Carbohydrates are really just polyhydroxyaldehydes, the aldoses,
or polyhydroxyketones, the ketoses.
So, for a carbohydrate to be an aldose it needs an aldehyde
group.
Remember that an aldehyde group, here shown as CH=O, is also often written as CHO.
And to be a ketose it needs a ketone group.
The critical centers are marked in the diagrams with *
The assignment is quite apparent in the acyclic form of the sugar, see
above.
In the cyclic form, find the anomeric center (*)
and then look at the substituents there. If one is an H, then it is an
aldose (see glucose example above).
The systems are further classified depending on how many C atoms
there are (see above for examples)
