Fatty acid desaturase that introduces a cis double bond at the 5-position in 20-carbon polyunsaturated fatty acids incorporated in a glycerolipid that contain a Delta(8) double bond. Involved in the conversion of di-homo-Delta-linolenic acid to arachidonic acid. Essential in the production of eicosanoids. an (8Z,11Z,14Z)-icosatrienoyl-containing glycerolipid + 2 Fe(II)-[cytochrome b5] + 2 H(+) + O2 = (5Z,8Z,11Z,14Z)-eicosatetraenoyl-containing glycerolipid + 2 Fe(III)-[cytochrome b5] + 2 H2O an (8Z,11Z,14Z,17Z)-eicosatetraenoyl-containing glycerolipid + 2 Fe(II)-[cytochrome b5] + 2 H(+) + O2 = a (5Z,8Z,11Z,14Z,17Z)-eicosapentaenoyl-containing glycerolipid + 2 Fe(III)-[cytochrome b5] + 2 H2O The cytochrome b5 heme-binding domain acts as the direct electron donor to the active site of the desaturase, and does not require an external cytochrome. Belongs to the fatty acid desaturase type 1 family.