The Major Lipids in Mammalian Membranes Are Phospholipids Glycosphingolipids Cholesterol

A. Phospholipids

Of the two major phospholipid classes present in membranes, phosphoglycerides are the more common and consist of a glycerol backbone to which are attached two fatty acids in ester linkage and a phosphorylated alcohol (Figure 41-2). The fatty acid constituents are usually even-numbered carbon molecules, most commonly containing 16 or 18 carbons. They are un-branched and can be saturated or unsaturated. The simplest phosphoglyceride is phosphatidic acid, which is

Fatty acids

II I

Glycerol

Alcohol

Figure 41-2. A phosphoglyceride showing the fatty acids (R1 and R2), glycerol, and phosphorylated alcohol components. In phosphatidic acid, R3 is hydrogen.

1,2-diacylglycerol 3-phosphate, a key intermediate in the formation of all other phosphoglycerides (Chapter 24). In other phosphoglycerides, the 3-phosphate is es-terified to an alcohol such as ethanolamine, choline, serine, glycerol, or inositol (Chapter 14).

The second major class of phospholipids is composed of sphingomyelin, which contains a sphingosine backbone rather than glycerol. A fatty acid is attached by an amide linkage to the amino group of sphingosine, forming ceramide. The primary hydroxyl group of sphingosine is esterified to phosphorylcholine. Sphin-gomyelin, as the name implies, is prominent in myelin sheaths.

The amounts and fatty acid compositions of the various phospholipids vary among the different cellular membranes.

B. Glycosphingolipids

The glycosphingolipids (GSLs) are sugar-containing lipids built on a backbone of ceramide; they include galactosyl- and glucosylceramide (cerebrosides) and the gangliosides. Their structures are described in Chapter 14. They are mainly located in the plasma membranes of cells.

C. Sterols

The most common sterol in membranes is cholesterol (Chapter 14), which resides mainly in the plasma membranes of mammalian cells but can also be found in lesser quantities in mitochondria, Golgi complexes, and nuclear membranes. Cholesterol intercalates among the phospholipids of the membrane, with its hydroxyl group at the aqueous interface and the remainder of the molecule within the leaflet. Its effect on the fluidity of membranes is discussed subsequently.

All of the above lipids can be separated from one another by techniques such as column, thin layer, and gas-liquid chromatography and their structures established by mass spectrometry.

Each eukaryotic cell membrane has a somewhat different lipid composition, though phospholipids are the major class in all.

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