The lengthening of fatty acids by the addition of two carbon units (acetyl groups). Elongation occurs in either the endoplasmic reticulum or the mitochondria. The reaction differs depending on where it occurs. In the endoplasmic reticulum, the reaction sequence is similar to that described for the cytosolic fatty acid synthase complex. The source of the two-carbon unit is malonyl coenzyme A (CoA), and NADPH provides the reducing power. The intermediates are CoA esters and not the acyl carrier protein 4′-phosphopantetheine. The reaction sequence produces stearic acid (18:0) in all tissues that make fatty acids except the brain. In the brain, elongation can proceed further by producing fatty acids containing up to 24 carbons. In the mitochondria, elongation uses acetyl CoA rather than malonyl CoA as the source of the two- carbon unit. It uses either NADH+H+ or NADPH+H^ as the source of reducing equivalents and uses, as substrate, carbon chains of less than 16 carbons. Mitochondrial elongation is the reversal of fatty acid oxidation, which also occurs in this organelle.