Phospholipids Of the two major phospholipid classes present in mem branes, phosphoglycerides are the more common and con sist of a glycerol-phosphate backbone to which are attached two fatty acids in ester linkages and an alcohol (Figure 1). The fatty acid constituents are usually even-numbered car bon molecules, most commonly containing 16 or 18 carbons. They are unbranched and can be saturated or unsaturated with one or more double bonds. The simplest phosphoglyceride is phosphatidic acid, a 1,2-diacylglycerol 3-phosphate, a key intermediate in the formation of other phosphoglycerides. In most phosphoglycerides present in membranes, the 3-phosphate is esterified to an alcohol such as choline, ethanolamine, glycerol, inositol, or serine. Phosphatidylcholine is generally the major phosphoglyceride by mass in the membranes of human cells.

Fig1. A phosphoglyceride showing the fatty acids (R1 and R2 ), glycerol, and a phosphorylated alcohol component. Saturated fatty acids are usually attached to carbon 1 of glycerol, and unsaturated fatty acids to carbon 2. In phosphatidic acid, R3 is hydrogen.

The second major class of phospholipids comprises sphingomyelin, a phospholipid that contains a sphingosine rather than a glycerol backbone. A fatty acid is attached by an amide linkage to the amino group of sphingosine, forming ceramide. When the primary hydroxyl group of sphingosine is esterified to phosphorylcholine, sphingomyelin is formed. As the name suggests, sphingomyelin is prominent in myelin sheaths.

Glycosphingolipids

 The glycosphingolipids (GSLs) are sugar-containing lipids built on a backbone of ceramide. GSLs include galactosyl and glucosyl-ceramides (cerebrosides) and the gangliosides, and are mainly located in the plasma membranes of cells, displaying their sugar components to the exterior of the cell.

Sterols

The most common sterol in the membranes of animal cells is cholesterol. The majority of cholesterol resides within plasma membranes, but smaller amounts are found within mitochondrial, Golgi complex, and nuclear membranes. Cholesterol intercalates among the phospholipids of the membrane, with its hydrophilic hydroxyl group at the aqueous interface and the remainder of the molecule buried within the lipid bilayer leaflet. From a nutritional viewpoint, it is important to know that cholesterol is not present in plants.

Lipids can be separated from one another and quantified by techniques such as column, thin-layer, and gas-liquid chromatography and their structures established by mass spectrometry and other techniques.

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