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IntroductionLipids are a group of fat and fat-like substances, rich in carbon and hydrogen, that dissolve in organic solvents. They are necessary for maintaining the structural integrity of membranes and constitute approximately 40 % of the total dry weight of membranes, the remaining 60 % being proteins.The kinds and proportions of lipids to proteins vary with the membrane and the physiological status of the cell. The principal lipid classes of the plasma membrane include phospholipids (65 % ), glycolipids (20 % ) and sterols (5 % ). The four most abundant phospholipids are phosphotidyl choline, phosphotidyl ethanolamine, phosphotidyl glycerol and phosphotidyl inositol. Mono-galactosyl diglyceride and digalactosyl-diglyceride, stigmasterol and sitosterol are the commonest glycolipids and sterols respectively. The distribution of these compounds differ with the organelles, depending on their function (Harwood and Russel 1984). Lipid Composition of Different MembranesThe lipid composition of the membranes varies with organelles depending on their structure and function. In plasma membranes approximately half of the dry weight is lipid. The major components include phospholipids (65%), glycolipids (20%) and sterols (5 % ). In the case of mitochondria, phospholipids comprise a greater portion of total membrane lipid (upto 98 %) with phosphotidylcholine and phosphotidylethanolamine as major components. Phosphotidylinositol is concentrated in the M. N. V. Prasad et al., Heavy Metal Stress in Plants

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