Membrane lipids are a group of compounds (structurally similar to fats and oils) which form the double-layered surface of all cells. The three major classes of membrane lipids are phospholipids, glycolipids, and cholesterol. Like magnets, membrane lipids have polarity with one end that is soluble in water ('polar') and an ending that is soluble in fat ('nonpolar'). By forming a double layer with the polar ends pointing outwards and the nonpolar ends pointing inwards membrane lipids can form a 'lipid bilayer' which keeps the watery interior of the cell separate from the watery exterior. The arrangements of lipids and various proteins, acting as receptors and channel pores in the membrane, control the entry and exit of other molecules as part of the cell's metabolism.
The heads of phospholipids are phosphorylated and they consist of either:
- Glycerol (and hence the name phosphoglycerides given to this group of lipids).
- Sphingosine (with only one member - sphingomyelin).
The heads of glycolipids contain a sphingosine with one or several sugar units attached to it. The hydrophobic chains belong either to:
Fatty acids 
The fatty acids in phospho- and glycolipids usually contain an even number of carbon atoms, typically between 14 and 24. The 16- and 18-carbon FAs are the most common ones. FAs may be saturated or unsaturated, with the configuration of the double bonds nearly always cis. The length and the degree of unsaturation of FAs chains have a profound effect on membranes' fluidity.
In phosphoglycerides, the hydroxyl groups at C-1 and C-2 of glycerol are esterified to the carboxyl groups of the FAs. The C-3 hydroxyl group is esterified to phosphoric acid. The resulting compound, called phosphatidate, is the simplest phosphoglycerate. Only small amounts of phosphatidate are present in membranes. However, it is a key intermediate in the biosynthesis of the other phosphoglycerides.
Sphingosine is an amino alcohol that contains a long, unsaturated hydrocarbon chain. In sphingomyelin and glycolipids, the amino group of sphingosine is linked to FAs by an amide bond. In sphingomyelin the primary hydroxyl group of sphingosine is esterified to phosphoryl choline. In glycolipids, the sugar component is attached to this group. The simplest glycolipid is cerebroside, in which there is only one sugar residue, either Glc or Gal. More complex glycolipids, such as gangliosides, contain a branched chain of as many as seven sugar residues.
Cholesterol occurs naturally in eukaryote cell membranes where it is bio-synthesised from mevalonate via a squalene cyclisation of terpenoids. It associated preferentially with sphingolipids (see diagram) in cholesterol-rich lipid rafts areas of the membranes in eukaryotic cells. Hopanoids serve a similar function in prokaryotes.
Cell membranes require high levels - typically an average of 20% cholesterol molecular in the whole membrane, increasing locally in raft areas up to 50% cholesterol (- % is molecular ratio)  Formation of lipid rafts promotes aggregation of peripheral and transmembrane proteins including docking of SNARE and VAMP proteins
See also 
- ¦Author= Chen, H;Born, E;Mathur, S N;Field, F J ¦Journal= J Lipid Res ¦Month= Dec ¦Number= 12 ¦Pages= 2159-67 ¦Title= Cholesterol and sphingomyelin syntheses are regulated independently in cultured human intestinal cells, CaCo-2: role of membrane cholesterol and sphingomyelin content ¦Volume= 34 ¦Year= 1993 ¦PMID=8301234 ¦ISBN=0022-2275
- de Meyer F, Smit B. Effect of cholesterol on the structure of a phospholipid bilayer. Proc Natl Acad Sci U S A 2009; 106: 3654-8.
- Lang T, Bruns D, Wenzel D, Riedel D, Holroyd P, Thiele C, Jahn R. SNAREs are concentrated in cholesterol-dependent clusters that define docking and fusion sites for exocytosis EMBO J 2001;20:2202-13.