Ribulose is a ketopentose, a monosaccharide containing five carbon atoms and a ketone functional group. It is synthesized in the pentose phosphate pathway and plays a role in the formation of various bioactive compounds. It is a structural isomer of ribose and exists as two enantiomers, D-ribulose and L-ribulose. A double phosphate ester of D-ribulose, ribulose-1,5-bisphosphate combines with carbon dioxide at the beginning of photosynthesis.