A major characteristic of eukaryotic cells is the presence of cellular organelles. These organelles are bound by one or more membranes that serve as a barrier to the free passage of molecules - thus maintaining the unique composition and environment of individual organelles. Not only do individual organelles have a characteristic complement of enzymes (and, therefore, metabolic pathways) they also have membranes made up of a special blend of lipid molecules. In a few cases, such as the diphos-
phatidylglycerol of the inner mitochondrial membrane, these lipid distributions are so specific that they can be used to aid identification of subcellular fractions. In some instances, organelles can be induced or caused to undergo functional development or morphological change. By the use of such systems, the role of lipids in membrane biogenesis and function can be studied.
Most cellular membranes are composed predominantly of proteins and phosphoglycerides. Therefore, phosphoglycerides are significant components of prokaryotes and eukaryotes. In animals and non-photosynthetic cells of higher plants, phosphatidylcholine is the main phosphoglyceride. This molecule is not, however, a major component in bacteria. Phosphatidylethanolamine is a major component in most organisms while phosphati-dylglycerol is common in prokaryotes and is the dominant (in many cases exclusive) phosphogly-ceride of the photosynthetic membranes of oxygen-evolving species. As well as these molecules, phosphatidylinositol is quantitatively important forming a smaller but significant percentage of the membrane lipids in many cases. Phosphorylated
Acyl-acid (wax ester)
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