Membrane lipids are a group of compounds (structurally similar to fats and oils) which form thelipid bilayer of thecell membrane. The three major classes of membranelipids arephospholipids,glycolipids, andcholesterol. Lipids areamphiphilic: they have 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 and ions as part of the cell's metabolism. In order to perform physiological functions,membrane proteins are facilitated to rotate and diffuse laterally in two dimensional expanse of lipid bilayer by the presence of a shell of lipids closely attached to protein surface, calledannular lipid shell.

The bilayer formed by membrane lipids serves as a containment unit of a living cell. Membrane lipids also form a matrix in whichmembrane proteins reside. Historically lipids were thought to merely serve a structural role. Functional roles of lipids are in fact many: They serve as regulatory agents incell growth andadhesion. They participate in thebiosynthesis of otherbiomolecules. They can serve to increase enzymatic activities ofenzymes.^[1]

Non-bilayer forming lipid like monogalactosyl diglyceride (MGDG) predominates the bulk lipids inthylakoid membranes, which when hydrated alone, forms reverse hexagonal cylindrical phase. However, in combination with other lipids andcarotenoids/chlorophylls of thylakoid membranes, they too conform together as lipid bilayers.^[2]

Phospholipids and glycolipids consist of two long, nonpolar (hydrophobic)hydrocarbon chains linked to ahydrophilic head group.

The heads of phospholipids arephosphorylated and they consist of either:

• Glycerol (and hence the namephosphoglycerides given to this group of lipids), or
• Sphingosine (e.g.sphingomyelin andceramide).

Glycerol dialkyl glycerol tetraether (GDGT) is helpingto study ancient environmental factors.^[3]

The heads of glycolipids (glyco- stands for sugar) contain asphingosine withone or several sugar units attached to it. The hydrophobic chains belong either to:

• twofatty acids (FA) – in the case of the phosphoglycerides, or
• one FA and the hydrocarbon tail of sphingosine – in the case of sphingomyelin and the glycolipids.

Galactolipids – monogalactosyl diglyceride (MGDG) and digalactosyl diglycreride (DGDG) form the predominant lipids in higher plant chloroplast thylakoid membranes; liposomal structures formed by total lipid extract of thylakoid membranes have been found sensitive to sucrose as it turns bilayers into micellar structures.^[4]

The fatty acids in phospho- and glycolipids usually contain an even number, typically between 14 and 24, ofcarbon atoms, with 16- and 18-carbon being the most common. FAs may be saturated or unsaturated, with the configuration of thedouble bonds nearly alwayscis. The length and the degree ofunsaturation of FAs chains have a profound effect onmembranes' fluidity.Plantthylakoid membranes maintain high fluidity, even at relatively cold environmental temperatures, due to the abundance of 18-carbon fatty acyl chains with three double bonds,linolenic acid, as has been revealed by 13-C NMR studies.^[5]

Inphosphoglycerides, the hydroxyl groups at C-1 and C-2 of glycerol areesterified to thecarboxyl groups of the FAs. The C-3 hydroxyl group is esterified to phosphoric acid. The resulting compound, calledphosphatidate, is the simplestphosphoglycerate. Only small amounts of phosphatidate are present in membranes. However, it is a key intermediate in the biosynthesis of the other phosphoglycerides.

Sphingosine is anamino alcohol that contains a long,unsaturated hydrocarbon chain. In sphingomyelin and glycolipids, the amino group of sphingosine is linked to FAs by anamide bond. In sphingomyelin the primaryhydroxyl group of sphingosine is esterified to phosphorylcholine.

In glycolipids, the sugar component is attached to this group. The simplest glycolipid iscerebroside, in which there is only one sugar residue, eitherGlc orGal. More complex glycolipids, such asgangliosides, contain a branched chain of as many as seven sugar residues.

The best knownsterol ischolesterol, which is found in humans. Cholesterol also occurs naturally in other eukaryotecell membranes. Sterols have a hydrophobic four-membered fused ring rigid structure, and a small polar head group.

Cholesterol is bio-synthesised frommevalonate via a squalene cyclisation ofterpenoids. Cell membranes require high levels of cholesterol – typically an average of 20% cholesterol in the whole membrane, increasing locally in raft areas up to 50% cholesterol (- % is molecular ratio).^[6] It associates preferentially withsphingolipids (see diagram) in cholesterol-richlipid rafts areas of the membranes in eukaryotic cells.^[7] Formation of lipid rafts promotes aggregation of peripheral andtransmembrane proteins including docking ofSNARE andVAMP proteins.^[8]Phytosterols, such as sitosterol and stigmasterol, andhopanoids serve a similar function in plants andprokaryotes.

• Homeoviscous adaptation
• Protein-lipid interaction

• Membrane+lipids at the U.S. National Library of MedicineMedical Subject Headings (MeSH)