Their Structure

Carbohydrates can exist as single molecules, monosaccharides, or polymers, polysaccharides, made up of the basic repeating monosaccharide unit. 

Monosaccharides General Formula:  Cx(H2O)y

(with anywhere from 3-6 carbon atoms forming a backbone)

Containing either an aldehyde group (-CHO) or a ketone group (C=O) with several hydroxyl(-OH) groups.

 Monosaccharides are reducing sugars because the ketone or aldehyde functional group can act as a reducing agent in a chemical reaction, by undergoing oxidation. Aldose sugarsare monosaccharides that contain an aldehyde functional group. 

Those monosaccharides containing a ketone functional group are therefore known as ketose sugars. Below is an example of fructose, a ketohexose, that is found in many fruits.

https://www.youtube.com/watch?v=mUo4c2f9rpI