|MadSci Network: Biochemistry|
Phospholipids are the building blocks of cell membranes in all creatures from bacteria to plants to insects to people. Eukaryotic (organisms above bacteria) cells have multiple subcellular organelles, like nuclei, mitochondria, chloroplasts, vacuoles, ER's, etc., which rely on phospholipid membranes to isolate them and to carry out many of their functions. Since all cells have membranes composed mostly of phospholipids, all organic material that you eat will contain some phospholipids.
Phospholipids come in several flavors, but they are all very closely related. At the heart of most phospholipids is Phosphatidic Acid (PA), which is a diglyceride composed of a molecule of glycerol with fatty acids attached to the first two carbons and a phosphate attached to the third. By attaching various 'head groups' to the phosphate of PA, you can make most of the phospholipids: PA + Inositol = phosphatidyl inositol (PI); PA + Ethanolamine = phosphatidyl ethanolamine (PE, also called cephalin); PA + Choline = phosphatidyl choline (PC, also called lecithin); and so on. The other major class of phospholipids is based around Ceremide, which is like PA but with serine at its heart in place of glycerol: Ceremide + choline = Sphingomyelin (SM); Ceremide + sugar = Ganglioside; and so on. The head groups affect the biochemistry of the membranes, so some organelles may have much more PE than the cell membrane which is higher in SM. They also vary tissue to tissue: liver cell membranes are high in PC; brain cell membranes are high in gangliosides; bacterial membranes are mostly PE.
Of all of the fat in food, only about 7% is diglycerides (including phospholipids), which may make them appear valuable. However, before these phospholipids can get to the small intestines, where fat absorption occurs, they are assaulted by lipases, phospho- lipases, and other enzymes which remove their fatty acid chains and break down the other components. By the time fat absorption occurs, the components of the phospholipids are indistinguishable from the components of other lipids. Some head groups, like choline and inositol, are hard for the body to make, so absorption of these components from food is important for a healthy diet. Choline and inositol are also available from other dietary sources, so the ingestion of the corresponding phospho- lipids is a good source of these substances but is not vital.
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