Introduction to Membrane Physiology Lipids in aqueous solutions Some lipids and especially those comprised of saturated long chain fatty acids are solid at room temperature The main difference between fats and oils in biological terms is that fats are solid while oils are liquid at room temperature although both are largely comprised of triacylglycerols Butter which is solid at room temperature is comprised of triacylglycerols that contain 75 saturated and 25 unsaturated predominantly monounsaturated acyl chains In contrast olive oil contains triacylglycerols with 15 saturated 70 monounsaturated and 15 polyunsaturated fatty acyl chains Oils being liquids can be mixed with water however the two components tend to rapidly separate out into regions containing only water and only the oil This is predominantly due to the hydrophobic effect the water is most stable when it minimizes its contact with the hydrophobic molecules in the oil Single lipid molecules have a tendency to aggregate However aggregation is associated with a decrease in entropy for the lipid molecules Therefore at low concentrations of lipid individual lipid molecules may remain in solution As the lipid concentration increases the hydrophobic effect becomes more important and tends to push the carbon backbones of the lipid molecules together If the lipid contains a hydrophilic and especially a charged head group as is found in fatty acids the polar group will attempt to remain in contain with the aqueous solution This results in the formation of a spherical aggregate called a micelle O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O Stearic acid O O O O O O O O O O O O The shape of a micelle is a consequence of the shape of its lipid constituents Free fatty acids result in spherical micelles because the van der Waals surface of a free fatty acid is essentially conical note that the carboxylic acid at the end is slightly more bulky than the hydrocarbon tail and more importantly is negatively charged which limits the possible packing density of the carboxyl end of the fatty acid Micelle formation is cooperative because it requires the presence of enough amphipathic lipid molecules to form the spherical structure At low concentrations micelles cannot form Once a high enough concentration occurs essentially all any additional lipid will also form micelles The limiting concentration is called the Copyright 2000 2011 Mark Brandt Ph D 95 critical micellar concentration CMC the CMC is a function of the properties of the lipid molecules and will be different for each type of amphipathic lipid Both the CMC and the size of the micelle depend on the length of the acyl chain and the nature of the head group Longer acyl chains result in a lower CMC because both the hydrophobic effect and the strength of van der Waals interactions between the lipids are greater for longer hydrocarbon chains Phospholipids have a van der Waals surface that is similar to a rectangular solid As a result phospholipids try to form larger micelles However this presents a problem because it usually results in empty space somewhere in the particle A much more favorable arrangement is the formation of a planar bilayer structure which allows the optimum packing of the phospholipids Phosphatidylcholine O CH3 O P O CH2 CH2 N CH3 O CH3 CH2 CH CH2 O O C C O CH3 O O P O CH2 CH2 N CH3 O O O C O O C CH3 O P O CH2 CH2 N CH3 O CH3 CH3 CH2 CH CH2 O CH2 CH CH2 C O C O CH3 O O P O CH2 CH2 N CH3 O CH3 O C O C CH3 O P O CH2 CH2 N CH3 O CH2 CH CH2 O OH CH3 CH2 CH CH2 O O C OH O C O CH3 O P O CH2 CH2 N CH3 O CH3 CH2 CH CH2 O O C OH O C O CH3 O P O CH2 CH2 N CH3 O CH3 CH2 CH CH2 O O C OH O C O CH3 O P O CH2 CH2 N CH3 O CH3 CH2 CH CH2 O O C OH O C O CH3 O P O CH2 CH2 N CH3 O CH3 CH2 CH CH2 O O C OH O C O CH3 O P O CH2 CH2 N CH3 O CH3 CH2 CH CH2 O O C OH O C O CH3 O P O CH2 CH2 N CH3 O CH3 CH2 CH CH2 O O C OH O C O CH3 O O P O CH2 CH2 N CH3 O CH3 O C O C CH3 O P O CH2 CH2 N CH3 O CH2 CH CH2 O OH CH3 CH2 CH CH2 O O C OH O C O CH3 O P O CH2 CH2 N CH3 O CH3 CH2 CH CH2 O O C OH O C O OH O O C O OH O C C O O CH2 CH CH2 O O OH O C C O O CH2 CH CH2 CH3 O P O CH2 CH2 N CH3 CH3 O O OH O C C O O CH2 CH CH2 CH3 O O P O CH2 CH2 N CH3 CH3 O OH O C C O O CH2 CH CH2 CH3 O P O CH2 CH2 N CH3 CH3 O O OH O C C O O CH2 CH CH2 CH3 O P O CH2 CH2 N CH3 CH3 O O OH O C C O O CH2 CH CH2 CH3 O P O CH2 CH2 N CH3 CH3 O O OH O C C O O CH2 CH CH2 CH3 O P O CH2 CH2 N CH3 CH3 O O OH O C C O O CH2 CH CH2 CH3 O P O CH2 CH2 N CH3 CH3 O O OH O C C O O CH2 CH CH2 CH3 O P O CH2 CH2 N CH3 CH3 O O OH O C C O O CH2 CH CH2 CH3 O P O CH2 CH2 N CH3 CH3 O O OH O C C O O CH2 CH CH2 CH3 O O P O CH2 CH2 N CH3 CH3 O OH O C O CH2 CH CH2 CH3 O P O CH2 CH2 N CH3 CH3 O CH3 O P O CH2 CH2 N CH3 O CH3 This presents a minor problem in that the ends of the plane must have some protection from the aqueous solvent Alternatively the structure can form a large bilayer sphere with the planar structure slowly bending around to enclose a significant volume This is how the plasma membrane of a cell is constructed Emulsifiers An emulsion is a mixture of two or more materials that are ordinarily immiscible When a mixture of oil and water is agitated vigorously the mixture will form an emulsion However in most cases the oil and water will rapidly separate back into a two phase system if the shaking stops An emulsifier is a compound that assists in stabilizing an emulsion …