Please note that the content of this book primarily consists of articles available from Wikipedia or other free sources online. A tetrose is a monosaccharide with 4 carbon atoms. They either have an aldehyde functional group in position 1 (aldotetroses) or a ketone functional group in position 2 (ketotetroses). The aldotetroses have two chiralatrial centres ("asymmetric carbon atoms") and so 4 different stereoisomers are possible. The naturally occurring aldotetroses are: D-Erythrose, D-Threose.