Palmitate is the precursor of stearate and longer-chain saturated fatty acids as well as palmitoleate and oleate (Pollard and Stumpf, 1980). The palmitic acid gets modified further and lengthened to form stearate (18:0) or even to longer saturated fatty acids (oleiceate, linealate, etc.) by further additions of acetyl groups through the action of fatty acid elongation systems present in the smooth endoplasmic reticulum (ER) and in mitochondria (Thelen and Ohlrogge, 2002). The mechanism of elongation in the ER is identical to palmitate synthesis, which involves donation of two carbons by malonyl-CoA, followed by reduction, dehydra­tion, and reduction to the saturated 18-carbon product, stearoyl-CoA. Figure 8.4 shows the formation of higher fatty acids from the palmitic acid through different steps of chain elonga­tion. In algae, oleate (from stearoyl-CoA) gets converted to the a and g linolenates (Thelen and Ohlrogge, 2002). a-linolenate further getsconverted to other polyunsaturated fatty acids, while g-linolenate converts to the eicosatrienoate and further arachidonate. Mammals cannot

convert oleate to linoleate or linolenate because of the lack of enzymes to introduce double bonds at carbon atoms beyond C9 (Nelson and Cox, 2009). All fatty acids containing a double bond at positions beyond C9 have to be supplied in the diet and are called essential fatty acids.