Carrier of the growing fatty acid chain in fatty acid biosynthesis. Lipid metabolism; fatty acid biosynthesis. 4'-phosphopantetheine is transferred from CoA to a specific serine of apo-ACP by acpS. This modification is essential for activity because fatty acids are bound in thioester linkage to the sulfhydryl of the prosthetic group (By similarity). Belongs to the acyl carrier protein (ACP) family.