Palmitate, the principal product of the fatty acid synthase system in animal cells, is the precursor of other long-chain fatty acids. It may be lengthened to form stearate (18:0) or even longer saturated fatty acids by further additions of acetyl groups, through the action of fatty acid elongation systems present in the smooth endoplasmic reticulum and in mitochondria.

The more active elongation system of the ER extends the 16-carbon chain of palmitoyl-CoA by two carbons, forming stearoyl-Co Although different enzyme systems are used, and coenzyme A rather than ACP is the acyl carrier in the reaction, the mechanism of elongation in the ER is otherwise identical to that in palmitate synthesis: donation of two carbons by malonyl-CoA, followed by reduction, dehydration, and reduction to the saturated 18-carbon product, stearoyl-Co

Two key products of elongation pathways are linoleate, an ω-6 fatty acid, and α-linolenate, an ω-3 fatty acid. These are precursors for two extensive families of derivative unsaturated fatty acids, the ω-6 and ω-3 families. Humans cannot synthesize linoleate and α-linolenate and must obtain them in the diet.

The ratio of ω-6 to ω-3 fatty acids in the diet, if too high, can lead to cardiovascular disease. The importance of this ratio may reflect the multitude of signaling molecules in the ω-6 and ω-3 families, with their equally complex physiological effects.