| Structure of phospholipids
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| The term 'phospholipid', introduced in Chapter 7, commonly refers to the major class of membrane lipids, glycerophospholipids, or phosphoglycerides. They contain a diacylglycerol (DAG) phosphate (phosphatidic acid) backbone. In addition to phosphatidylcholine, phosphatidylethanolamine, and phosphatidylserine, there are a number of more complicated structures, such as phosphatidylglycerol and diphosphatidylglycerol (cardiolipin), which are found in mitochondrial membranes, and phosphatidylinositol, which has a role in signal transduction and in anchoring proteins in the plasma membrane (Fig. 26.1).
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| Phospholipids from any natural source contain a spectrum of fatty acids. For example, phosphatidylcholine usually contains palmitic acid (C-16:0) or stearic acid (C-18:0) at its carbon-1 position and an 18-carbon, unsaturated fatty acid (e.g. oleic, linoleic, or linolenic) at its carbon-2 position (C-16:0 represents a 16-carbon fatty acid with no double bonds, while C-18:1 represents an 18-carbon fatty acid with one unsaturated (C=C) bond). Phosphatidylethanolamine usually has a longer-chain fatty acid at carbon-2, such as arachidonic acid (C-20:4). In addition to forming the backbone of the membrane, these complex lipids contribute charge to the membrane: phosphatidylcholine and phosphatidylethanolamine are zwitterionic at physiologic pH and have no net charge, but the other glycerophos-pholipids and the major species of sphingolipids are anionic in nature.
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