Biochemistry: The Chemical Reactions of Living Cells, Volume 1 - D. Metzler 1980

Membranes and Cell Walls
Membrane Structure
Lipid Components of Membranes

About 1500 different Lipids have been identified in the myelin of the human Central Nervous system, 30 of which are present in significant amounts [12]. Elucidating the General Principles of membrane composition is significantly complicated by the fact that membranes of different origins vary widely in their content of various lipid types. However, virtually all membranes, regardless of origin, contain Phospholipids, which account for 40 to 90% of the total Membrane Lipids (Table 5-1).

Five types of phospholipids dominate in membranes: phosphatidylcholine, phosphatidylethanolamine, phosphatidylserine, cardiolipin (diphosphatidylglycerol), and sphingomyelin; phosphatidylinositol is also found in small amounts. Significant amounts of cardiolipin are found exclusively in bacterial membranes and The inner mitochondrial membranes. Sphingomyelin is absent from mitochondrial membranes. A characteristic sphingolipid of invertebrates is ceramide aminoethylphosphonate. Chloroplast membranes contain a specific sulfolipid with negatively charged HEAD groups (its Structure is shown in Chapter 2, Section D).

Glycolipids are essential components of Plasma Membranes, myelin (which is derived from plasma membranes), as well as the membranes of the Endoplasmic reticulum and METABOLISM/14.html">Chloroplasts. Cerebrosides are particularly abundant in myelin, which also contains sulfate esters of cerebrosides (sulfatides). Plant Cell membranes primarily contain glycerol derivatives (galactosyl diglycerides) rather than sphingosine.

Plasma membranes also contain Cholesterol, with its content in myelin reaching as high as 17%. In contrast, cholesterol is found only in trace amounts in Mitochondria and is virtually absent in bacterial membranes. Sterol esters, which serve as the transport form of sterols, do not occur in membranes. Triglycerides are likewise absent from membranes—these compounds are found exclusively as cytoplasmic lipid droplets. Minor membrane components (which, despite their low Abundance, perform crucial biological Functions) include ubiquinone, localized in the inner mitochondrial membrane (Chapter 10, Section G), tocopherols (Chapter 10, Section G), and polyprenyl alcohols (Chapter 12, Section 3). The chloroplast membranes of plant Cells contain chlorophyll, carotenes, and several other fat-soluble pigments.

Class="center">Table 5-1 Approximate Chemical composition of certain membranes

^а Dewey M. M., Barr L., Curr. Top. Membr. Transp., 1, 6 (1970).

⁶ Kaback H. R., Curr. Top. Membr. Transp., 1, 35–99 (1970).

^в Mizushima S., Yamada H., BBA, 375, 44–53 (1975).

^r Yamato I., Anraku Y., Hirosawa H., J. Biochem. (Tokyo), 77, 705–718, 1975. These authors found 67% protein, 21% lipids, 10% CARBOHYDRATES, and 2% RNA.

^д Lichtenthaler H. K., Park R. B., Nature (London), 198, 1070–1072 (1963).

^е Approximately 14% is accounted for by chlorophyll, carotenoids, and Quinones.

^ж Diphosphatidylglycerol (Table 2-8).