Microsomal (ER) omega-3 fatty acid desaturase introduces the third double bond in the biosynthesis of 18:3 fatty acids, important constituents of plant membranes. It is thought to use cytochrome b5 as an electron donor and to act on fatty acids esterified to phosphatidylcholine and, possibly, other phospholipids. a (9Z,12Z)-octadecadienoyl-containing glycerolipid + 2 Fe(II)-[cytochrome b5] + 2 H(+) + O2 = (9Z,12Z,15Z)-octadecatrienoyl-containing glycerolipid + 2 Fe(III)-[cytochrome b5] + 2 H2O Lipid metabolism; polyunsaturated fatty acid biosynthesis. A number of isoforms are produced. According to EST sequences. Abundant in leaves and seedlings. Barely detectable in root tissue. The histidine box domains may contain the active site and/or be involved in metal ion binding. Belongs to the fatty acid desaturase type 1 family.