Fig. 1.

Self-assembly of biological membranes. Lipids are amphiphilic molecules. In aqueous environments, they self-assemble to form micelles or bilayers such that the hydrophilic head groups interface with water while the hydrophobic tails are protected. Left: free lipids in water have their hydrophobic tails exposed. The hydrogen bonds within water are broken or rearranged to account for the new hydrophobic entities. Center: the lipids can arrange themselves into a micelle, so that all the hydrophobic groups are protected from exposure to water. Right: the fatty acid chains in 2-tailed lipids are often too long to fit in micelles without exposure of the hydrophobic groups to water. Therefore, lipids often form bilayers and water forms a hydrogen bond cage along the surface of the head groups.