Involved in the biosynthesis of dimorphecolic acid (9-OH-18:2(10E,12E)). Converts oleic acid (18:1(9Z)) into 18:2(9Z,12E) and probably palmitoleic acid (16:1(9Z)) into 16:2(9Z,12E). Very limited ability to catalyze (Z)-delta(12) desaturation. a (9Z)-octadecenoyl-containing glycerolipid + 2 Fe(II)-[cytochrome b5] + 2 H(+) + O2 = a (9Z,12E)-octadecadienoyl-containing glycerolipid + 2 Fe(III)-[cytochrome b5] + 2 H2O a (9Z)-hexadecenoyl-containing glycerolipid + 2 Fe(II)-[cytochrome b5] + 2 H(+) + O2 = a (9Z,12E)-hexadecadienoyl-containing glycerolipid + 2 Fe(III)-[cytochrome b5] + 2 H2O Belongs to the fatty acid desaturase type 1 family.