of this interaction and consequently decreasing the lipid mobility. Thus, at a given temperature, a short-tailed lipid will be more fluid than an otherwise identical long-tailed lipid. Another way of expressing this would be to say that the gel to liquid phase transition temperature increases with increasing number of carbons in the lipid alkane chains. Saturated phosphatidylcholine lipids with tails longer than 14 carbons are solid at room temperature, while those with fewer than 14 are liquid. This phenomenon is analogous to the fact that paraffin wax, which is composed of long alkanes, is solid at room temperature, while octane (gasoline), a short alkane, is liquid. Aside from chain length, transition temperature can also be affected by the degree of unsaturation of the lipid tails. An unsaturated double bond can produce a kink in the alkane chain, disrupting the regular periodic structure. This disruption creates extra free space within the bilayer which allows additional flexibility in the adjacent chains. It is this disruption of packing that leads to lower transition temperatures with increasing double bonds. This is a particularly powerful effect; decreasing the overall chain length by one carbon usually alters the transition temperature of a lipid by ten degrees Celsius or less, but adding a single double bond can decrease the transition temperature by fifty degrees or more (see table). An example of this effect can be noted in everyday life as butter, which has a large percentage saturated fats, is solid at room temperature while vegetable oil, which is mostly unsaturated, is liquid. Mixed systems Bilayers need not be composed of a single type of lipid and, in fact, most natural membranes are a complex mixture of different lipid molecules. Such mixtures often exhibit properties intermediate to their components, but are also capable of a phenomenon not seen in single component systems: phase separation. If some of the components are liquid at a given temperature while others are in the gel phase, the two phases can coexist in spatially separated populations. This phase separation plays a critical role in biochemical phenomena because membrane components such as proteins can partition into one or the other phase and thus be locally concentrated or activated. Cholesterol The presence of cholesterol exerts a profound but complicated influence on lipid bilayer properties because of its unique physical characteristics. Although it is a lipid, cholesterol bears little resemblance to a phospholipid. The hydrophilic domain