Carrier of the growing fatty acid chain in fatty acid biosynthesis. May be involved in the synthesis of very-long-chain fatty acids. Accessory and non-catalytic subunit of the mitochondrial membrane respiratory chain NADH dehydrogenase (Complex I), which functions in the transfer of electrons from NADH to the respiratory chain (By similarity). Lipid metabolism; fatty acid biosynthesis. Complex I is composed of about 45 different subunits. 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.