The hydroxyl group is highly polar, and usually are either split from their molecules or they link onto larger molecular assemblies. The two most frequent interactions are condensation, which is when the elements of a water molecule get released as subunits, and hydrolysis, where the elements of a water molecule are incorporated as subunits which are then split from a larger molecule. The hydroxyl group is the group that forms the reactive part of the alcohol group. A hydroxyl is bonded to the carbon, which also has another hydrogen molecule attached. The alcohol group easily reduces to aldehydes and ketones, because the hydroxyl segment forms linkages very easily.
Alcohol is part of many carbohydrate molecules.
The carbonyl group is very weakly polar, and highly reactive. It is especially susceptible to attack by bases. Thus group is part of both aldehydes and ketones. Aldehydes and ketones are important as building blocks of fats and carbohydrates. In a ketone, the carbonyl group is in the middle of the molecule, while in an aldehyde, it exists at the end.