A pentose is a monosaccharide with five carbon atoms. They either have an aldehyde functional group in position 1 (aldopentoses), or a ketone functional group in position 2 (ketopentoses). The aldopentoses have three chiral centres ("asymmetric carbon atoms") and so 8 different stereoisomers are possible. The 4 D-aldopentoses are: CH=O CH=O CH=O CH=O | | | | HC-OH HO-CH HC-OH HO-CH | | | | HC-OH HC-OH HO-CH HO-CH | | | | HC-OH HC-OH HC-OH HC-OH | | | | CH2OH CH2OH CH2OH CH2OH D-Ribose D-Arabinos… (More on Pentose)