on 05-Nov-2016 (Sat)

At still higher concentrations, phospholipids spontaneously form bilayers

In phospholipid bilayers, the phospholipid molecules arrange themselves into two parallel sheets or leaflets that face each other tail to tail

An individual phospholipid molecule is free to diffuse within the entire layer of the membrane where it resides

The rate at which this two-dimensional diffusion occurs is extremely temperature dependent. At high temperatures, the thermal energy of any given lipid molecule is greater than the interaction energy that would tend to hold adjacent lipid molecules together. Under these con- ditions, lateral diffusion can proceed rapidly, and the lipid is said to be in the sol state. At lower temperatures, interac- tion energies exceed the thermal energies of most individual molecules. Thus, phospholipids diffuse slowly because they lack the energy to free themselves from the embraces of their neighbors. This behavior is characteristic of the gel state

The temperature at which the bilayer membrane converts from the gel to the sol phase (and vice versa) is referred to as the transition temperature.

The transition temperature is another characteristic that depends on the chemical makeup of the phospholipids in the bilayer. Phospholipids with long, saturated fatty acid chains can extensively interact with one another. Consequently, a fair amount of thermal energy is required to overcome these interactions and permit diffusion. Not surprisingly, such bilayers have relatively high transition temperatures. For example, the transition tem- perature for dioctadecanoic phosphatidylcholine (which has two 18-carbon fatty acid chains, fully saturated) is 55.5°C. In contrast, phospholipids that have shorter fatty acid chains or double bonds (which introduce kinks) cannot line up next to each other as well and hence do not interact as well. Con- siderably less energy is required to induce them to partici- pate in diffusion. For example, if we reduce the length of the carbon chain from 18 to 14, the transition temperature falls to 23°C. If we retain 18 carbons but introduce a single, double bond (making the fatty acid chains monounsaturated), the transition temperature also falls dramatically

The glycerol-based phospholipids, the most common membrane lipids, include the phosphatidyletha- nolamines described earlier (Fig. 2-1A) as well as the phos- phatidylinositols (Fig. 2-2A), phosphatidylserines (Fig. 2-2B), and phosphatidylcholines

sphingolipids (deriva- tives of sphingosine), are made up of three subgroups: sphin- gomyelins (Fig. 2-2D), glycosphingolipids such as the galactocerebrosides (Fig. 2-2E), and gangliosides

Cholesterol molecule rigid steroid ring binds to and partially immobilizes fatty acid side chains.

Therefore, at modest con- centrations, cholesterol decreases fl uidity. However, when it is present in high concentrations, cholesterol can substan- tially disrupt the ability of the phospholipids to interact among themselves, which increases fl uidity and lowers the gel-sol transition temperature.