Phospholipid scramblase that acts downstream of ced-9 and caspase ced-3 to promote phosphatidylserine exposure on apoptotic cell surface (PubMed:24225442, PubMed:23845944). Phosphatidylserine is a specific marker only present at the surface of apoptotic cells and acts as a specific signal for engulfment (PubMed:24225442, PubMed:23845944). Regulates apoptosis kinetics during embryonic development (PubMed:10882128, PubMed:1936965, PubMed:24225442, PubMed:23845944). Not required for engulfment of germ cell corpses (PubMed:9927601). a 1,2-diacyl-sn-glycero-3-phospho-L-serine(in) = a 1,2-diacyl-sn-glycero-3-phospho-L-serine(out) Cleavage by ced-3 activates ced-8 function in promoting phosphatidylserine exposure at the surface of apoptotic cells. Delayed cell death process. Belongs to the XK family.