Ribulose is a ketopentose — a monosaccharide containing five carbon atoms, and including a ketone functional group. It has chemical formula C5H10O5. Two enantiomers are possible, D-ribulose (D-erythro-pentulose) and L-ribulose (L-erythro-pentulose). D-Ribulose is the diastereomer of D-xylulose. Ribulose sugars are composed in the pentose phosphate pathway. They are important in the formation of many bioactive substances. For example, D-ribulose is an intermediate in the fungal pathway for D-arabitol production. Also, as the 1,5-bisphosphate, D-ribulose combines with carbon dioxide at the start of the photosynthesis process in green plants (carbon dioxide trap).