cellular membranes

In animal cells the plasma membrane physically separates the intracellular components from the extracellular environment. All eukaryotic cells are compartmentalized into membrane-bound organelles which carry out different functions.

Components

The glycerophospholipids are the main structural component of biological membranes, such as the cellular plasma membrane and the intracellular membranes of organelles.

Glycerophospholipids are amphipathic molecules that contain a glycerol core linked to two fatty acid-derived "tails" by ester or, more rarely, ether linkages and to one "head" group by a phosphate ester linkage.

While glycerophospholipids are the major component of biological membranes, other non-glyceride lipid components such as sphingomyelin and sterols (mainly cholesterol in animal cell membranes) are also found in biological membranes.

Types

 plasmic membranes
 intracellular membranes

Components

For lipids present in biological membranes, the hydrophilic head is from one of three classes:

 glycolipids, whose heads contain an oligosaccharide with 1-15 saccharide residues.
 phospholipids, whose heads contain a positively charged group that is linked to the tail by a negatively charged phosphate group.
 sterols, whose heads contain a planar steroid ring, for example, cholesterol.

Lipid bilayer and liposome

A biological membrane is a form of lipid bilayer, as is a liposome. The formation of lipid bilayers is an energetically-preferred process when the glycerophospholipids described above are in an aqueous environment.

In an aqueous system, the polar heads of lipids orientate towards the polar, aqueous environment, while the hydrophobic tails minimise their contact with water.

The lipophilic tails of lipids (U) tend to cluster together, forming a lipid bilayer (1) or a micelle (2). Other aggregations are also observed and form part of the polymorphism of amphiphile (lipid) behaviour.

The polar heads (P) face the aqueous environment, curving away from the water. Phase behaviour is a complicated area within biophysics and is the subject of current academic research. Micelles and bilayers form in the polar medium by a process known as the hydrophobic effect.

When dissolving a lipophilic or amphiphilic substance in a polar environment, the polar molecules (i.e. water in an aqueous solution) become more ordered around the dissolved lipophilic substance, since the polar molecules cannot form hydrogen bonds to the lipophilic areas of the amphiphile.

So in an aqueous environment the water molecules form an ordered "clathrate" cage around the dissolved lipophilic molecule.

References

 Boes M, Cuvillier A, Ploegh H. Membrane specializations and endosome maturation in dendritic cells and B cells. Trends Cell Biol. 2004 Apr;14(4):175-83. PMID: 15066635