These reactions occur in a large membrane-bound cellular organelle called the anammoxosome, which contains an electron transport chain and an ATPase that pumps protons back into the cytoplasm from the anammoxosome lumen. It functions much like a mitochondrion in eukaryotic cells. The anammoxosome membrane is invaginated (folded in upon itself) to increase its surface area. The existence of membrane-bound cellular organelles is very unusual in prokaryotes, and appears to be limited to the members of the phylum Planctomycetota.
