Copyright © 2000-2011 Mark Brandt, Ph.D. 95 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. 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 OOOOStearic acidOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOO OCopyright © 2000-2011 Mark Brandt, Ph.D. 96 “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. 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. Detergents are amphipathic molecules (such as fatty acids and related molecules) that act as emulsifiers by forming micelles and trapping more hydrophobic molecules in the non-polar center of the micelle. Digesting fat requires the ability to emulsify the fat so that it can be absorbed. Animals use detergents, and especially the cholesterol-derived bile acids, to solubilize the dietary fat to begin the absorption process. Three examples of bile acids are shown below. COOOPOOOCH2CH CH2OCOCH2CH2NCH3CH3CH3PhosphatidylcholineCOOOPOOOCH2CH CH2OCOCH2CH2NCH3CH3CH3COHOOPOOOCH2CH CH2OCOCH2CH2NCH3CH3CH3COHOOPOOOCH2CH CH2OCOCH2CH2NCH3CH3CH3COHOOPOOOCH2CH CH2OCOCH2CH2NCH3CH3CH3COHOOPOOOCH2CH CH2OCOCH2CH2NCH3CH3CH3COHOOPOOOCH2CH CH2OCOCH2CH2NCH3CH3CH3COHOOPOOOCH2CH CH2OCOCH2CH2NCH3CH3CH3COHOOPOOOCH2CH CH2OCOCH2CH2NCH3CH3CH3COHOOPOOOCH2CH CH2OCOCH2CH2NCH3CH3CH3COHOOPOOOCH2CH CH2OCOCH2CH2NCH3CH3CH3COHOOPOOOCH2CH CH2OCOCH2CH2NCH3CH3CH3COHOOPOOOCH2CH CH2OCOCH2CH2NCH3CH3CH3COHOOPOOOCH2CH CH2OCOCH2CH2NCH3CH3CH3COHOOPOOOCH2CH CH2OCOCH2CH2NCH3CH3CH3COHOOPOOOCH2CH CH2OCOCH2CH2NCH3CH3CH3COHOOPOOOCH2CH CH2OCOCH2CH2NCH3CH3CH3COHOOPOOOCH2CH CH2OCOCH2CH2NCH3CH3CH3COHOOPOOOCH2CH CH2OCOCH2CH2NCH3CH3CH3COHOOPOOOCH2CH CH2OCOCH2CH2NCH3CH3CH3COHOOPOOOCH2CH CH2OCOCH2CH2NCH3CH3CH3COHOOPOOOCH2CH CH2OCOCH2CH2NCH3CH3CH3COHOOPOOOCH2CH CH2OCOCH2CH2NCH3CH3CH3COHOOPOOOCH2CH CH2OCOCH2CH2NCH3CH3CH3COHOOPOOOCH2CH CH2OCOCH2CH2NCH3CH3CH3COHOOPOOOCH2CH CH2OCOCH2CH2NCH3CH3CH3HO OHOHOOCholic acidHO OHOHNHOTaurocholic acidSOOOHOOHOODeoxycholic acidCopyright © 2000-2011 Mark Brandt, Ph.D. 97 Saponification Another method for forming an emulsion is to convert a hydrophobic molecule into an amphipathic molecule. Hydrolysis at high pH results in release of free fatty acids and glycerol from any type of fatty acid ester, including membrane phospholipids and the triacylglycerols found in oils and fats. Soap is produced by addition of sodium hydroxide or potassium hydroxide to oil or membranes. Agents such as sodium hydroxide and hydrolytic enzymes are emulsifiers. Bilayers If a system contains large amounts of lipid, the lipid must either form many small micelles