<P> The first region on either side of the bilayer is the hydrophilic headgroup . This portion of the membrane is completely hydrated and is typically around 0.8 - 0.9 nm thick . In phospholipid bilayers the phosphate group is located within this hydrated region, approximately 0.5 nm outside the hydrophobic core . In some cases, the hydrated region can extend much further, for instance in lipids with a large protein or long sugar chain grafted to the head . One common example of such a modification in nature is the lipopolysaccharide coat on a bacterial outer membrane, which helps retain a water layer around the bacterium to prevent dehydration . </P> <P> Next to the hydrated region is an intermediate region that is only partially hydrated . This boundary layer is approximately 0.3 nm thick . Within this short distance, the water concentration drops from 2M on the headgroup side to nearly zero on the tail (core) side . The hydrophobic core of the bilayer is typically 3 - 4 nm thick, but this value varies with chain length and chemistry . Core thickness also varies significantly with temperature, in particular near a phase transition . </P> <P> In many naturally occurring bilayers, the compositions of the inner and outer membrane leaflets are different . In human red blood cells, the inner (cytoplasmic) leaflet is composed mostly of phosphatidylethanolamine, phosphatidylserine and phosphatidylinositol and its phosphorylated derivatives . By contrast, the outer (extracellular) leaflet is based on phosphatidylcholine, sphingomyelin and a variety of glycolipids, In some cases, this asymmetry is based on where the lipids are made in the cell and reflects their initial orientation . The biological functions of lipid asymmetry are imperfectly understood, although it is clear that it is used in several different situations . For example, when a cell undergoes apoptosis, the phosphatidylserine--normally localised to the cytoplasmic leaflet--is transferred to the outer surface: There, it is recognised by a macrophage that then actively scavenges the dying cell . </P> <P> Lipid asymmetry arises, at least in part, from the fact that most phospholipids are synthesised and initially inserted into the inner monolayer: those that constitute the outer monolayer are then transported from the inner monolayer by a class of enzymes called flippases . Other lipids, such as sphingomyelin, appear to be synthesised at the external leaflet . Flippases are members of a larger family of lipid transport molecules that also includes floppases, which transfer lipids in the opposite direction, and scramblases, which randomize lipid distribution across lipid bilayers (as in apoptotic cells). In any case, once lipid asymmetry is established, it does not normally dissipate quickly because spontaneous flip - flop of lipids between leaflets is extremely slow . </P>

Why does the arrangement of the phospholipid bilayer make sense