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. 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. Defects in the fatty acid composition. Belongs to the fatty acid desaturase type 1 family.