS
Tendency of solutes to diffuse from area of high concentra=on to area of low concentra=on thermodynamically favoured due to increase in entropy
“down” a concentra=on gradient
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DIALYSIS IN THE LAB
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DIALYSIS IN THE CLINIC
Kidney dialysis pumps blood through a machine with a semi-‐permeable membrane enclosing a dialysate fluid to collect crea=ne and urea (present at high concentra=ons in blood); excess water also removed. “Cleansed” blood is pumped back into the pa=ent.
15-‐01-‐26 S.A. McFarland©2012 10
Water All aspects of cell structure and func=on are adapted to the physical and chemical proper=es of water
• Biological solvent • Par=cipant in chemical reac=ons • Biological template • Buffered solu=ons
