Arrangement The phospholipids are
amphiphilic. The hydrophilic end usually contains a negatively charged phosphate group, and the hydrophobic end usually consists of two "tails" that are long
fatty acid residues. In aqueous solutions, phospholipids are driven by
hydrophobic interactions, which result in the fatty acid tails aggregating to minimize interactions with the water molecules. The result is often a
phospholipid bilayer: a membrane that consists of two layers of oppositely oriented phospholipid molecules, with their heads exposed to the liquid on both sides, and with the tails directed into the membrane. That is the dominant structural motif of the membranes of all
cells and of some other biological structures, such as
vesicles or
virus coatings. In biological membranes, the phospholipids often occur with other molecules (e.g.,
proteins,
glycolipids,
sterols) in a
bilayer such as a
cell membrane. Lipid bilayers occur when hydrophobic tails line up against one another, forming a membrane of hydrophilic heads on both sides facing the water.
Dynamics These specific properties allow phospholipids to play an important role in the cell membrane. Their movement can be described by the
fluid mosaic model, which describes the membrane as a mosaic of lipid molecules that act as a solvent for all the substances and proteins within it, so proteins and lipid molecules are then free to diffuse laterally through the lipid matrix and migrate over the membrane.
Sterols contribute to membrane fluidity by hindering the packing together of phospholipids. However, this model has now been superseded, as through the study of
lipid polymorphism it is now known that the behaviour of lipids under physiological (and other) conditions is not simple. ==Main phospholipids==