A carbohydrate   is composed   of carbon (carbo-), and hydrogen   and oxygen (-hydrate).  The  easiest  carbohydrates  are  the  monosaccharides that  have  the common  formula  (CH2O)n    where  n is 3 or more.  A simple sugar or monosaccharide  ,  having  of  a  carbon  chain  with  a  number  of  hydroxyl  (OH)  groups and either one aldehyde  group  frequently written  as -CHO)  or one ketone group  .  A  sugar  which  bears  an  aldehyde  group  is  called as  an  aldose whereas a sugar with a ketone group is a ketose. The smallest carbohydrates, for that n = 3, are called as trioses. The terms can be combined. Thus glyceraldehyde is a triose which has an aldehyde group and so is an aldose. Thus it can also be called an aldotriose. Similarly, dihydroxyacetone shown in the below figure is a ketotriose.