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, -ribulose (-erythro-pentulose) and -ribulose (-erythro-pentulose). -Ribulose is the diastereomer of -xylulose. Ribulose sugars are composed in the pentose phosphate pathway from arabinose. They are important in the formation of many bioactive substances. For example, -ribulose is an intermediate in the fungal pathway for -arabitol production. Also, as the 1,5-bisphosphate, -ribulose combines with carbon dioxide at the start of the photosynthesis process in green plants (carbon dioxide trap). Ribulose has the same stereochemistry at carbons 3 and 4 as the five-carbon aldoses ribose and arabinose.