Catalyzes the formation of phosphatidylethanolamine (PtdEtn) from phosphatidylserine (PtdSer) (PubMed:30488656, PubMed:30858161). Plays a central role in phospholipid metabolism and in the interorganelle trafficking of phosphatidylserine. May be involved in lipid droplet biogenesis at the endoplasmic reticulum membrane (By similarity). {ECO:0000250|UniProtKB:A0A8H4BVL9, ECO:0000255|HAMAP- Rule:MF_03208, ECO:0000269|PubMed:30488656, ECO:0000269|PubMed:30858161}. This is the function of PISD (phosphatidylserine decarboxylase, Ensembl gene identifier ENSG00000241878).