Formation of double bonds:

In eukaryotes  the SER has enzymes  able to introduce  double  bonds into fatty acyl CoA molecules  in an oxidation  reaction  which uses molecular  oxygen.  This reaction is catalyzed through a membrane-bound complex of three enzymes: NADH- cytochrome cytochrome b5, b5 reductase, and a desaturase. The whole reaction is:

The reaction may be repeated to introduce more than one double bond into a fatty acid.

Mammals lack the enzymes to insert twice bonds at carbon atoms beyond C-9 in the fatty acid chain. Therefore they cannot synthesize linoleate and linolenate, both of which have twice bonds later in the chain than C-9 (linoleate has cis Δ⁹,  cis, Δ¹²  double bonds, and linolenate  has all-cis Δ¹², Δ⁹ , Δ¹⁵  double bonds). Therefore, in mammals linoleate and linolenate are called essential fatty acids since they have to be supplied in the diet. That two unsaturated fatty acids are also the beginning points for the synthesis of other unsaturated fatty acids, like as arachidonate.  That C20:4 fatty acid is the precursor of various biologically important molecules, involving the prostacyclins, thromboxanes, prostaglandins, and leukotrienes.